From beb16cf634c83e6d66316df814b24461cda4bb8a Mon Sep 17 00:00:00 2001
From: Martin Goik <goik@hdm-stuttgart.de>
Date: Tue, 30 Apr 2019 17:17:23 +0200
Subject: [PATCH] Slide cosmetics, new method def slide, exercise moved beyond
constructor mention
---
Doc/Sd1/objectsClasses.xml | 2881 ++++++++++++++++++------------------
1 file changed, 1456 insertions(+), 1425 deletions(-)
diff --git a/Doc/Sd1/objectsClasses.xml b/Doc/Sd1/objectsClasses.xml
index 56b5c4831..1c9b00280 100644
--- a/Doc/Sd1/objectsClasses.xml
+++ b/Doc/Sd1/objectsClasses.xml
@@ -817,1827 +817,1858 @@ System.out.println("Perimeter=" + r.getPerimeter());</programlisting></td>
</informaltable>
</figure>
- <qandaset defaultlabel="qanda" xml:id="sd1_qanda_geometry_Rectangle">
- <title>Modeling geometry objects: Rectangles</title>
-
- <qandaentry>
- <question>
- <para>We want to represent rectangles being defined by width and
- height to allow for the subsequently demonstrated operations:</para>
-
- <programlisting language="java">final Rectangle r = new Rectangle(8, 5); <co
- linkends="sw1CalloutRectangle-1" xml:id="constructor"/>
-
-System.out.println("Perimeter:" + r.getPerimeter()<co
- linkends="sw1CalloutRectangle-2" xml:id="getPerimeter"/>);
-System.out.println("Area:" + r.getArea()<co linkends="sw1CalloutRectangle-3"
- xml:id="getArea"/>);
-
-r.setWidth(4); <co linkends="sw1CalloutRectangle-4" xml:id="setWidth"/>
-r.setHeight(7);
-
-System.out.println("Perimeter:" + r.getPerimeter()); <co
- linkends="sw1CalloutRectangle-5"
- xml:id="sw1CalloutRectangle-5-co"/>
-System.out.println("Area:" + r.getArea());</programlisting>
-
- <calloutlist>
- <callout arearefs="constructor" xml:id="sw1CalloutRectangle-1">
- <para>Creating an instance of class
- <classname>Rectangle</classname> by calling a non-default
- constructor which allows for providing width (8) and height
- (5).</para>
- </callout>
+ <section xml:id="sd1_sect_accessControl">
+ <title>Encapsulation and access control</title>
- <callout arearefs="getPerimeter" xml:id="sw1CalloutRectangle-2">
- <para>Returning the rectangle's perimeter,</para>
- </callout>
+ <figure xml:id="sd1_fig_accessObjectives">
+ <title>Access control: Overall objectives</title>
- <callout arearefs="getArea" xml:id="sw1CalloutRectangle-3">
- <para>Returning the rectangle's area.</para>
- </callout>
+ <itemizedlist>
+ <listitem>
+ <para>Fine-grained control on attributes and methods.</para>
+ </listitem>
- <callout arearefs="setWidth" xml:id="sw1CalloutRectangle-4">
- <para>Setting with and height to new values.</para>
- </callout>
+ <listitem>
+ <para>Support <link
+ xlink:href="https://en.wikipedia.org/wiki/Encapsulation_(computer_programming)">encapsulation</link>
+ / <link
+ xlink:href="https://en.wikipedia.org/wiki/Information_hiding">Information
+ hiding</link>.</para>
- <callout arearefs="sw1CalloutRectangle-5-co"
- xml:id="sw1CalloutRectangle-5">
- <para>Write (possibly) changed perimeter and area values.</para>
- </callout>
- </calloutlist>
+ <para>Purpose: Hide implementation details within class or
+ package</para>
+ </listitem>
+ </itemizedlist>
+ </figure>
- <para>You may start from the following
- <classname>Rectangle</classname> class dummy implementation:</para>
+ <figure xml:id="sd1_fig_timeImplementation">
+ <title>Example: Implementing time and date</title>
- <programlisting language="java">/**
- * Representing rectangular shapes.
- *
- */
-public class Rectangle {
+ <informaltable border="0">
+ <tr>
+ <td valign="top"><programlisting language="java">public class Time {
+ private long <co linkends="sd1_callout_publicPrivateInterface-1"
+ xml:id="sd1_callout_publicPrivateInterface-1-co"/> secondsSince_2000_01_01;
- /**
- *
- * @param width The rectangle's width
- * @param heigth The rectangle's height
- */
- public Rectangle (double width, double heigth) {
- //TODO
+ public int getSecond() { <co linkends="sd1_callout_publicPrivateInterface-2"
+ xml:id="sd1_callout_publicPrivateInterface-2-co"/>
+ return
+ (int) secondsSince_2000_01_01 % (60);
}
- /**
- * @return The rectangle's area.
- */
- public double getArea() {
- return 0; // TODO
+ public int getMinute() {<coref
+ linkend="sd1_callout_publicPrivateInterface-2-co"/>
+ return (int)
+ (secondsSince_2000_01_01 / 60) % 60;
}
+ // ...
+}</programlisting></td>
- /**
- * @return The rectangle's perimeter.
- */
- public double getPerimeter() {
- return 0; // TODO
- }
+ <td valign="top"><programlisting language="java">public class Time {
+ private int second, minute,<coref
+ linkend="sd1_callout_publicPrivateInterface-1-co"/>
+ hour,day, month, year;
- /**
- * @return The rectangle's width.
- */
- public double getWidth() {
- return 0; // TODO
+ public int getSecond() { <coref
+ linkend="sd1_callout_publicPrivateInterface-2-co"/>
+ return second;
}
- /**
- * @param width The rectangle's new width
- */
- public void setWidth(double width) {
- // TODO
+ public int getMinute() { <coref
+ linkend="sd1_callout_publicPrivateInterface-2-co"/>
+ return minute;
}
+ // ...
+}</programlisting></td>
+ </tr>
+ </informaltable>
- /**
- * @return The rectangle's height.
- */
- public double getHeight() {
- return 0; // TODO
- }
+ <calloutlist role="slideExclude">
+ <callout arearefs="sd1_callout_publicPrivateInterface-1-co"
+ xml:id="sd1_callout_publicPrivateInterface-1">
+ <para>Private attributes only accessible inside class.</para>
+ </callout>
- /**
- * @param height The rectangle's new height
- */
- public void setHeight(double height) {
- // TODO
+ <callout arearefs="sd1_callout_publicPrivateInterface-2-co"
+ xml:id="sd1_callout_publicPrivateInterface-2">
+ <para>Public interface accessible by arbitrary classes.</para>
+ </callout>
+ </calloutlist>
+ </figure>
+
+ <figure xml:id="sd1_fig_timeAccessViolation">
+ <title>Access violation</title>
+
+ <programlisting language="java">public class Q {
+ public static void main(String[] args) {
+
+ Time t = new Time();
+
+ // Error: 'secondsSince_2000_01_01' has private access in 'Time'
+ t.secondsSince_2000_01_01 = 33445354L;
}
}</programlisting>
- </question>
+ </figure>
- <answer>
- <para>First we define two instance variables <code
- language="java">width</code> and <code language="java">height</code>
- representing a <classname>Rectangle</classname>'s corresponding two
- parameters <code language="java">width</code> and <code
- language="java">height</code>:</para>
+ <figure xml:id="sd1_fig_AccessRules">
+ <title>Access rules</title>
- <programlisting language="java">public class Rectangle {
+ <informaltable border="1">
+ <colgroup width="20%"/>
- // Instance variables representing a rectangle's parameters
- private double <emphasis role="bold">width</emphasis>, <emphasis role="bold">height</emphasis>;
-...
-}</programlisting>
+ <colgroup width="20%"/>
- <para>Next we allow for changing these two parameters:</para>
+ <colgroup width="20%"/>
- <programlisting language="java">public class Rectangle {
+ <colgroup width="20%"/>
- // Instance variables representing a rectangle's parameters
- private double width, height;
+ <colgroup width="20%"/>
-...
- /**
- * @param width Changing the rectangle's width
- */
- public void setWidth(double w) {
- <emphasis role="bold">width = w;</emphasis>
- }
+ <tr>
+ <td><link
+ xlink:href="https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html">Access
+ Level</link></td>
- /**
- * @param width Changing the rectangle's height
- */
- public void setHeight(double height) {
- <emphasis role="bold">this.height = height;</emphasis>
- }
-...
-}</programlisting>
+ <td>Other package</td>
- <para>Note the subtle implementation difference between
- <methodname>setWidth(...)</methodname> and
- <methodname>setHeight(...)</methodname>:</para>
+ <td>Child class</td>
- <glosslist>
- <glossentry xml:id="glossMethodDiff1">
- <glossterm><methodname>setWidth(double
- w)</methodname></glossterm>
+ <td>Same package</td>
- <glossdef>
- <para>We use the formal parameter name <quote>w</quote>. Its
- name does not conflict with the instance variable name
- <quote>width</quote> being defined at class level. We can
- simply assign this value to our corresponding instance
- variable using <code language="java">width = w;</code>.</para>
- </glossdef>
- </glossentry>
+ <td>Same class</td>
+ </tr>
- <glossentry xml:id="xmlMethodDiff2">
- <glossterm><methodname>setHeight(double
- height)</methodname></glossterm>
+ <tr>
+ <td><code language="java">public</code></td>
- <glossdef>
- <para>The method's formal parameter <quote>height</quote>
- shadows the instance variable's name being defined at class
- level. We need the <quote>this</quote> keyword in <code
- language="java">this.height = height</code> to resolve the
- ambiguity.</para>
- </glossdef>
- </glossentry>
- </glosslist>
+ <td>yes</td>
- <para>Both ways are perfectly legal. The complete implementation
- including all remaining methods reads:</para>
+ <td>yes</td>
- <programlisting language="java">/**
- * Representing rectangular shapes.
- *
- */
-public class Rectangle {
+ <td>yes</td>
- // Instance variables representing a rectangle's parameters
- private double width, height;
+ <td>yes</td>
+ </tr>
- /**
- *
- * @param width The rectangle's width
- * @param heigth The rectangle's height
- */
- public Rectangle (double width, double height) {
- setWidth(width);
- setHeight(height);
- }
- /**
- * @return The rectangle's area.
- */
- public double getArea() {
- return width * height;
- }
+ <tr>
+ <td><code language="java">protected</code></td>
- /**
- * @return The rectangle's perimeter.
- */
- public double getPerimeter() {
- return 2 * (width + height);
- }
+ <td><emphasis role="red">no</emphasis></td>
- /**
- * @return The rectangle's width.
- */
- public double getWidth() {
- return width;
- }
- /**
- * @param width The rectangle's new width
- */
- public void setWidth(double w) {
- width = w;
- }
+ <td>yes</td>
- /**
- * @return The rectangle's height.
- */
- public double getHeight() {
- return height;
- }
+ <td>yes</td>
- /**
- * @param width The rectangle's new height
- */
- public void setHeight(double height) {
- this.height = height;
- }
-}</programlisting>
- </answer>
- </qandaentry>
- </qandaset>
+ <td>yes</td>
+ </tr>
- <qandaset defaultlabel="qanda" xml:id="qandasetGeometry">
- <title>Modeling circles</title>
+ <tr>
+ <td><emphasis role="bold">Default</emphasis></td>
- <qandadiv>
- <qandaentry>
- <question>
- <para>This exercise is very similar to <xref
- linkend="sd1_qanda_geometry_Rectangle"/>. With respect to the
- upcoming section on inheritance its presence will be justified
- later by <xref linkend="sw1SectGeometryInherit"/>.</para>
+ <td><emphasis role="red">no</emphasis></td>
- <para>We provide a corresponding class
- <classname>Circle</classname> dummy implementation:</para>
+ <td><emphasis role="red">no</emphasis></td>
- <programlisting language="java">package step1.dummy;
+ <td>yes</td>
-/**
- * A circle of given radius
- *
- */
-public class Circle {
-
- /**
- * A new circle
- *
- * @param radius
- * The desired radius.
- */
- public Circle(double radius) {
- // TODO
- }
+ <td>yes</td>
+ </tr>
- /**
- * @return The circle's area.
