<?xml version="1.0" encoding="UTF-8"?>
<chapter annotations="slide" version="5.1" xml:id="sw1ChapterInheritance"
xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xila="http://www.w3.org/2001/XInclude/local-attributes"
xmlns:xi="http://www.w3.org/2001/XInclude"
xmlns:svg="http://www.w3.org/2000/svg"
xmlns:ns="http://docbook.org/ns/transclusion"
xmlns:m="http://www.w3.org/1998/Math/MathML"
xmlns:html="http://www.w3.org/1999/xhtml"
xmlns:db="http://docbook.org/ns/docbook">
<title>Inheritance</title>
<figure xml:id="sd1_inherit_fig_guessWho">
<title>Guess who's inheriting the money</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/guessWho.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sd1_inherit_fig_biologicalInherit">
<title>Biology and inheritance</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/biologicalInheritance.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sda_inherit_fig_DuplicateCode">
<title>Duplicate code</title>
<informaltable border="0">
<tr>
<td valign="top"><programlisting language="none">public class Rectangle{
// Center coordinate
<emphasis role="red">private double x</emphasis>; <co
linkends="sda_inherit_fig_DuplicateCode-1"
xml:id="sda_inherit_fig_DuplicateCode-1-co"/>
<emphasis role="red">private double y</emphasis>;
<emphasis role="red">public void move
(double dx, double dy){</emphasis> <co
linkends="sda_inherit_fig_DuplicateCode-2"
xml:id="sda_inherit_fig_DuplicateCode-2-co"/>
<emphasis role="red">x += dx;
y += dy;
}</emphasis>
private double width, height; <co linkends="sda_inherit_fig_DuplicateCode-3"
xml:id="sda_inherit_fig_DuplicateCode-3-co"/> ...
}</programlisting></td>
<td valign="top"><programlisting language="none">public class Circle {
// Center coordinate
<emphasis role="red">private double x</emphasis>; <coref
linkend="sda_inherit_fig_DuplicateCode-1-co"/>
<emphasis role="red">private double y</emphasis>;
<emphasis role="red">public void move
(double dx, double dy){</emphasis> <coref
linkend="sda_inherit_fig_DuplicateCode-2-co"/>
<emphasis role="red">x += dx;
y += dy;
}</emphasis>
private double radius; <coref linkend="sda_inherit_fig_DuplicateCode-3-co"/> ...
}</programlisting></td>
</tr>
</informaltable>
<calloutlist role="slideExclude">
<callout arearefs="sda_inherit_fig_DuplicateCode-1-co"
xml:id="sda_inherit_fig_DuplicateCode-1">
<para>The center coordinate (<code language="java">x</code>|<code
language="java">y)</code> appears both in
<classname>Rectangle</classname> and
<classname>Circle</classname>.</para>
</callout>
<callout arearefs="sda_inherit_fig_DuplicateCode-2-co"
xml:id="sda_inherit_fig_DuplicateCode-2">
<para>The move(...) method is being defined <emphasis
role="bold">identically</emphasis> both in
<classname>Rectangle</classname> and
<classname>Circle</classname>!</para>
</callout>
<callout arearefs="sda_inherit_fig_DuplicateCode-3-co"
xml:id="sda_inherit_fig_DuplicateCode-3">
<itemizedlist>
<listitem>
<para><property>width</property> and <property>height</property>
only appear in class <classname>Rectangle</classname>.</para>
</listitem>
<listitem>
<para><property>radius</property> only appears in class
Circle.</para>
</listitem>
</itemizedlist>
</callout>
</calloutlist>
</figure>
<figure xml:id="sda_inherit_fig_ideaFactorOutCommonCode">
<title>Idea: Centralize common code</title>
<itemizedlist>
<listitem>
<para>Create a parent class <classname>Shape</classname> containing
common code portions.</para>
</listitem>
<listitem>
<para>Relate both <classname>Rectangle</classname> and
<classname>Circle</classname> to <classname>Shape</classname>.</para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sda_inherit_fig_shapeCommonSpecific">
<title>Common and specific properties</title>
<informaltable border="0">
<colgroup width="40%"/>
<colgroup width="20%"/>
<colgroup width="40%"/>
<tr>
<th align="center" colspan="3" valign="top">Common
<classname>Rectangle</classname> and <classname>Circle</classname>