- */
- public double getArea() {
- return 0; // TODO
- }
+ <tr>
+ <td><code language="java">private</code></td>
- /**
- * @return The circle's perimeter.
- */
- public double getPerimeter() {
- return 0; // TODO
- }
+ <td><emphasis role="red">no</emphasis></td>
- /**
- * @return The circle's radius.
- */
- public double getRadius() {
- return 0; // TODO
- }
+ <td><emphasis role="red">no</emphasis></td>
- /**
- * @param radius
- * Setting the circle's radius to a new value.
- */
- public void setRadius(double radius) {
- // TODO
- }
-}</programlisting>
+ <td><emphasis role="red">no</emphasis></td>
- <para>Instances of this class shall be usable in the following
- fashion:</para>
+ <td>yes</td>
+ </tr>
+ </informaltable>
+ </figure>
- <programlisting language="java">public static void main(String[] args) {
- final Circle c = new Circle(2.3);
+ <figure xml:id="sd1_fig_tipChoseAccessLevel">
+ <title><quote
+ xlink:href="https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html#visibility">Tips
+ on Choosing an Access Level</quote></title>
- System.out.println("Radius:" + c.getRadius());
- System.out.println("Perimeter:" + c.getPerimeter());
- System.out.println("Area:" + c.getArea());
+ <itemizedlist>
+ <listitem>
+ <para>Use the most restrictive access level that makes sense for a
+ particular member.</para>
+ </listitem>
- // Changing the circle's radius to a different value
- c.setRadius(4.7);
+ <listitem>
+ <para>Use <code language="java">private</code> unless you have a
+ good reason not to.</para>
+ </listitem>
- System.out.println("Radius:" + c.getRadius());
- System.out.println("Perimeter:" + c.getPerimeter());
- System.out.println("Area:" + c.getArea());
-}</programlisting>
+ <listitem>
+ <para>Avoid <code language="java">public</code> fields except for
+ constants. Public fields tend linking to a particular
+ implementation and limit your flexibility in changing your
+ code.</para>
+ </listitem>
+ </itemizedlist>
+ </figure>
- <para>Hint: Obviously you'll have to define an instance variable
- within Circle to keep track of its current radius value. All
- methods mentioned above simply depend on this single value.</para>
- </question>
+ <qandaset defaultlabel="qanda" xml:id="sd1_qanda_oracleAccessExample">
+ <title>Understanding access control</title>
- <answer>
- <para>We define an instance variable <code
- language="java">radius</code> inside our class
- <classname>Circle</classname>:</para>
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>Follow the example given in <quote
+ xlink:href="https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html#figure:classes-access.gif">Classes
+ and Packages of the Example Used to Illustrate Access
+ Levels</quote> and define two respective classes
+ <classname>Alpha</classname> and <classname>Beta</classname> in
+ different packages. Supply suitable dummy methods and fields
+ illustrating both legal and illegal access.</para>
+ </question>
- <programlisting language="java">public class Circle {
+ <answer>
+ <informaltable border="1">
+ <tr>
+ <th><classname>Alpha</classname></th>
- double radius;
-...
-}</programlisting>
+ <th><classname>Beta</classname></th>
- <para>Next we implement our method to change a circle's
- radius:</para>
+ <th><classname>AlphaSub</classname></th>
- <programlisting language="java">public void setRadius(double r) {
- radius = r;
-}</programlisting>
+ <th><classname>Gamma</classname></th>
+ </tr>
- <para>Note that we have chosen a different value for the method's
- formal radius parameter to be <quote>r</quote> rather than
- <quote>radius</quote>. Many people prefer to use radius here
- making it easier for a programmer to recognize the expected name
- in the generated <xref linkend="glo_Javadoc"/>:</para>
+ <tr>
+ <td valign="top"><programlisting language="java">package package_one;
- <programlisting language="java">public void setRadius(double radius) {
-<emphasis role="red"> this.</emphasis>radius = radius;
-}</programlisting>
+public class Alpha {
- <para>This requires the usage of the <code
- language="java">this</code> keyword to distinguish the formal
- parameter in <methodname>setRadius(double radius)</methodname>
- from the instance variable previously being defined within our
- class <classname>Circle</classname>. In other words: We have to
- resolve a name shadowing conflict.</para>
+ public int attribPublic;
+ protected int attribProtected;
+/*Default*/ int attribDefault;
+ private int attribPrivate;
- <para>The rest of the implementation is (quite) straightforward. A
- complete class reads:</para>
+ void dummy(/* same class */) {
+ int v;
+ v = attribPublic;
+ v = attribProtected;
+ v = attribDefault;
+ v = attribPrivate;
+ }
+}</programlisting></td>
- <programlisting language="java">package step1;
+ <td valign="top"><programlisting language="java">package package_one;
-/**
- * A circle of given radius
- *
- */
-public class Circle {
+public class Beta {
- double radius;
- /**
- * A new circle
- * @param radius The desired radius.
- */
- public Circle(double radius) {
- setRadius(radius);
- }
- /**
- * @return The circle's area.
- */
- public double getArea() {
- return radius * radius * Math.PI;
- }
- /**
- * @return The circle's perimeter.
- */
- public double getPerimeter() {
- return 2 * Math.PI * radius;
- }
- /**
- * @return The circle's radius.
- */
- public double getRadius() {
- return radius;
- }
- /**
- * @param radius Setting the circle's radius to a new value.
- */
- public void setRadius(double radius) {
- this.radius = radius;
+ void dummy(Alpha a) {
+ int v;
+ v = a.attribPublic;
+ v = a.attribProtected;
+ v = a.attribDefault;
+ v = a.<emphasis role="red">attribPrivate</emphasis>;
}
-}</programlisting>
- </answer>
- </qandaentry>
- </qandadiv>
- </qandaset>
-
- <qandaset defaultlabel="qanda" xml:id="sdQandaSvgAndTranslation">
- <title>Adding translations and <xref linkend="glo_SVG"/> export.</title>
+}</programlisting></td>
- <qandadiv>
- <qandaentry>
- <question>
- <para>We want to add more features to tour
- <classname>Circle</classname> and <classname>Rectangle</classname>
- classes:</para>
+ <td valign="top"><programlisting language="java">//different package
+package package_two;
- <glosslist>
- <glossentry>
- <glossterm>Translations</glossterm>
+import package_one.Alpha;
- <glossdef>
- <para>Add two more instance variables x and y and
- corresponding setter methods to account for a shape's
- translation vector with respect to the origin (0,0). The
- following hint may be helpful:</para>
+public class AlphaSub
+ extends Alpha {
- <programlisting language="java">/**
- * @param x The circle's x center coordinate value
- */
-public void setX(double x) {
- // TODO
-}
-/**
- * @param y The circle's y center coordinate value
- */
-public void setY(double y) {
- // TODO
-}</programlisting>
- <para>You may as well extend the constructors of
- <classname>Rectangle</classname> and
- <classname>Circle</classname> to accept center coordinates
- as well:</para>
+ void dummy(/* Inherited */) {
+ int v;
+ v = attribPublic;
+ v = attribProtected;
+ v = <emphasis role="red">attribDefault</emphasis>;
+ v = <emphasis role="red">attribPrivate</emphasis>;
+ }
+}</programlisting></td>
- <programlisting language="java">public class Rectangle {
+ <td valign="top"><programlisting language="java">//different package
+package package_two;
+import package_one.Alpha;
-...
- /**
- * Rectangle having center coordinates x, y, width and height.
- *
- * @param x The rectangle center's x-coordinate
- * @param y The rectangle center's y-coordinate
- * @param width The rectangle's width
- * @param height The rectangle's height
- */
- public Rectangle(double x, double y, double width, double height) {
- ...
- }
- ...
-}</programlisting>
+public class Gamma {
- <programlisting language="java">public class Circle {
- ...
- /**
- * Circle having center coordinates x, y and radius r.
- *
- * @param x The circle center's x-coordinate
- * @param y The circle center's y-coordinate
- * @param radius The circle's radius
- */
- public Circle(double x, double y, double radius) {
- ...
- }
- ...
-}</programlisting>
- </glossdef>
- </glossentry>
- <glossentry>
- <glossterm><xref linkend="glo_SVG"/> export</glossterm>
- <glossdef>
- <para>We would like Rectangle and Circle instances to be
- visualized as <xref linkend="glo_SVG"/> graphics. <link
- xlink:href="http://www.w3schools.com/graphics/svg_examples.asp">SVG
- Examples</link> provides editor samples both for <link
- xlink:href="http://www.w3schools.com/graphics/svg_circle.asp">circles</link>
- and <link
- xlink:href="http://www.w3schools.com/graphics/svg_rect.asp">rectangles</link>.
- Add a method <methodname>void writeSvg()</methodname> to
- both of your classes which allows for <xref
- linkend="glo_SVG"/> code being written to standard output.
- Use <methodname>System.out.println(...)</methodname> calls
- to create the desired <xref linkend="glo_SVG"/> output. You
- may need <code language="java">\"</code> to escape double
- quotes as in the subsequent example or use single attribute
- quotes instead:</para>
-
- <programlisting language="java">System.out.println("<rect width=\"20\"" ...</programlisting>
- </glossdef>
- </glossentry>
- </glosslist>
+ void dummy(Alpha a) {
+ int v;
+ v = a.attribPublic;
+ v = a.<emphasis role="red">attribProtected</emphasis>;
+ v = a.<emphasis role="red">attribDefault</emphasis>;
+ v = a.<emphasis role="red">attribPrivate</emphasis>;
+ }
+}</programlisting></td>
+ </tr>
+ </informaltable>
- <para>The following code snippet may serve to illustrate the
- intended use of <methodname>void writeSvg()</methodname>:</para>
-
- <programlisting language="java">public class Driver {
-
- public static void main(String[] args) {
+ <note>
+ <para>Notice the related follow up <link
+ linkend="sd1_qanda_protectedAccessProblem">inheritance
+ exercise</link>.</para>
+ </note>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
- System.out.println("<!DOCTYPE html><html><body>");
- System.out.println(" <svg width='300' height='200' >");
+ <qandaset defaultlabel="qanda" xml:id="sd1_qanda_whyTimeMinuteCast">
+ <title>Explaining times</title>
- // Draw a rectangle as SVG
- final Rectangle r = new Rectangle(5, 4);
- r.setX(2);
- r.setY(1);
- r.writeSvg();
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>In <xref linkend="sd1_fig_timeImplementation"/> we
+ have:</para>
- // Draw a circle as SVG
- final Circle c = new Circle(3);
- c.setX(3);
- c.setY(1);
- c.writeSvg();
- System.out.println(" </svg >");
- System.out.println("</body></html>");
- }
-}</programlisting>
+ <programlisting language="java">public int getMinute() {
+ return
+ (int) (secondsSince_2000_01_01 / 60) % 60;
+ }</programlisting>
- <para>Implement the method <methodname>void
- writeSvg()</methodname> in both classes
- <classname>Rectangle</classname> and
- <classname>Circle</classname>. The following sample export is
- intended for 300x200 pixel requiring to scale
- <abbrev>e.g.</abbrev> the above circle's radius of three by a
- factor of 20 resulting in an effective <tag
- class="emptytag">circle r='60' ...</tag> value. This scaling is
- being required for all parameters:</para>
-
- <programlisting language="java"><!DOCTYPE html>
-<html>
- <body>
- <svg width='300' height='200' >
- <emphasis role="bold"><rect width='100.0' height='80.0' x='40.0' y='20.0'
- style='fill:rgb(0,255,0);stroke-width:3;stroke:rgb(0,0,0)'/></emphasis>
- <emphasis role="bold"><circle r='60.0' cx='60.0' cy='20.0'
- style='fill:rgb(0,0,255);stroke-width:3;stroke:rgb(0,0,0)'/></emphasis>
- </svg >
- </body>
-</html></programlisting>
+ <orderedlist>
+ <listitem>
+ <para>Explain the underlying idea of the term <code
+ language="java">(secondsSince_2000_01_01 / 60) %
+ 60</code>.</para>
+ </listitem>
- <para>You may copy this output into a file
- <filename>svg.html</filename>. A web browser should visualize this
- output as:</para>
+ <listitem>
+ <para>Explain the <code language="java">(int)</code> cast's
+ necessity. Why is there no overflow error about to
+ happen?</para>
+ </listitem>
+ </orderedlist>
+ </question>
- <informalfigure>
- <mediaobject>
- <imageobject>
- <imagedata fileref="Ref/Fig/svgGeometry.png"/>
- </imageobject>
- </mediaobject>
- </informalfigure>
- </question>
+ <answer>
+ <orderedlist>
+ <listitem>
+ <para>Every 60 seconds a new minute begins and seconds start
+ from zero again. Every 60 minutes a new our will start and
+ both seconds and minutes start from zero again.</para>
- <answer>
- <programlisting language="java">public class Rectangle {
+ <para>The term secondsSince_2000_01_01 / 60 amounts to the
+ number of minutes passed since 2000/01/01. Building the
+ remainder to 60 provides the current minute.</para>
- private double x, y, width, height;
- ...
- /**
- * @param x The rectangle's x center coordinate value
- */
- public void setX(double x) {
- this.x = x;
- }
- /**
- * @param y The rectangle's y center coordinate value
- */
- public void setY(double y) {
- this.y = y;
- }
+ <para>Example: If there are 4450 seconds elapsed this
+ amounts to 4450 / 3600 == 1 hour, (4450 / 60) % 60 == 14
+ minutes and 4450 % 60 == 10 seconds.</para>
+ </listitem>
-public void writeSvg() {
- final int scale = 20;
- System.out.println(
- "<rect width='" + scale * width +"' height='" + scale * height +
- "' x='" + scale * x + "'" + " y='" + scale * y + "'" +
- "' style='fill:rgb(0,255,0);stroke-width:3;stroke:rgb(0,0,0)'/>");
- }
-}</programlisting>
+ <listitem>
+ <para>The expression <code
+ language="java">(secondsSince_2000_01_01 / 60) % 60</code>
+ is of type long due to the presence of a <code
+ language="java">long</code> variable. However the
+ <quote>%</quote> operator guarantees the result to be in the
+ integer range 0 ... 59 which may thus be safely narrowed to
+ an <code language="java">int</code> value.</para>
+ </listitem>
+ </orderedlist>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
+ </section>
- <programlisting language="java">public class Circle {
+ <section xml:id="sd1_sect_getterSetter">
+ <title>Getter and setter methods</title>
- double x, y, radius;
-...
+ <figure xml:id="sd1_fig_directVsSetterGetter">
+ <title>Direct access <abbrev>vs.</abbrev> setter method</title>
- /**
- * @param x The circle's x center coordinate value
- */
- public void setX(double x) {
- this.x = x;
- }
- /**
- * @param y The circle's y center coordinate value
- */
- public void setY(double y) {
- this.y = y;
- }
+ <informaltable border="1">
+ <tr>
+ <td valign="top"><programlisting language="java">package hour_second.direct;
-public void writeSvg() {
- final int scale = 20;
+public class Time {
+ public int hour, minute;
+}</programlisting><programlisting language="java">package hour_second;
+import hour_second.direct.Time;
- System.out.println(
- "<circle r='" + scale * radius +
- "' cx='" + scale * x + "'" + " cy='" + scale * y +
- "' style='fill:rgb(0,0,255);stroke-width:3;stroke:rgb(0,0,0)'/>");
+public class DirectAccess {
+ Time time = new Time();
+ void init() {
+ time.hour = 17;
+ time.minute = 45;
+ }
+}</programlisting></td>
- }
-}</programlisting>
- </answer>
- </qandaentry>
- </qandadiv>
- </qandaset>
+ <td valign="top"><programlisting language="java">package hour_second.setter;
- <section xml:id="sd1_sect_accessControl">
- <title>Encapsulation and access control</title>
+public class Time {
+ private int hour, minute;
+ public void setTime(int h, int m) {
+ minute = m;
+ hour = h;
+ }
+}</programlisting><programlisting language="java">package hour_second;
+import hour_second.setter.Time;
+public class SetterAccess {
+ Time time = new Time();
+ void init() { time.setTime(17, 45);}
+}</programlisting></td>
+ </tr>
+ </informaltable>
+ </figure>
- <figure xml:id="sd1_fig_accessObjectives">
- <title>Access control: Overall objectives</title>
+ <figure xml:id="sd1_fig_whySetters">
+ <title>Why adding setter methods?</title>
<itemizedlist>
<listitem>
- <para>Fine-grained control on attributes and methods.</para>
+ <para>Allow for change of implementation.</para>
</listitem>
<listitem>
- <para>Support <link
- xlink:href="https://en.wikipedia.org/wiki/Encapsulation_(computer_programming)">encapsulation</link>
- / <link
- xlink:href="https://en.wikipedia.org/wiki/Information_hiding">Information
- hiding</link>.</para>
+ <para>Allow for non-business logic concerns. Examples:</para>
- <para>Purpose: Hide implementation details within class or
- package</para>
+ <itemizedlist>
+ <listitem>
+ <para>Logging</para>
+ </listitem>
+
+ <listitem>
+ <para>Adding persistence</para>
+ </listitem>
+ </itemizedlist>
</listitem>
</itemizedlist>
</figure>
- <figure xml:id="sd1_fig_timeImplementation">
- <title>Example: Implementing time and date</title>
+ <figure xml:id="sd1_fig_implementJustSeconds">
+ <title>Implementation by minutes only</title>
- <informaltable border="0">
+ <informaltable border="1">
<tr>
- <td valign="top"><programlisting language="java">public class Time {
- private long <co linkends="sd1_callout_publicPrivateInterface-1"
- xml:id="sd1_callout_publicPrivateInterface-1-co"/> secondsSince_2000_01_01;
-
- public int getSecond() { <co linkends="sd1_callout_publicPrivateInterface-2"
- xml:id="sd1_callout_publicPrivateInterface-2-co"/>
- return
- (int) secondsSince_2000_01_01 % (60);
- }
- public int getMinute() {<coref
- linkend="sd1_callout_publicPrivateInterface-2-co"/>
- return (int)
- (secondsSince_2000_01_01 / 60) % 60;
+ <td valign="top"><programlisting language="java">package only_minute.direct;
+public class Time {
+ // Minutes since 00:00
+ public int minute;
+}</programlisting><programlisting language="java">package only_minute;
+import only_minute.direct.Time;
+public class DirectAccess {
+ Time time = new Time();
+ void init() {
+ <emphasis role="red">time.minute = 1065;</emphasis>// 17:45
}
- // ...
}</programlisting></td>
- <td valign="top"><programlisting language="java">public class Time {
- private int second, minute,<coref
- linkend="sd1_callout_publicPrivateInterface-1-co"/>
- hour,day, month, year;
-
- public int getSecond() { <coref
- linkend="sd1_callout_publicPrivateInterface-2-co"/>
- return second;
- }
- public int getMinute() { <coref
- linkend="sd1_callout_publicPrivateInterface-2-co"/>
- return minute;
+ <td valign="top"><programlisting language="java">package only_minute.setter;
+public class Time {
+ private int minute;
+ public void setTime(int h, int m) {
+ minute = m + 60 * h;
}
- // ...
+}</programlisting><programlisting language="java">package only_minute;
+import only_minute.setter.Time;
+public class SetterAccess {
+ Time time = new Time();
+ void init() { <emphasis role="red">time.setTime(0, 0);</emphasis>}
}</programlisting></td>
</tr>
</informaltable>
+ </figure>
- <calloutlist role="slideExclude">
- <callout arearefs="sd1_callout_publicPrivateInterface-1-co"
- xml:id="sd1_callout_publicPrivateInterface-1">
- <para>Private attributes only accessible inside class.</para>
- </callout>
+ <qandaset defaultlabel="qanda"
+ xml:id="sd1QandaEmployeeFilesystemRepresentation">
+ <title>File system representation</title>
- <callout arearefs="sd1_callout_publicPrivateInterface-2-co"
- xml:id="sd1_callout_publicPrivateInterface-2">
- <para>Public interface accessible by arbitrary classes.</para>
- </callout>
- </calloutlist>
- </figure>
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>In <xref linkend="sd1_qanda_refineAccessEmployee"/> you
+ moved the <classname>Employee</classname> class to a package
+ <package>model</package>.</para>
- <figure xml:id="sd1_fig_timeAccessViolation">
- <title>Access violation</title>
+ <para>How are the corresponding <xref linkend="glo_Java"/> files
+ <filename>Employee.java</filename> and
+ <filename>Driver.java</filename> being represented? What about
+ the generated byte code files
+ <filename>Employee.</filename>class and
+ <filename>Driver.</filename>class?</para>
- <programlisting language="java">public class Q {
- public static void main(String[] args) {
+ <tip>
+ <para>Use your OS's file system explorer.</para>
+ </tip>
+ </question>
- Time t = new Time();
+ <answer>
+ <para>Depending on your type of project (Java, Maven,...) both
+ your <filename>.java</filename> source files and generated
+ <filename>.class</filename> files will be organized below their
+ respective entry level directories. The following typical layout
+ depends on your Java development environment / <xref
+ linkend="glo_IDE"/>:</para>
- // Error: 'secondsSince_2000_01_01' has private access in 'Time'
- t.secondsSince_2000_01_01 = 33445354L;
- }
-}</programlisting>
- </figure>
+ <glosslist>
+ <glossentry>
+ <glossterm><filename>.java</filename> source
+ files</glossterm>
- <figure xml:id="sd1_fig_AccessRules">
- <title>Access rules</title>
+ <glossdef>
+ <para>Below folder <filename>src</filename>.</para>
+ </glossdef>
+ </glossentry>
- <informaltable border="1">
- <colgroup width="20%"/>
+ <glossentry>
+ <glossterm><filename>.class</filename> byte code
+ files</glossterm>
- <colgroup width="20%"/>
+ <glossdef>
+ <para>Below folder <filename>bin</filename>.</para>
+ </glossdef>
+ </glossentry>
+ </glosslist>
- <colgroup width="20%"/>
+ <para>Packages give rise to further sub directories both for
+ source code and their generated byte code
+ <quote>partners</quote>. The following example is a minor
+ variation of our employee example. The package
+ <package>model</package> has been transformed into
+ <package>company.model</package>:</para>
- <colgroup width="20%"/>
+ <figure xml:id="sd1FigPackageHierarchy">
+ <title>Package hierarchy of the employee example</title>
- <colgroup width="20%"/>
+ <screenshot>
+ <info>
+ <title>On filesystem level the package
+ <package>company.model</package> becomes a subdirectory
+ <filename>model</filename> of sub directory
+ <filename>company</filename> for both our source
+ (<filename>src</filename>) and bytecode
+ (<filename>bin</filename>) file hierarchy.</title>
+ </info>
- <tr>
- <td><link
- xlink:href="https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html">Access
- Level</link></td>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="Ref/Fig/packageHierarchy.png"/>
+ </imageobject>
+ </mediaobject>
+ </screenshot>
+ </figure>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
+ </section>
- <td>Other package</td>
+ <section xml:id="sd1_sect_methodSignature">
+ <title>Signatures</title>
- <td>Child class</td>
+ <figure xml:id="sd1_fig_typeSignatureDef">
+ <title>Defining type signatures</title>
- <td>Same package</td>
+ <programlisting language="java">boolean <co
+ linkends="sd1_fig_typeSignatureDef-1"
+ xml:id="sd1_fig_typeSignatureDef-1-co"/> <link
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#startsWith(java.lang.String,int)">startsWith</link>(String prefix, int toffset) <co
+ linkends="sd1_fig_typeSignatureDef-2"
+ xml:id="sd1_fig_typeSignatureDef-2-co"/> </programlisting>
+
+ <calloutlist>
+ <callout arearefs="sd1_fig_typeSignatureDef-1-co"
+ xml:id="sd1_fig_typeSignatureDef-1">
+ <para>Return type boolean</para>
+ </callout>
- <td>Same class</td>
- </tr>
+ <callout arearefs="sd1_fig_typeSignatureDef-2-co"
+ xml:id="sd1_fig_typeSignatureDef-2">
+ <para>Number of arguments among with their respective types and
+ order:</para>
- <tr>
- <td><code language="java">public</code></td>
+ <para>Two argument types: String followed by <code
+ language="java">int</code>. Consider:</para>
- <td>yes</td>
+ <programlisting language="java">boolean startsWith(int toffset, String prefix) </programlisting>
+ </callout>
+ </calloutlist>
+ </figure>
- <td>yes</td>