attributes:</th>
</tr>
<tr>
<td valign="top"/>
<td valign="top"><programlisting language="java">double x;
double y;</programlisting></td>
<td valign="top"/>
</tr>
<tr>
<th align="center" valign="top"><classname>Rectangle</classname>
attributes</th>
<td/>
<th align="center" valign="top"><classname>Circle</classname>
attributes</th>
</tr>
<tr>
<td valign="top"><programlisting language="java">double width;
double height;</programlisting></td>
<td/>
<td valign="top"><programlisting language="java">double radius;</programlisting></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sda_inherit_fig_shapeInheritPrinciple">
<title>Basic shape inheritance</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/shapeBasicInherit.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sda_inherit_fig_intro">
<title>Inheritance</title>
<itemizedlist>
<listitem>
<para>Derived classes inherit state and behaviour.</para>
</listitem>
<listitem>
<para>Refinement, specialization.</para>
</listitem>
<listitem>
<para><quote>is-A</quote> relationship:</para>
<itemizedlist>
<listitem>
<para>A rectangle <emphasis role="red">is a</emphasis>
shape.</para>
</listitem>
<listitem>
<para>A circle <emphasis role="red">is a</emphasis> shape.</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sda_inherit_fig_shapeSimpleImplement">
<title>Implementing <classname>Shape</classname> hierarchy</title>
<informaltable border="0">
<colgroup width="25%"/>
<colgroup width="25%"/>
<colgroup width="25%"/>
<colgroup width="25%"/>
<tr>
<td valign="top"/>
<td colspan="2" valign="top"><programlisting language="java">public class Shape {
private double x, y;
}</programlisting></td>
<td valign="top"/>
</tr>
<tr>
<td colspan="2" valign="top"><programlisting language="java">public class Rectangle extends Shape {
private double width;
private double height;
}</programlisting></td>
<td colspan="2" valign="top"><programlisting language="java">public class Circle extends Shape {
private double radius;
}</programlisting></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sda_inherit_fig_createShapesInstances">
<title>Creating instances</title>
<programlisting language="java">final double x = 2, y = 3;
final Shape shape = new Shape(x, y);
final double width = 2, height = 5;
final Rectangle r = new Rectangle(x, y , width, height);
final double radius = 2.5;
final Circle circle = new Circle(x, y , radius);</programlisting>
</figure>
<figure xml:id="sda_inherit_fig_createShapesConstruct">
<title><classname>Shape</classname> constructor</title>
<programlisting language="java">/**
* Creating a shape located at center coordinate.
* @param x The center's x component.
* @param y The center's y component.
*/
public Shape(double x,double y) {
this.x = x;
this.y = y;
}</programlisting>
</figure>
<figure xml:id="sda_inherit_fig_createRectangle">
<title>Creating <classname>Rectangle</classname> instances</title>
<programlisting language="java">final Rectangle r =
new Rectangle(x, y <co linkends="sda_inherit_fig_createRectangle-1"
xml:id="sda_inherit_fig_createRectangle-1-co"/>,
width, height <co
linkends="sda_inherit_fig_createRectangle-2"
xml:id="sda_inherit_fig_createRectangle-2-co"/>);</programlisting>
<calloutlist>
<callout arearefs="sda_inherit_fig_createRectangle-1-co"
xml:id="sda_inherit_fig_createRectangle-1">
<para>Center coordinate components <quote>belonging</quote> to
superclass <classname>Shape</classname>.</para>
</callout>
<callout arearefs="sda_inherit_fig_createRectangle-2-co"
xml:id="sda_inherit_fig_createRectangle-2">
<para>width and height <quote>belonging</quote> to class
<classname>Rectangle</classname>.</para>
</callout>
</calloutlist>
<para>Solution: Nested constructor call. Coming soon ...</para>
</figure>
<figure xml:id="sda_inherit_fig_RectangleConstructor">
<title><classname>Rectangle</classname> constructor</title>
<programlisting language="java">/**
* Creating a rectangle at (x|y) of given width and height.
* @param x Center's x component.
* @param y Center's y component.
* @param width Rectangle's width.
* @param height Rectangle's height.
*/
public <link
xlink:href="https://gitlab.mi.hdm-stuttgart.de/goik/GoikLectures/blob/master/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Rectangle.java">Rectangle</link>(double x, double y,
double width, double height) {
super(x, y) <co linkends="sda_inherit_fig_RectangleConstructor-1"
xml:id="sda_inherit_fig_RectangleConstructor-1-co"/>;
this.width = width; this.height = height <co
linkends="sda_inherit_fig_RectangleConstructor-2"
xml:id="sda_inherit_fig_RectangleConstructor-2-co"/>;
}</programlisting>
<calloutlist role="slideExclude">
<callout arearefs="sda_inherit_fig_RectangleConstructor-1-co"
xml:id="sda_inherit_fig_RectangleConstructor-1">
<para>Passing center coordinate to superclass
<classname>Shape</classname>'s constructor.</para>
<note>
<para>This <emphasis>must</emphasis> be the first statement within
<classname>Rectangle</classname>'s constructor.</para>
</note>
</callout>
<callout arearefs="sda_inherit_fig_RectangleConstructor-2-co"
xml:id="sda_inherit_fig_RectangleConstructor-2">
<para>Processing <code language="java">width</code> and <code
language="java">height</code> in <quote>current</quote>
<classname>Rectangle</classname> constructor.</para>
</callout>
</calloutlist>
</figure>
<section xml:id="sd1_inherit_sect_overrideEquals">
<title>Overriding <methodname>equals()</methodname> and
<methodname>hashCode()</methodname></title>
<figure xml:id="sda_inherit_fig_ShapeEquals">
<title><classname>Shape</classname>.<methodname>equals()</methodname></title>
<programlisting language="java">public abstract class Shape {
...
@Override <co linkends="sda_inherit_fig_ShapeEquals-1"
xml:id="sda_inherit_fig_ShapeEquals-1-co"/> public boolean equals(final Object o) {
if (o instanceof Shape <emphasis role="red">s</emphasis> <co
linkends="sda_inherit_fig_ShapeEquals-2"
xml:id="sda_inherit_fig_ShapeEquals-2-co"/>) {
return x == <emphasis role="red">s</emphasis>.x && y == <emphasis
role="red">s</emphasis>.y; <co
linkends="sda_inherit_fig_ShapeEquals-4"
xml:id="sda_inherit_fig_ShapeEquals-4-co"/>
} else {
return false; <co linkends="sda_inherit_fig_ShapeEquals-5"
xml:id="sda_inherit_fig_ShapeEquals-5-co"/>
} ...</programlisting>
<calloutlist role="slideExclude">
<callout arearefs="sda_inherit_fig_ShapeEquals-1-co"
xml:id="sda_inherit_fig_ShapeEquals-1">
<para>Promise: The current method overrides a superclass
method.</para>
</callout>
<callout arearefs="sda_inherit_fig_ShapeEquals-2-co"
xml:id="sda_inherit_fig_ShapeEquals-2">
<para>Other instance is a <classname>Shape</classname>
object?</para>
</callout>
<callout arearefs="sda_inherit_fig_ShapeEquals-4-co"
xml:id="sda_inherit_fig_ShapeEquals-4">
<para>Return <code language="java">true</code> if and only if both
center coordinate pairs are equal.</para>
</callout>
<callout arearefs="sda_inherit_fig_ShapeEquals-5-co"
xml:id="sda_inherit_fig_ShapeEquals-5">
<para>Other object distinct from class
<classname>Shape</classname>.</para>
</callout>
</calloutlist>
</figure>
<figure xml:id="sda_inherit_fig_RectangleEquals">
<title><classname>Rectangle</classname>.<methodname>equals()</methodname></title>
<programlisting language="java">public class Rectangle extends Shape {
...