+ <figure xml:id="sd1_fig_typeSignature">
+ <title>Type signature examples</title>
- <td>yes</td>
+ <informaltable border="1">
+ <colgroup width="81%"/>
- <td>yes</td>
- </tr>
+ <colgroup width="11%"/>
+
+ <colgroup width="8%"/>
<tr>
- <td><code language="java">protected</code></td>
+ <th valign="top">Method</th>
- <td><emphasis role="red">no</emphasis></td>
+ <th valign="top">Return type</th>
- <td>yes</td>
+ <th valign="top">Argument type list</th>
+ </tr>
- <td>yes</td>
+ <tr>
+ <td><methodname>void print()</methodname></td>
- <td>yes</td>
+ <td>void</td>
+
+ <td>(void)</td>
</tr>
<tr>
- <td><emphasis role="bold">Default</emphasis></td>
+ <td><methodname>int add (int a, int b) </methodname></td>
- <td><emphasis role="red">no</emphasis></td>
+ <td>int</td>
- <td><emphasis role="red">no</emphasis></td>
+ <td>(int, int)</td>
+ </tr>
- <td>yes</td>
+ <tr>
+ <td><methodname>int maximum (int a, int b)</methodname></td>
- <td>yes</td>
+ <td>int</td>
+
+ <td>(int, int)</td>
</tr>
<tr>
- <td><code language="java">private</code></td>
+ <td><methodname>void print (int a, float b)</methodname></td>
- <td><emphasis role="red">no</emphasis></td>
+ <td>void</td>
- <td><emphasis role="red">no</emphasis></td>
+ <td>(int, float)</td>
+ </tr>
- <td><emphasis role="red">no</emphasis></td>
+ <tr>
+ <td><methodname>void display (float a, int b)</methodname></td>
- <td>yes</td>
+ <td>void</td>
+
+ <td>(float, int)</td>
</tr>
</informaltable>
</figure>
- <figure xml:id="sd1_fig_tipChoseAccessLevel">
- <title><quote
- xlink:href="https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html#visibility">Tips
- on Choosing an Access Level</quote></title>
-
- <itemizedlist>
- <listitem>
- <para>Use the most restrictive access level that makes sense for a
- particular member.</para>
- </listitem>
-
- <listitem>
- <para>Use <code language="java">private</code> unless you have a
- good reason not to.</para>
- </listitem>
+ <figure xml:id="sd1_fig_typeSignatureDetail">
+ <title>Defining method signatures</title>
+
+ <programlisting language="java">boolean <link
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#startsWith(java.lang.String,int)">startsWith</link><co
+ linkends="sd1_fig_typeSignatureDef-1.2"
+ xml:id="sd1_fig_typeSignatureDef-1.2-co"/>(String prefix, int toffset) <co
+ linkends="sd1_fig_typeSignatureDef-2.2"
+ xml:id="sd1_fig_typeSignatureDef-2.2-co"/> </programlisting>
+
+ <calloutlist>
+ <callout arearefs="sd1_fig_typeSignatureDef-1.2-co"
+ xml:id="sd1_fig_typeSignatureDef-1.2">
+ <para>Method name <code language="java">startsWith</code>.</para>
+ </callout>
- <listitem>
- <para>Avoid <code language="java">public</code> fields except for
- constants. Public fields tend linking to a particular
- implementation and limit your flexibility in changing your
- code.</para>
- </listitem>
- </itemizedlist>
+ <callout arearefs="sd1_fig_typeSignatureDef-2.2-co"
+ xml:id="sd1_fig_typeSignatureDef-2.2">
+ <para>Number of arguments among with their respective types and
+ order.</para>
+ </callout>
+ </calloutlist>
</figure>
- <qandaset defaultlabel="qanda" xml:id="sd1_qanda_oracleAccessExample">
- <title>Understanding access control</title>
+ <figure xml:id="sd1_fig_methodSignature">
+ <title>Method signature examples</title>
- <qandadiv>
- <qandaentry>
- <question>
- <para>Follow the example given in <quote
- xlink:href="https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html#figure:classes-access.gif">Classes
- and Packages of the Example Used to Illustrate Access
- Levels</quote> and define two respective classes
- <classname>Alpha</classname> and <classname>Beta</classname> in
- different packages. Supply suitable dummy methods and fields
- illustrating both legal and illegal access.</para>
- </question>
+ <informaltable border="1">
+ <colgroup width="81%"/>
- <answer>
- <informaltable border="1">
- <tr>
- <th><classname>Alpha</classname></th>
+ <colgroup width="11%"/>
- <th><classname>Beta</classname></th>
+ <colgroup width="8%"/>
- <th><classname>AlphaSub</classname></th>
+ <tr>
+ <th valign="top">Method</th>
- <th><classname>Gamma</classname></th>
- </tr>
+ <th valign="top">Method name</th>
- <tr>
- <td valign="top"><programlisting language="java">package package_one;
+ <th colspan="1" valign="top">Method signature</th>
+ </tr>
-public class Alpha {
+ <tr>
+ <td><methodname>void print() </methodname></td>
- public int attribPublic;
- protected int attribProtected;
-/*Default*/ int attribDefault;
- private int attribPrivate;
+ <td>print</td>
- void dummy(/* same class */) {
- int v;
- v = attribPublic;
- v = attribProtected;
- v = attribDefault;
- v = attribPrivate;
- }
-}</programlisting></td>
+ <td>(void)</td>
+ </tr>
- <td valign="top"><programlisting language="java">package package_one;
+ <tr>
+ <td><methodname>int add (int a, int b)</methodname></td>
-public class Beta {
+ <td>add</td>
+ <td>(int, int)</td>
+ </tr>
+ <tr>
+ <td><methodname>int maximum (int a, int b)</methodname></td>
+ <td>maximum</td>
+ <td>(int, int)</td>
+ </tr>
+ <tr>
+ <td><methodname>void print (int a, float b)</methodname></td>
- void dummy(Alpha a) {
- int v;
- v = a.attribPublic;
- v = a.attribProtected;
- v = a.attribDefault;
- v = a.<emphasis role="red">attribPrivate</emphasis>;
- }
-}</programlisting></td>
+ <td>print</td>
- <td valign="top"><programlisting language="java">//different package
-package package_two;
+ <td>(int, float)</td>
+ </tr>
-import package_one.Alpha;
+ <tr>
+ <td><methodname>void display (float a, int b)</methodname></td>
-public class AlphaSub
- extends Alpha {
+ <td>display</td>
+ <td>(float, int)</td>
+ </tr>
+ </informaltable>
+ </figure>
+ </section>
- void dummy(/* Inherited */) {
- int v;
- v = attribPublic;
- v = attribProtected;
- v = <emphasis role="red">attribDefault</emphasis>;
- v = <emphasis role="red">attribPrivate</emphasis>;
- }
-}</programlisting></td>
+ <section xml:id="sd1_sect_methodOverloading">
+ <title>Method overloading</title>
- <td valign="top"><programlisting language="java">//different package
-package package_two;
-import package_one.Alpha;
+ <figure xml:id="sd1_fig_overloadingPrinciple">
+ <title>Method overloading: Same name, different signature</title>
-public class Gamma {
+ <informaltable border="0">
+ <tr>
+ <td rowspan="2" valign="top"><programlisting language="java">public class Print {
+ public void print() { // (void)
+ System.out.println("No argument");
+ }
+ public void print(int i) { // (int)
+ System.out.println("int value " + i);
+ }
+ public void print(double d) { // (double)
+ System.out.println("double value " + d);
+ }
+ public void print(int i, int j) { // (int, int)
+ System.out.println("Two int values "+
+ i + " and " + j);
+ }
+}</programlisting></td>
+ <td valign="top"><programlisting language="java">Print p = new Print();
+p.print();
+p.print(33);
+p.print(4.333);
+p.print(-1, 7);</programlisting></td>
+ </tr>
+ <tr>
+ <td valign="top"><screen>No argument
+int value 33
+double value 4.333
+Two int values -1 and 7</screen></td>
+ </tr>
+ </informaltable>
+ </figure>
+ <figure xml:id="sd1_fig_overloadingAlternateNames">
+ <title>Overloading, alternate names</title>
- void dummy(Alpha a) {
- int v;
- v = a.attribPublic;
- v = a.<emphasis role="red">attribProtected</emphasis>;
- v = a.<emphasis role="red">attribDefault</emphasis>;
- v = a.<emphasis role="red">attribPrivate</emphasis>;
- }
-}</programlisting></td>
- </tr>
- </informaltable>
+ <itemizedlist>
+ <listitem>
+ <para>Static polymorphism.</para>
+ </listitem>
- <note>
- <para>Notice the related follow up <link
- linkend="sd1_qanda_protectedAccessProblem">inheritance
- exercise</link>.</para>
- </note>
- </answer>
- </qandaentry>
- </qandadiv>
- </qandaset>
+ <listitem>
+ <para>Compile time binding.</para>
+ </listitem>
- <qandaset defaultlabel="qanda" xml:id="sd1_qanda_whyTimeMinuteCast">
- <title>Explaining times</title>
+ <listitem>
+ <para>Early binding.</para>
+ </listitem>
+ </itemizedlist>
+ </figure>
- <qandadiv>
- <qandaentry>
- <question>
- <para>In <xref linkend="sd1_fig_timeImplementation"/> we
- have:</para>
+ <figure xml:id="sd1_fig_noOverloadingInC">
+ <title>No such concept in C</title>
- <programlisting language="java">public int getMinute() {
- return
- (int) (secondsSince_2000_01_01 / 60) % 60;
- }</programlisting>
+ <programlisting language="c">void print() {
+ printf("No argument\n");
+}
- <orderedlist>
- <listitem>
- <para>Explain the underlying idea of the term <code
- language="java">(secondsSince_2000_01_01 / 60) %
- 60</code>.</para>
- </listitem>
+void print(int i) { <emphasis role="bold"> /* Error: redefinition of ‘print’ */</emphasis>
+ printf("int value %d\n", i);
+}
- <listitem>
- <para>Explain the <code language="java">(int)</code> cast's
- necessity. Why is there no overflow error about to
- happen?</para>
- </listitem>
- </orderedlist>
- </question>
+void main(void) {
+ print();
+ print(33);
+}</programlisting>
+ </figure>
- <answer>
- <orderedlist>
- <listitem>
- <para>Every 60 seconds a new minute begins and seconds start
- from zero again. Every 60 minutes a new our will start and
- both seconds and minutes start from zero again.</para>
+ <figure xml:id="sd1_fig_distinctReturnType">
+ <title>What about return type?</title>
- <para>The term secondsSince_2000_01_01 / 60 amounts to the
- number of minutes passed since 2000/01/01. Building the
- remainder to 60 provides the current minute.</para>
+ <programlisting language="java">public class Person {
+ String getDetails() { return "dummy";}
+ int getDetails() { return 1;} // Error: 'getDetails()' is already
+ // defined in 'Person'
+}</programlisting>
- <para>Example: If there are 4450 seconds elapsed this
- amounts to 4450 / 3600 == 1 hour, (4450 / 60) % 60 == 14
- minutes and 4450 % 60 == 10 seconds.</para>
- </listitem>
+ <informaltable border="1">
+ <tr>
+ <th rowspan="2" valign="top">Return type</th>
- <listitem>
- <para>The expression <code
- language="java">(secondsSince_2000_01_01 / 60) % 60</code>
- is of type long due to the presence of a <code
- language="java">long</code> variable. However the
- <quote>%</quote> operator guarantees the result to be in the
- integer range 0 ... 59 which may thus be safely narrowed to
- an <code language="java">int</code> value.</para>
- </listitem>
- </orderedlist>
- </answer>
- </qandaentry>
- </qandadiv>
- </qandaset>
- </section>
+ <th colspan="2">Method signature</th>
+ </tr>
- <section xml:id="sd1_sect_getterSetter">
- <title>Getter and setter methods</title>
+ <tr>
+ <th>Method name</th>
- <figure xml:id="sd1_fig_directVsSetterGetter">
- <title>Direct access <abbrev>vs.</abbrev> setter method</title>
+ <th>Argument type list</th>
+ </tr>
- <informaltable border="1">
<tr>
- <td valign="top"><programlisting language="java">package hour_second.direct;
+ <td>String</td>
-public class Time {
- public int hour, minute;
-}</programlisting><programlisting language="java">package hour_second;
-import hour_second.direct.Time;
+ <td>getDetails</td>
-public class DirectAccess {
- Time time = new Time();
- void init() {
- time.hour = 17;
- time.minute = 45;
- }
-}</programlisting></td>
+ <td>(void)</td>
+ </tr>
- <td valign="top"><programlisting language="java">package hour_second.setter;
+ <tr>
+ <td>int</td>
-public class Time {
- private int hour, minute;
- public void setTime(int h, int m) {
- minute = m;
- hour = h;
- }
-}</programlisting><programlisting language="java">package hour_second;
-import hour_second.setter.Time;
-public class SetterAccess {
- Time time = new Time();
- void init() { time.setTime(17, 45);}
-}</programlisting></td>
+ <td>getDetails</td>
+
+ <td>(void)</td>
</tr>
</informaltable>
+
+ <para>Return type irrelevant, only method signature support in <xref
+ linkend="glo_Java"/>.</para>
</figure>
- <figure xml:id="sd1_fig_whySetters">
- <title>Why adding setter methods?</title>
+ <figure xml:id="sd1_fig_methodSignatureRatio">
+ <title>Method signatures rationale</title>
- <itemizedlist>
- <listitem>
- <para>Allow for change of implementation.</para>
- </listitem>
+ <para>In <xref linkend="glo_Java"/> method signatures allow for
+ uniquely addressing a method within a given class e.g.:</para>
- <listitem>
- <para>Allow for non-business logic concerns. Examples:</para>
+ <para>The method named <code language="java">print</code> having an
+ int argument followed by a double:</para>
- <itemizedlist>
- <listitem>
- <para>Logging</para>
- </listitem>
+ <programlisting language="java">print(int, double)</programlisting>
+ </figure>
- <listitem>
- <para>Adding persistence</para>
- </listitem>
- </itemizedlist>
- </listitem>
- </itemizedlist>
+ <figure xml:id="sd1_fig_chooseByMethodSignature">
+ <title>Method signatures rationale</title>
+
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="Ref/ObjectsAndClasses/chooseOverloaded.svg"/>
+ </imageobject>
+ </mediaobject>
</figure>
- <figure xml:id="sd1_fig_implementJustSeconds">
- <title>Implementation by minutes only</title>
+ <figure xml:id="sd1_fig_sysoutPrintlnOverload">
+ <title>Example: System.out.print(...)</title>
- <informaltable border="1">
+ <informaltable border="0">
<tr>
- <td valign="top"><programlisting language="java">package only_minute.direct;
-public class Time {
- // Minutes since 00:00
- public int minute;
-}</programlisting><programlisting language="java">package only_minute;
-import only_minute.direct.Time;
-public class DirectAccess {
- Time time = new Time();
- void init() {
- <emphasis role="red">time.minute = 1065;</emphasis>// 17:45
- }
-}</programlisting></td>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-boolean-">print(boolean
+ b)</methodname></td>
- <td valign="top"><programlisting language="java">package only_minute.setter;
-public class Time {
- private int minute;
- public void setTime(int h, int m) {
- minute = m + 60 * h;
- }
-}</programlisting><programlisting language="java">package only_minute;
-import only_minute.setter.Time;
-public class SetterAccess {
- Time time = new Time();
- void init() { <emphasis role="red">time.setTime(0, 0);</emphasis>}
-}</programlisting></td>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-char-">print(char
+ c)</methodname></td>
</tr>
- </informaltable>
- </figure>
-
- <qandaset defaultlabel="qanda"
- xml:id="sd1QandaEmployeeFilesystemRepresentation">
- <title>File system representation</title>
- <qandadiv>
- <qandaentry>
- <question>
- <para>In <xref linkend="sd1_qanda_refineAccessEmployee"/> you
- moved the <classname>Employee</classname> class to a package
- <package>model</package>.</para>
-
- <para>How are the corresponding <xref linkend="glo_Java"/> files
- <filename>Employee.java</filename> and
- <filename>Driver.java</filename> being represented? What about
- the generated byte code files
- <filename>Employee.</filename>class and
- <filename>Driver.</filename>class?</para>
-
- <tip>
- <para>Use your OS's file system explorer.</para>
- </tip>
- </question>
-
- <answer>
- <para>Depending on your type of project (Java, Maven,...) both
- your <filename>.java</filename> source files and generated
- <filename>.class</filename> files will be organized below their
- respective entry level directories. The following typical layout
- depends on your Java development environment / <xref
- linkend="glo_IDE"/>:</para>
-
- <glosslist>
- <glossentry>
- <glossterm><filename>.java</filename> source
- files</glossterm>
+ <tr>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-char:A-">print(char[]
+ s)</methodname></td>
- <glossdef>
- <para>Below folder <filename>src</filename>.</para>
- </glossdef>
- </glossentry>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-double-">print(double
+ d)</methodname></td>
+ </tr>
- <glossentry>
- <glossterm><filename>.class</filename> byte code
- files</glossterm>
+ <tr>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-float-">print(float
+ f)</methodname></td>
- <glossdef>
- <para>Below folder <filename>bin</filename>.</para>
- </glossdef>
- </glossentry>
- </glosslist>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-int-">print(int
+ i)</methodname></td>
+ </tr>
- <para>Packages give rise to further sub directories both for
- source code and their generated byte code
- <quote>partners</quote>. The following example is a minor
- variation of our employee example. The package
- <package>model</package> has been transformed into
- <package>company.model</package>:</para>
+ <tr>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-long-">print(long
+ l)</methodname></td>
- <figure xml:id="sd1FigPackageHierarchy">
- <title>Package hierarchy of the employee example</title>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-java.lang.Object-">print(Object
+ obj)</methodname></td>
+ </tr>
- <screenshot>
- <info>
- <title>On filesystem level the package
- <package>company.model</package> becomes a subdirectory
- <filename>model</filename> of sub directory
- <filename>company</filename> for both our source
- (<filename>src</filename>) and bytecode
- (<filename>bin</filename>) file hierarchy.</title>
- </info>
+ <tr>
+ <td><methodname
+ xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-java.lang.String-">print(String
+ s)</methodname></td>
- <mediaobject>
- <imageobject>
- <imagedata fileref="Ref/Fig/packageHierarchy.png"/>
- </imageobject>
- </mediaobject>
- </screenshot>
- </figure>
- </answer>
- </qandaentry>
- </qandadiv>
- </qandaset>
+ <td/>
+ </tr>
+ </informaltable>
+ </figure>
</section>
- <section xml:id="sd1_sect_methodSignature">
- <title>Signatures</title>
-
- <figure xml:id="sd1_fig_typeSignature">
- <title>Type signatures</title>
+ <section xml:id="sd1_sect_constructors">
+ <title>Constructors</title>
- <para>A method's return and arguments type list. Examples:</para>
+ <figure xml:id="sd1_fig_createRectangleConstructor">
+ <title>Creating and initializing rectangles</title>
<informaltable border="1">
- <colgroup width="75%"/>
-
- <colgroup width="12%"/>
+ <colgroup width="30%"/>
- <colgroup width="13%"/>
+ <colgroup width="70%"/>
<tr>
- <th rowspan="2" valign="top">Method</th>
+ <td valign="top"><programlisting language="java">int a;
+a = 33;</programlisting></td>
- <th colspan="2">Type signature</th>
+ <td valign="top"><programlisting language="java">Rectangle r = new Rectangle();
+
+r.width = 28;
+r.height = 10;
+r.hasSolidBorder = false;</programlisting></td>
</tr>
<tr>
- <th>Return type</th>
-
- <th>Argument type list</th>
+ <th colspan="2"><para>Combining statements desired:</para></th>
</tr>
<tr>
- <td><methodname>void print() {...}</methodname></td>
-
- <td>void</td>
+ <td valign="top"><programlisting language="java">int a = 33; // works!</programlisting></td>
- <td>(void)</td>
+ <td valign="top"><programlisting language="java">Rectangle r = new Rectangle(28, 10, false); // <emphasis
+ role="red">how ???</emphasis></programlisting></td>
</tr>
+ </informaltable>
- <tr>
- <td><methodname>int add (int a, int b) {...}</methodname></td>
+ <para>How to get this work?</para>
+ </figure>
- <td>int</td>
+ <figure xml:id="sd1_fig_createRectangleDefineConstruct">
+ <title>Defining a constructor</title>
- <td>(int, int)</td>
- </tr>
+ <programlisting language="java">public class Rectangle {
+ int width, height;
+ boolean hasSolidBorder;
+ ...
+ public <co linkends="sd1_callout_defineConstructor-1"
+ xml:id="sd1_callout_defineConstructor-1-co"/> Rectangle <co
+ linkends="sd1_callout_defineConstructor-2"
+ xml:id="sd1_callout_defineConstructor-2-co"/> (int width, int height, boolean hasSolidBorder){
+ this.width = width;
+ this.height = height;
+ this.hasSolidBorder = hasSolidBorder;
+ }
+}</programlisting>
+ </figure>
- <tr>
- <td><methodname>int maximum (int a, int b){...}</methodname></td>
+ <para>Similar to <xref linkend="fig_methodSyntax"/> but:</para>
- <td>int</td>
+ <calloutlist>
+ <callout arearefs="sd1_callout_defineConstructor-1-co"
+ xml:id="sd1_callout_defineConstructor-1">
+ <para>No return type declaration.</para>
+ </callout>
- <td>(int, int)</td>
- </tr>
+ <callout arearefs="sd1_callout_defineConstructor-2-co"
+ xml:id="sd1_callout_defineConstructor-2">
+ <para>Constructor's name equals class name.</para>
+ </callout>
+ </calloutlist>
- <tr>
- <td><methodname>void print (int a, float b){...}</methodname></td>
+ <figure xml:id="sd1_fig_constructorSyntax">
+ <title>Constructor syntax</title>
- <td>void</td>
+ <programlisting language="java">constructorName (listOfArguments) {
+ [constructor body]
+}</programlisting>
- <td>(int, float)</td>
- </tr>
+ <glosslist>
+ <glossentry>
+ <glossterm>Empty argument list</glossterm>
- <tr>
- <td><methodname>void display (float a, int
- b){...}</methodname></td>
+ <glossdef>
+ <para>Default constructor e.g. <code language="java">obj = new
+ MyClass()</code>.</para>
+ </glossdef>
+ </glossentry>
- <td>void</td>
+ <glossentry>
+ <glossterm>Non-empty argument list</glossterm>
- <td>(float, int)</td>
- </tr>
- </informaltable>
+ <glossdef>
+ <para>Non-default constructor, e.g. <code language="java">obj =
+ new MyClass("xyz")</code>.</para>
+ </glossdef>
+ </glossentry>
+ </glosslist>
</figure>
- <figure xml:id="sd1_fig_typeSignatureDetail">
- <title>Defining type signatures</title>
+ <figure xml:id="sd1_fig_constructorInfo">
+ <title>Constructors</title>
<itemizedlist>
<listitem>
- <para>Return type, e.g. <code language="java">void</code></para>
+ <para>Can only be executed on object creation.</para>
</listitem>
<listitem>
- <para>Number of arguments, e.g. 2.</para>
+ <para>Are being called prior to any non-constructor method.</para>
</listitem>
<listitem>
- <para>Type of arguments among with their respective order, e.g.