@Override public boolean equals(final Object o) {
if (o instanceof Rectangle <emphasis role="red">r</emphasis>) {
return super.equals(o) <co linkends="sda_inherit_fig_RectangleEquals-1"
xml:id="sda_inherit_fig_RectangleEquals-1-co"/> &&
width == <emphasis role="red">r</emphasis>.width && height == <emphasis
role="red">r</emphasis>.height <co
linkends="sda_inherit_fig_RectangleEquals-2"
xml:id="sda_inherit_fig_RectangleEquals-2-co"/>;
} else {
return false;
} ...</programlisting>
<calloutlist role="slideExclude">
<callout arearefs="sda_inherit_fig_RectangleEquals-1-co"
xml:id="sda_inherit_fig_RectangleEquals-1">
<para>Including superclass method
<classname>Shape</classname>.<methodname>equals()</methodname>.</para>
</callout>
<callout arearefs="sda_inherit_fig_RectangleEquals-2-co"
xml:id="sda_inherit_fig_RectangleEquals-2">
<para>Return <code language="java">true</code> if and only if both
<code language="java">width</code> and <code
language="java">height</code> pairs are equal.</para>
</callout>
</calloutlist>
</figure>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_stringEqualProblem">
<title>Let me pass, please!</title>
<qandadiv>
<qandaentry>
<question>
<para>Consider the following snippet:</para>
<programlisting language="java">final Scanner scan = new Scanner(System.in));
do {
System.out.print("Please enter password: ");
final String password = scan.nextLine();
if (password == "secret") {
break; // Leave enclosing do ... while loop
} else {
System.out.println("Sorry, please try again");
}
} while (true);
System.out.println("You made it!");
// ...</programlisting>
<para>Describe the above code's intended behaviour. Will it
succeed? Execute the above code and provide a clue for correcting
the underlying flaw.</para>
</question>
<answer>
<para>The user is being asked for a password. Only when entering
<code>"secret"</code> access shall be granted.</para>
<para>Unfortunately comparing the user's input and the
corresponding password is seriously flawed:</para>
<programlisting language="java">...
if (password == "secret")
...</programlisting>
<para>On execution the string literal "secret" will be represented
as a <classname xlink:href="javaapi://String">String</classname>
object in memory. Each new user input string will be represented
as a <emphasis>different</emphasis> <classname
xlink:href="javaapi://String">String</classname> object in memory
as well.</para>
<para>Unfortunately the <quote>==</quote> operator only works as
expected for the eight built in primitive <xref
linkend="glo_Java"/> types. With respect to class instances
variables hold references to objects rather than to the objects'
values. The <quote>==</quote> operator compares for object
identity rather than for object equality.</para>
<para>Two different <classname
xlink:href="javaapi://String">String</classname> instances may off
course be equal with respect to their <quote>payload</quote>
namely the values they both represent. Comparing for object
equality rather than for object identity in <xref
linkend="glo_Java"/> requires using the
<methodname>equals(...)</methodname> method instead. A
prerequisite for this to work requires the class authors
overriding the <methodname
xlink:href="javaapi://Object#equals(java.lang.Object)">Object.equals(Object
o)</methodname> method accordingly. This override does exist in
class <classname xlink:href="javaapi://String">String</classname>.