- (int, double) differing from (double, int).</para>
+ <para>Only one of potentially multiple constructors will be
+ executed exactly one time.</para>
</listitem>
</itemizedlist>
</figure>
- <figure xml:id="sd1_fig_methodSignature">
- <title>A method signature</title>
-
- <para>A method's name and arguments type list:</para>
+ <figure xml:id="sd1_fig_rectangleThreeConstructors">
+ <title>Multiple constructors by overloading</title>
<informaltable border="1">
- <colgroup width="75%"/>
-
- <colgroup width="12%"/>
-
- <colgroup width="13%"/>
-
- <tr>
- <th valign="top">Method</th>
-
- <th>Method name</th>
-
- <th colspan="1">Method signature</th>
- </tr>
-
<tr>
- <td><methodname>void print() {...}</methodname></td>
-
- <td>print</td>
+ <td valign="top"><programlisting language="java">public class Rectangle {
+ int width, height;
- <td>(void)</td>
- </tr>
-
- <tr>
- <td><methodname>int add (int a, int b) {...}</methodname></td>
-
- <td>add</td>
-
- <td>(int, int)</td>
- </tr>
-
- <tr>
- <td><methodname>int maximum (int a, int b){...}</methodname></td>
-
- <td>maximum</td>
-
- <td>(int, int)</td>
- </tr>
-
- <tr>
- <td><methodname>void print (int a, float b){...}</methodname></td>
-
- <td>print</td>
-
- <td>(int, float)</td>
- </tr>
-
- <tr>
- <td><methodname>void display (float a, int
- b){...}</methodname></td>
-
- <td>display</td>
+ public Rectangle() {
+ width = height = 1;
+ }
+ public Rectangle(int width, int height){
+ this.width = width;
+ this.height = height;
+ }
+ public Rectangle(int widthAndHeight) {
+ width = height = widthAndHeight;
+ }
+}</programlisting></td>
- <td>(float, int)</td>
+ <td><mediaobject>
+ <imageobject>
+ <imagedata fileref="Ref/ObjectsAndClasses/rectangleThreeConstruct.svg"
+ valign="top"/>
+ </imageobject>
+ </mediaobject></td>
</tr>
</informaltable>
</figure>
- </section>
-
- <section xml:id="sd1_sect_methodOverloading">
- <title>Method overloading</title>
- <figure xml:id="sd1_fig_overloadingPrinciple">
- <title>Method overloading: Same name, different signature</title>
+ <figure xml:id="sd1_fig_rectangle_constructorCallsConstructor">
+ <title>Constructor calls within constructor</title>
- <informaltable border="0">
+ <informaltable border="1">
<tr>
- <td rowspan="2" valign="top"><programlisting language="java">public class Print {
- public void print() { // (void)
- System.out.println("No argument");
- }
- public void print(int i) { // (int)
- System.out.println("int value " + i);
+ <td valign="top"><programlisting language="java">public class Rectangle {
+ int width, height;
+
+ public Rectangle(int width,
+ int height){
+ this.width = width;
+ this.height = height;
}
- public void print(double d) { // (double)
- System.out.println("double value " + d);
+ public Rectangle() {
+ width = height = 1;
}
- public void print(int i, int j) { // (int, int)
- System.out.println("Two int values "+
- i + " and " + j);
+ public Rectangle(
+ int widthAndHeight) {
+ width = height =
+ widthAndHeight;
}
}</programlisting></td>
- <td valign="top"><programlisting language="java">Print p = new Print();
-p.print();
-p.print(33);
-p.print(4.333);
-p.print(-1, 7);</programlisting></td>
- </tr>
+ <td><programlisting language="java">public class Rectangle {
+ int width, height;
- <tr>
- <td valign="top"><screen>No argument
-int value 33
-double value 4.333
-Two int values -1 and 7</screen></td>
+ public Rectangle(int width,
+ int height){
+ this.width = width;
+ this.height = height;
+ }
+ public Rectangle() {
+ this(1, 1); <co linkends="sd1_fig_rectangle_constructorCallsConstructor-1"
+ xml:id="sd1_fig_rectangle_constructorCallsConstructor-1-co"/>
+ }
+ public Rectangle(
+ int widthAndHeight) {
+ this(widthAndHeight,
+ widthAndHeight); <co
+ linkends="sd1_fig_rectangle_constructorCallsConstructor-2"
+ xml:id="sd1_fig_rectangle_constructorCallsConstructor-2-co"/>
+ }
+}</programlisting></td>
</tr>
</informaltable>
- </figure>
-
- <figure xml:id="sd1_fig_overloadingAlternateNames">
- <title>Overloading, alternate names</title>
-
- <itemizedlist>
- <listitem>
- <para>Static polymorphism.</para>
- </listitem>
- <listitem>
- <para>Compile time binding.</para>
- </listitem>
+ <calloutlist role="slideExclude">
+ <callout arearefs="sd1_fig_rectangle_constructorCallsConstructor-1-co"
+ xml:id="sd1_fig_rectangle_constructorCallsConstructor-1">
+ <para>Reusing constructor <methodname>Rectangle(int width, int
+ height)</methodname> with parameters <code language="java">width =
+ height = 1</code>.</para>
+ </callout>
- <listitem>
- <para>Early binding.</para>
- </listitem>
- </itemizedlist>
+ <callout arearefs="sd1_fig_rectangle_constructorCallsConstructor-2-co"
+ xml:id="sd1_fig_rectangle_constructorCallsConstructor-2">
+ <para>Reusing constructor <methodname>Rectangle(int width, int
+ height)</methodname> with parameters <code language="java">width =
+ height = widthAndHeight</code>.</para>
+ </callout>
+ </calloutlist>
</figure>
- <figure xml:id="sd1_fig_noOverloadingInC">
- <title>No such concept in C</title>
-
- <programlisting language="c">void print() {
- printf("No argument\n");
-}
+ <figure xml:id="sd1_fig_rectangleThreeConstructorsInstances">
+ <title>Instances by overloaded constructors</title>
-void print(int i) { <emphasis role="bold"> /* Error: redefinition of ‘print’ */</emphasis>
- printf("int value %d\n", i);
-}
+ <informaltable border="1">
+ <tr>
+ <td valign="top"><programlisting language="java">Rectangle standard = new Rectangle(); // 1 x 1
+Rectangle square = new Rectangle(2); // 2 x 2
+Rectangle individual = new Rectangle(2, 7); // 2 x 7</programlisting></td>
-void main(void) {
- print();
- print(33);
-}</programlisting>
+ <td valign="top"><mediaobject>
+ <imageobject>
+ <imagedata fileref="Ref/ObjectsAndClasses/rectangleThreeConstruct.svg"/>
+ </imageobject>
+ </mediaobject></td>
+ </tr>
+ </informaltable>
</figure>
- <figure xml:id="sd1_fig_distinctReturnType">
- <title>What about return type?</title>
-
- <programlisting language="java">public class Person {
- String getDetails() { return "dummy";}
- int getDetails() { return 1;} // Error: 'getDetails()' is already
- // defined in 'Person'
-}</programlisting>
+ <figure xml:id="sd1_fig_createRectangleNoConstructor">
+ <title>No constructor <abbrev>vs.</abbrev> default constructor</title>
<informaltable border="1">
<tr>
- <th rowspan="2" valign="top">Return type</th>
-
- <th colspan="2">Method signature</th>
+ <th colspan="2">Equivalent: <code language="java">Rectangle r =
+ new Rectangle();</code></th>
</tr>
<tr>
- <th>Method name</th>
+ <td valign="top"><programlisting language="java">public class Rectangle {
+ int width, height;
+ boolean hasSolidBorder;
- <th>Argument type list</th>
+ // Default constructor, empty body.
+ public Rectangle ( ){}
+}</programlisting></td>
+
+ <td valign="top"><programlisting language="java">public class Rectangle {
+ int width, height;
+ boolean hasSolidBorder;
+}</programlisting></td>
</tr>
+ </informaltable>
+ </figure>
+ <figure xml:id="sd1_fig_createRectangleMissingDefaultConstructor">
+ <title>Absent default constructor</title>
+
+ <informaltable border="1">
<tr>
- <td>String</td>
+ <td rowspan="2"><programlisting language="java">public class Rectangle {
+ int width, height;
+ boolean hasSolidBorder;
- <td>getDetails</td>
+ public Rectangle(int width,
+ int height,
+ boolean hasSolidBorder){
+ this.width = width;
+ this.height = height;
+ this.hasSolidBorder =
+ hasSolidBorder;
+ }
+}</programlisting></td>
- <td>(void)</td>
+ <td valign="top"><programlisting language="java">Rectangle r =
+ new Rectangle(3, 6, false);</programlisting><para>o.K.: Using non-default
+ constructor.</para></td>
</tr>
<tr>
- <td>int</td>
-
- <td>getDetails</td>
-
- <td>(void)</td>
+ <td valign="top"><programlisting language="java">Rectangle r = new Rectangle();</programlisting><para>Wrong:
+ Default constructor undefined, but non-default constructor
+ present.</para></td>
</tr>
</informaltable>
-
- <para>Return type irrelevant, only method signatures in <xref
- linkend="glo_Java"/>.</para>
</figure>
- <figure xml:id="sd1_fig_methodSignatureRatio">
- <title>Method signatures rationale</title>
+ <qandaset defaultlabel="qanda" xml:id="sd1_qanda_geometry_Rectangle">
+ <title>Modeling geometry objects: Rectangles</title>
- <para>In <xref linkend="glo_Java"/> method signatures allow for
- uniquely addressing a method within a given class e.g.:</para>
+ <qandaentry>
+ <question>
+ <para>We want to represent rectangles being defined by width and
+ height to allow for the subsequently demonstrated
+ operations:</para>
- <para>The method named <code language="java">print</code> having an
- int argument followed by a double:</para>
+ <programlisting language="java">final Rectangle r = new Rectangle(8, 5); <co
+ linkends="sw1CalloutRectangle-1" xml:id="constructor"/>
- <programlisting language="java">print(int, double)</programlisting>
- </figure>
+System.out.println("Perimeter:" + r.getPerimeter()<co
+ linkends="sw1CalloutRectangle-2" xml:id="getPerimeter"/>);
+System.out.println("Area:" + r.getArea()<co linkends="sw1CalloutRectangle-3"
+ xml:id="getArea"/>);
- <figure xml:id="sd1_fig_chooseByMethodSignature">
- <title>Method signatures rationale</title>
+r.setWidth(4); <co linkends="sw1CalloutRectangle-4" xml:id="setWidth"/>
+r.setHeight(7);
- <mediaobject>
- <imageobject>
- <imagedata fileref="Ref/ObjectsAndClasses/chooseOverloaded.svg"/>
- </imageobject>
- </mediaobject>
- </figure>
+System.out.println("Perimeter:" + r.getPerimeter()); <co
+ linkends="sw1CalloutRectangle-5"
+ xml:id="sw1CalloutRectangle-5-co"/>
+System.out.println("Area:" + r.getArea());</programlisting>
- <figure xml:id="sd1_fig_sysoutPrintlnOverload">
- <title>Example: System.out.print(...)</title>
+ <calloutlist>
+ <callout arearefs="constructor" xml:id="sw1CalloutRectangle-1">
+ <para>Creating an instance of class
+ <classname>Rectangle</classname> by calling a non-default
+ constructor which allows for providing width (8) and height
+ (5).</para>
+ </callout>
- <informaltable border="0">
- <tr>
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-boolean-">print(boolean
- b)</methodname></td>
-
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-char-">print(char
- c)</methodname></td>
- </tr>
-
- <tr>
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-char:A-">print(char[]
- s)</methodname></td>
-
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-double-">print(double
- d)</methodname></td>
- </tr>
-
- <tr>
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-float-">print(float
- f)</methodname></td>
+ <callout arearefs="getPerimeter" xml:id="sw1CalloutRectangle-2">
+ <para>Returning the rectangle's perimeter,</para>
+ </callout>
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-int-">print(int
- i)</methodname></td>
- </tr>
+ <callout arearefs="getArea" xml:id="sw1CalloutRectangle-3">
+ <para>Returning the rectangle's area.</para>
+ </callout>
- <tr>
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-long-">print(long
- l)</methodname></td>
+ <callout arearefs="setWidth" xml:id="sw1CalloutRectangle-4">
+ <para>Setting with and height to new values.</para>
+ </callout>
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-java.lang.Object-">print(Object
- obj)</methodname></td>
- </tr>
+ <callout arearefs="sw1CalloutRectangle-5-co"
+ xml:id="sw1CalloutRectangle-5">
+ <para>Write (possibly) changed perimeter and area
+ values.</para>
+ </callout>
+ </calloutlist>
- <tr>
- <td><methodname
- xlink:href="https://docs.oracle.com/javase/10/docs/api/java/io/PrintStream.html#print-java.lang.String-">print(String
- s)</methodname></td>
+ <para>You may start from the following
+ <classname>Rectangle</classname> class dummy
+ implementation:</para>
- <td/>
- </tr>
- </informaltable>
- </figure>
- </section>
+ <programlisting language="java">/**
+ * Representing rectangular shapes.
+ *
+ */
+public class Rectangle {
- <section xml:id="sd1_sect_constructors">
- <title>Constructors</title>
+ /**
+ *
+ * @param width The rectangle's width
+ * @param heigth The rectangle's height
+ */
+ public Rectangle (double width, double heigth) {
+ //TODO
+ }
+ /**
+ * @return The rectangle's area.
+ */
+ public double getArea() {
+ return 0; // TODO
+ }
- <figure xml:id="sd1_fig_createRectangleConstructor">
- <title>Creating and initializing rectangles</title>
+ /**
+ * @return The rectangle's perimeter.
+ */
+ public double getPerimeter() {
+ return 0; // TODO
+ }
- <informaltable border="1">
- <colgroup width="30%"/>
+ /**
+ * @return The rectangle's width.
+ */
+ public double getWidth() {
+ return 0; // TODO
+ }
+ /**
+ * @param width The rectangle's new width
+ */
+ public void setWidth(double width) {
+ // TODO
+ }
- <colgroup width="70%"/>
+ /**
+ * @return The rectangle's height.
+ */
+ public double getHeight() {
+ return 0; // TODO
+ }
- <tr>
- <td valign="top"><programlisting language="java">int a;
-a = 33;</programlisting></td>
+ /**
+ * @param height The rectangle's new height
+ */
+ public void setHeight(double height) {
+ // TODO
+ }
+}</programlisting>
+ </question>
- <td valign="top"><programlisting language="java">Rectangle r = new Rectangle();
+ <answer>
+ <para>First we define two instance variables <code
+ language="java">width</code> and <code
+ language="java">height</code> representing a
+ <classname>Rectangle</classname>'s corresponding two parameters
+ <code language="java">width</code> and <code
+ language="java">height</code>:</para>
-r.width = 28;
-r.height = 10;
-r.hasSolidBorder = false;</programlisting></td>
- </tr>
+ <programlisting language="java">public class Rectangle {
- <tr>
- <th colspan="2"><para>Combining statements desired:</para></th>
- </tr>
+ // Instance variables representing a rectangle's parameters
+ private double <emphasis role="bold">width</emphasis>, <emphasis role="bold">height</emphasis>;
+...
+}</programlisting>
- <tr>
- <td valign="top"><programlisting language="java">int a = 33; // works!</programlisting></td>
+ <para>Next we allow for changing these two parameters:</para>
- <td valign="top"><programlisting language="java">Rectangle r = new Rectangle(28, 10, false); // <emphasis
- role="red">how ???</emphasis></programlisting></td>
- </tr>
- </informaltable>
+ <programlisting language="java">public class Rectangle {
- <para>How to get this work?</para>
- </figure>
+ // Instance variables representing a rectangle's parameters
+ private double width, height;
- <figure xml:id="sd1_fig_createRectangleDefineConstruct">
- <title>Defining a constructor</title>
+...
+ /**
+ * @param width Changing the rectangle's width
+ */
+ public void setWidth(double w) {
+ <emphasis role="bold">width = w;</emphasis>
+ }
- <programlisting language="java">public class Rectangle {
- int width, height;
- boolean hasSolidBorder;
- ...
- public <co linkends="sd1_callout_defineConstructor-1"
- xml:id="sd1_callout_defineConstructor-1-co"/> Rectangle <co
- linkends="sd1_callout_defineConstructor-2"
- xml:id="sd1_callout_defineConstructor-2-co"/> (int width, int height, boolean hasSolidBorder){
- this.width = width;
- this.height = height;
- this.hasSolidBorder = hasSolidBorder;
- }
+ /**
+ * @param width Changing the rectangle's height
+ */
+ public void setHeight(double height) {
+ <emphasis role="bold">this.height = height;</emphasis>
+ }
+...
}</programlisting>
- </figure>
- <para>Similar to <xref linkend="fig_methodSyntax"/> but:</para>
-
- <calloutlist>
- <callout arearefs="sd1_callout_defineConstructor-1-co"
- xml:id="sd1_callout_defineConstructor-1">
- <para>No return type declaration.</para>
- </callout>
+ <para>Note the subtle implementation difference between
+ <methodname>setWidth(...)</methodname> and
+ <methodname>setHeight(...)</methodname>:</para>
- <callout arearefs="sd1_callout_defineConstructor-2-co"
- xml:id="sd1_callout_defineConstructor-2">
- <para>Constructor's name equals class name.</para>
- </callout>
- </calloutlist>
+ <glosslist>
+ <glossentry xml:id="glossMethodDiff1">
+ <glossterm><methodname>setWidth(double
+ w)</methodname></glossterm>
- <figure xml:id="sd1_fig_constructorSyntax">
- <title>Constructor syntax</title>
+ <glossdef>
+ <para>We use the formal parameter name <quote>w</quote>. Its
+ name does not conflict with the instance variable name
+ <quote>width</quote> being defined at class level. We can
+ simply assign this value to our corresponding instance
+ variable using <code language="java">width =
+ w;</code>.</para>
+ </glossdef>
+ </glossentry>
- <programlisting language="java">constructorName (listOfArguments) {
- [constructor body]
-}</programlisting>
+ <glossentry xml:id="xmlMethodDiff2">
+ <glossterm><methodname>setHeight(double
+ height)</methodname></glossterm>
- <glosslist>
- <glossentry>
- <glossterm>Empty argument list</glossterm>
+ <glossdef>
+ <para>The method's formal parameter <quote>height</quote>
+ shadows the instance variable's name being defined at class
+ level. We need the <quote>this</quote> keyword in <code
+ language="java">this.height = height</code> to resolve the
+ ambiguity.</para>
+ </glossdef>
+ </glossentry>
+ </glosslist>
- <glossdef>
- <para>Default constructor e.g. <code language="java">obj = new
- MyClass()</code>.</para>
- </glossdef>
- </glossentry>
+ <para>Both ways are perfectly legal. The complete implementation
+ including all remaining methods reads:</para>
- <glossentry>
- <glossterm>Non-empty argument list</glossterm>
+ <programlisting language="java">/**
+ * Representing rectangular shapes.
+ *
+ */
+public class Rectangle {
- <glossdef>
- <para>Non-default constructor, e.g. <code language="java">obj =
- new MyClass("xyz")</code>.</para>
- </glossdef>
- </glossentry>
- </glosslist>
- </figure>
+ // Instance variables representing a rectangle's parameters
+ private double width, height;
- <figure xml:id="sd1_fig_constructorInfo">
- <title>Constructors</title>
+ /**
+ *
+ * @param width The rectangle's width
+ * @param heigth The rectangle's height
+ */
+ public Rectangle (double width, double height) {
+ setWidth(width);
+ setHeight(height);
+ }
+ /**
+ * @return The rectangle's area.
+ */
+ public double getArea() {
+ return width * height;
+ }
- <itemizedlist>
- <listitem>
- <para>Can only be executed on object creation.</para>
- </listitem>
+ /**
+ * @return The rectangle's perimeter.
+ */
+ public double getPerimeter() {
+ return 2 * (width + height);
+ }
- <listitem>
- <para>Are being called prior to any non-constructor method.</para>
- </listitem>
+ /**
+ * @return The rectangle's width.
+ */
+ public double getWidth() {
+ return width;
+ }
+ /**
+ * @param width The rectangle's new width
+ */
+ public void setWidth(double w) {
+ width = w;
+ }
- <listitem>
- <para>Only one of potentially multiple constructors will be
- executed exactly one time.</para>
- </listitem>
- </itemizedlist>
- </figure>
+ /**
+ * @return The rectangle's height.
+ */
+ public double getHeight() {
+ return height;
+ }
- <figure xml:id="sd1_fig_rectangleThreeConstructors">
- <title>Multiple constructors by overloading</title>
+ /**
+ * @param width The rectangle's new height
+ */
+ public void setHeight(double height) {
+ this.height = height;
+ }
+}</programlisting>
+ </answer>
+ </qandaentry>
+ </qandaset>
- <informaltable border="1">
- <tr>
- <td valign="top"><programlisting language="java">public class Rectangle {
- int width, height;
+ <qandaset defaultlabel="qanda" xml:id="qandasetGeometry">
+ <title>Modeling circles</title>
- public Rectangle() {
- width = height = 1;
- }
- public Rectangle(int width, int height){
- this.width = width;
- this.height = height;
- }
- public Rectangle(int widthAndHeight) {
- width = height = widthAndHeight;
- }
-}</programlisting></td>
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>This exercise is very similar to <xref
+ linkend="sd1_qanda_geometry_Rectangle"/>. With respect to the
+ upcoming section on inheritance its presence will be justified
+ later by <xref linkend="sw1SectGeometryInherit"/>.</para>
- <td><mediaobject>
- <imageobject>
- <imagedata fileref="Ref/ObjectsAndClasses/rectangleThreeConstruct.svg"
- valign="top"/>
- </imageobject>
- </mediaobject></td>
- </tr>
- </informaltable>
- </figure>
+ <para>We provide a corresponding class
+ <classname>Circle</classname> dummy implementation:</para>
- <figure xml:id="sd1_fig_rectangle_constructorCallsConstructor">
- <title>Constructor calls within constructor</title>
+ <programlisting language="java">package step1.dummy;
- <informaltable border="1">
- <tr>
- <td valign="top"><programlisting language="java">public class Rectangle {
- int width, height;
+/**
+ * A circle of given radius
+ *
+ */
+public class Circle {
- public Rectangle(int width,
- int height){
- this.width = width;
- this.height = height;
+ /**
+ * A new circle
+ *
+ * @param radius
+ * The desired radius.
+ */
+ public Circle(double radius) {
+ // TODO
}
- public Rectangle() {
- width = height = 1;
+
+ /**
+ * @return The circle's area.
+ */
+ public double getArea() {
+ return 0; // TODO
}
- public Rectangle(
- int widthAndHeight) {
- width = height =
- widthAndHeight;
+
+ /**
+ * @return The circle's perimeter.
+ */
+ public double getPerimeter() {
+ return 0; // TODO
}
-}</programlisting></td>
- <td><programlisting language="java">public class Rectangle {
- int width, height;
+ /**
+ * @return The circle's radius.
+ */
+ public double getRadius() {
+ return 0; // TODO
+ }
- public Rectangle(int width,
- int height){
- this.width = width;
- this.height = height;
+ /**
+ * @param radius
+ * Setting the circle's radius to a new value.
+ */
+ public void setRadius(double radius) {
+ // TODO
}
- public Rectangle() {
- this(1, 1); <co linkends="sd1_fig_rectangle_constructorCallsConstructor-1"
- xml:id="sd1_fig_rectangle_constructorCallsConstructor-1-co"/>
+}</programlisting>
+
+ <para>Instances of this class shall be usable in the following
+ fashion:</para>
+
+ <programlisting language="java">public static void main(String[] args) {
+ final Circle c = new Circle(2.3);
+
+ System.out.println("Radius:" + c.getRadius());
+ System.out.println("Perimeter:" + c.getPerimeter());
+ System.out.println("Area:" + c.getArea());
+
+ // Changing the circle's radius to a different value
+ c.setRadius(4.7);
+
+ System.out.println("Radius:" + c.getRadius());
+ System.out.println("Perimeter:" + c.getPerimeter());
+ System.out.println("Area:" + c.getArea());
+}</programlisting>
+
+ <para>Hint: Obviously you'll have to define an instance variable
+ within Circle to keep track of its current radius value. All
+ methods mentioned above simply depend on this single
+ value.</para>
+ </question>
+
+ <answer>
+ <para>We define an instance variable <code
+ language="java">radius</code> inside our class
+ <classname>Circle</classname>:</para>
+
+ <programlisting language="java">public class Circle {
+
+ double radius;
+...
+}</programlisting>
+
+ <para>Next we implement our method to change a circle's
+ radius:</para>
+
+ <programlisting language="java">public void setRadius(double r) {
+ radius = r;
+}</programlisting>
+
+ <para>Note that we have chosen a different value for the
+ method's formal radius parameter to be <quote>r</quote> rather
+ than <quote>radius</quote>. Many people prefer to use radius
+ here making it easier for a programmer to recognize the expected
+ name in the generated <xref linkend="glo_Javadoc"/>:</para>
+
+ <programlisting language="java">public void setRadius(double radius) {
+<emphasis role="red"> this.</emphasis>radius = radius;
+}</programlisting>
+
+ <para>This requires the usage of the <code
+ language="java">this</code> keyword to distinguish the formal
+ parameter in <methodname>setRadius(double radius)</methodname>
+ from the instance variable previously being defined within our
+ class <classname>Circle</classname>. In other words: We have to
+ resolve a name shadowing conflict.</para>
+
+ <para>The rest of the implementation is (quite) straightforward.
+ A complete class reads:</para>
+
+ <programlisting language="java">package step1;
+
+/**
+ * A circle of given radius
+ *
+ */
+public class Circle {
+
+ double radius;
+
+ /**
+ * A new circle
+ * @param radius The desired radius.
+ */
+ public Circle(double radius) {
+ setRadius(radius);
}
- public Rectangle(
- int widthAndHeight) {
- this(widthAndHeight,
- widthAndHeight); <co
- linkends="sd1_fig_rectangle_constructorCallsConstructor-2"
- xml:id="sd1_fig_rectangle_constructorCallsConstructor-2-co"/>
+
+ /**
+ * @return The circle's area.
+ */
+ public double getArea() {
+ return radius * radius * Math.PI;
}
-}</programlisting></td>
- </tr>
- </informaltable>
- <calloutlist role="slideExclude">
- <callout arearefs="sd1_fig_rectangle_constructorCallsConstructor-1-co"
- xml:id="sd1_fig_rectangle_constructorCallsConstructor-1">
- <para>Reusing constructor <methodname>Rectangle(int width, int
- height)</methodname> with parameters <code language="java">width =
- height = 1</code>.</para>
- </callout>
+ /**
+ * @return The circle's perimeter.
+ */
+ public double getPerimeter() {
+ return 2 * Math.PI * radius;
+ }
- <callout arearefs="sd1_fig_rectangle_constructorCallsConstructor-2-co"
- xml:id="sd1_fig_rectangle_constructorCallsConstructor-2">
- <para>Reusing constructor <methodname>Rectangle(int width, int
- height)</methodname> with parameters <code language="java">width =
- height = widthAndHeight</code>.</para>
- </callout>
- </calloutlist>
- </figure>
+ /**
+ * @return The circle's radius.
+ */
+ public double getRadius() {
+ return radius;
+ }
- <figure xml:id="sd1_fig_rectangleThreeConstructorsInstances">
- <title>Instances by overloaded constructors</title>
+ /**
+ * @param radius Setting the circle's radius to a new value.
+ */
+ public void setRadius(double radius) {
+ this.radius = radius;
+ }
+}</programlisting>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
- <informaltable border="1">
- <tr>
- <td valign="top"><programlisting language="java">Rectangle standard = new Rectangle(); // 1 x 1
-Rectangle square = new Rectangle(2); // 2 x 2
-Rectangle individual = new Rectangle(2, 7); // 2 x 7</programlisting></td>
+ <qandaset defaultlabel="qanda" xml:id="sdQandaSvgAndTranslation">
+ <title>Adding translations and <xref linkend="glo_SVG"/>
+ export.</title>
- <td valign="top"><mediaobject>
- <imageobject>
- <imagedata fileref="Ref/ObjectsAndClasses/rectangleThreeConstruct.svg"/>
- </imageobject>
- </mediaobject></td>
- </tr>
- </informaltable>
- </figure>
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>We want to add more features to tour
+ <classname>Circle</classname> and
+ <classname>Rectangle</classname> classes:</para>
- <figure xml:id="sd1_fig_createRectangleNoConstructor">
- <title>No constructor <abbrev>vs.</abbrev> default constructor</title>
+ <glosslist>
+ <glossentry>
+ <glossterm>Translations</glossterm>
- <informaltable border="1">
- <tr>
- <th colspan="2">Equivalent: <code language="java">Rectangle r =
- new Rectangle();</code></th>
- </tr>
+ <glossdef>
+ <para>Add two more instance variables x and y and
+ corresponding setter methods to account for a shape's
+ translation vector with respect to the origin (0,0). The
+ following hint may be helpful:</para>
- <tr>
- <td valign="top"><programlisting language="java">public class Rectangle {
- int width, height;
- boolean hasSolidBorder;
+ <programlisting language="java">/**
+ * @param x The circle's x center coordinate value
+ */
+public void setX(double x) {
+ // TODO
+}
+/**
+ * @param y The circle's y center coordinate value
+ */
+public void setY(double y) {
+ // TODO
+}</programlisting>
- // Default constructor, empty body.
- public Rectangle ( ){}
-}</programlisting></td>
+ <para>You may as well extend the constructors of
+ <classname>Rectangle</classname> and
+ <classname>Circle</classname> to accept center coordinates
+ as well:</para>
- <td valign="top"><programlisting language="java">public class Rectangle {
- int width, height;
- boolean hasSolidBorder;
-}</programlisting></td>
- </tr>
- </informaltable>
- </figure>
+ <programlisting language="java">public class Rectangle {
- <figure xml:id="sd1_fig_createRectangleMissingDefaultConstructor">
- <title>Absent default constructor</title>
+...
+ /**
+ * Rectangle having center coordinates x, y, width and height.
+ *
+ * @param x The rectangle center's x-coordinate
+ * @param y The rectangle center's y-coordinate
+ * @param width The rectangle's width
+ * @param height The rectangle's height
+ */
+ public Rectangle(double x, double y, double width, double height) {
+ ...
+ }
+ ...
+}</programlisting>
- <informaltable border="1">
- <tr>
- <td rowspan="2"><programlisting language="java">public class Rectangle {
- int width, height;
- boolean hasSolidBorder;
+ <programlisting language="java">public class Circle {
+ ...
- public Rectangle(int width,
- int height,
- boolean hasSolidBorder){
- this.width = width;
- this.height = height;
- this.hasSolidBorder =
- hasSolidBorder;
+ /**
+ * Circle having center coordinates x, y and radius r.
+ *
+ * @param x The circle center's x-coordinate
+ * @param y The circle center's y-coordinate
+ * @param radius The circle's radius
+ */
+ public Circle(double x, double y, double radius) {
+ ...
+ }
+ ...
+}</programlisting>
+ </glossdef>
+ </glossentry>
+
+ <glossentry>
+ <glossterm><xref linkend="glo_SVG"/> export</glossterm>
+
+ <glossdef>
+ <para>We would like Rectangle and Circle instances to be
+ visualized as <xref linkend="glo_SVG"/> graphics. <link
+ xlink:href="http://www.w3schools.com/graphics/svg_examples.asp">SVG
+ Examples</link> provides editor samples both for <link
+ xlink:href="http://www.w3schools.com/graphics/svg_circle.asp">circles</link>
+ and <link
+ xlink:href="http://www.w3schools.com/graphics/svg_rect.asp">rectangles</link>.
+ Add a method <methodname>void writeSvg()</methodname> to
+ both of your classes which allows for <xref
+ linkend="glo_SVG"/> code being written to standard output.
+ Use <methodname>System.out.println(...)</methodname> calls
+ to create the desired <xref linkend="glo_SVG"/> output.
+ You may need <code language="java">\"</code> to escape
+ double quotes as in the subsequent example or use single
+ attribute quotes instead:</para>
+
+ <programlisting language="java">System.out.println("<rect width=\"20\"" ...</programlisting>
+ </glossdef>
+ </glossentry>
+ </glosslist>
+
+ <para>The following code snippet may serve to illustrate the
+ intended use of <methodname>void writeSvg()</methodname>:</para>
+
+ <programlisting language="java">public class Driver {
+
+ public static void main(String[] args) {
+
+ System.out.println("<!DOCTYPE html><html><body>");
+ System.out.println(" <svg width='300' height='200' >");
+
+ // Draw a rectangle as SVG
+ final Rectangle r = new Rectangle(5, 4);
+ r.setX(2);
+ r.setY(1);
+ r.writeSvg();
+
+ // Draw a circle as SVG
+ final Circle c = new Circle(3);
+ c.setX(3);
+ c.setY(1);
+ c.writeSvg();
+ System.out.println(" </svg >");
+ System.out.println("</body></html>");
}
-}</programlisting></td>
+}</programlisting>
- <td valign="top"><programlisting language="java">Rectangle r =
- new Rectangle(3, 6, false);</programlisting><para>o.K.: Using non-default
- constructor.</para></td>
- </tr>
+ <para>Implement the method <methodname>void
+ writeSvg()</methodname> in both classes
+ <classname>Rectangle</classname> and
+ <classname>Circle</classname>. The following sample export is
+ intended for 300x200 pixel requiring to scale
+ <abbrev>e.g.</abbrev> the above circle's radius of three by a
+ factor of 20 resulting in an effective <tag
+ class="emptytag">circle r='60' ...</tag> value. This scaling is
+ being required for all parameters:</para>
+
+ <programlisting language="java"><!DOCTYPE html>
+<html>
+ <body>
+ <svg width='300' height='200' >
+ <emphasis role="bold"><rect width='100.0' height='80.0' x='40.0' y='20.0'
+ style='fill:rgb(0,255,0);stroke-width:3;stroke:rgb(0,0,0)'/></emphasis>
+ <emphasis role="bold"><circle r='60.0' cx='60.0' cy='20.0'
+ style='fill:rgb(0,0,255);stroke-width:3;stroke:rgb(0,0,0)'/></emphasis>
+ </svg >
+ </body>
+</html></programlisting>
- <tr>
- <td valign="top"><programlisting language="java">Rectangle r = new Rectangle();</programlisting><para>Wrong:
- Default constructor undefined, but non-default constructor
- present.</para></td>
- </tr>
- </informaltable>
- </figure>
+ <para>You may copy this output into a file
+ <filename>svg.html</filename>. A web browser should visualize
+ this output as:</para>
+
+ <informalfigure>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="Ref/Fig/svgGeometry.png"/>
+ </imageobject>
+ </mediaobject>
+ </informalfigure>
+ </question>
+
+ <answer>
+ <programlisting language="java">public class Rectangle {
+
+ private double x, y, width, height;
+ ...
+ /**
+ * @param x The rectangle's x center coordinate value
+ */
+ public void setX(double x) {
+ this.x = x;
+ }
+ /**
+ * @param y The rectangle's y center coordinate value
+ */
+ public void setY(double y) {
+ this.y = y;
+ }
+
+public void writeSvg() {
+ final int scale = 20;
+ System.out.println(
+ "<rect width='" + scale * width +"' height='" + scale * height +
+ "' x='" + scale * x + "'" + " y='" + scale * y + "'" +
+ "' style='fill:rgb(0,255,0);stroke-width:3;stroke:rgb(0,0,0)'/>");
+ }
+}</programlisting>
+
+ <programlisting language="java">public class Circle {
+
+ double x, y, radius;
+...
+
+ /**
+ * @param x The circle's x center coordinate value
+ */
+ public void setX(double x) {
+ this.x = x;
+ }
+ /**
+ * @param y The circle's y center coordinate value
+ */
+ public void setY(double y) {
+ this.y = y;
+ }
+
+public void writeSvg() {
+ final int scale = 20;
+
+ System.out.println(
+ "<circle r='" + scale * radius +
+ "' cx='" + scale * x + "'" + " cy='" + scale * y +
+ "' style='fill:rgb(0,0,255);stroke-width:3;stroke:rgb(0,0,0)'/>");
+
+ }
+}</programlisting>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_extendEmployee">
<title>Extending the employee example.</title>
--
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