Within the given context we may thus simply use <methodname
xlink:href="javaapi://String#equals(java.lang.Object)">String.equals(Object
o)</methodname>:</para>
<programlisting language="none">final Scanner scan = new Scanner(System.in));
do {
System.out.print("Please enter password: ");
final String password = scan.nextLine();
<emphasis role="red">if (password.equals("secret"))</emphasis> {
break; // Leave enclosing do ... while loop
} else {
System.out.println("Sorry, please try again");
}
} while (true);
System.out.println("You made it!");
// ...</programlisting>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_enomNoEqualsRequired">
<title>Why is == correctly comparing <code language="java">enum</code>
instances?</title>
<qandadiv>
<qandaentry>
<question>
<para>The preceding exercise <xref
linkend="sd1_qanda_stringEqualProblem"/> told us to override
<methodname
xlink:href="javaapi://Object#equals(java.lang.Object)">Object.equals(Object
o)</methodname> when comparing different objects for equality. The
== operator on the other hand does compare for object identity
rather than semantic equality of objects based on their respective
attribute values. However comparing <code
language="java">Enum</code> instances for equality using the <code
language="java">==</code> operator seems to be sufficient<abbrev>
e.g.</abbrev>:</para>
<programlisting language="java">Day day = Day.Saturday;
// Sufficient using operator == ?
if (day == Day.FRIDAY) ...
// Or do we require equals?
if (day.equals(Day.FRIDAY)) ...</programlisting>
<para>Do we thus require an <methodname>equals(Object
o)</methodname> method in <code language="java">enum</code>
<classname>Day</classname> for comparisons? Give a precise
explanation.</para>
</question>
<answer>
<para>In case of <code language="java">enum</code> instances using
the <quote>==</quote> operator is sufficient. Due to the way <code
language="java">enum</code>'s are being constructed instance
creation is limited to the underlying <code
language="java">enum</code>'s scope by its implicitly <code
language="java">private</code> constructor. It is thus impossible
creating new instances outside the respective <code
language="java">enum</code>'s scope.</para>
<para>Therefore for any two given <code
language="java">enum</code> instances both object identity and
equality of their respective values is being guaranteed to yield
the very same result:</para>
<programlisting language="none">MyEnum instance1 = MyEnum.x, // MyEnum representing
instance2 = MyEnum.y; // some enum type.
final boolean
<emphasis role="red">objectIdentity</emphasis> = (instance1 == instance2), // Both boolean values will
<emphasis role="red">objectEquality</emphasis> = instance1.equals(instance2); // always be equal.</programlisting>
<para>As an aside: In case of <code language="java">null</code>
values using the <quote>==</quote> operator avoids
<classname>java.lang.NullPointerException</classname>
problems:</para>
<programlisting language="java">Day day = null;
if (day == Day.FRIDAY) ... // Just different, no problems
if (day.equals(Day.FRIDAY)) ... // Oops: NPE approaching ...</programlisting>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1_inherit_sect_overrideToString">
<title>Overriding toString()</title>
<figure xml:id="sda_inherit_fig_shapeInstanceLog">
<title>Printing a <classname>Shape</classname>'s info</title>
<informaltable border="0">
<tr>
<th>Code</th>
<th>Output</th>
</tr>
<tr>
<td valign="top"><programlisting language="none">package inherit;
public class Run {
public static void main(String[] args) {
final Shape shape =
new Shape(<emphasis role="red">2.0</emphasis>, <emphasis role="red">3.0</emphasis>); // Center coordinates
System.out.println(shape); </programlisting></td>
<td valign="top"><screen>inherit.Shape@37d31475</screen><para>Desired:</para><screen><emphasis
role="red">(2.0|3.0)</emphasis></screen></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sda_inherit_fig_shapeToStringRedefine">
<title>Overwriting <methodname
xlink:href="javaapi://Object#toString--">toString()</methodname></title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/shapeOverrideToStringIde.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sda_inherit_fig_shapeExtendObject">
<title><classname>Shape</classname> extending
<classname>Object</classname></title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/shapeRedefineToString.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sda_inherit_fig_RectangleLogging">
<title>Logging <classname>Rectangle</classname> instances</title>
<informaltable border="0">
<tr>
<th>Code</th>
<th>Output</th>
</tr>
<tr>
<td valign="top"><programlisting language="java">final Rectangle r = // Center coordinates,
new Rectangle(2.0, 3.0, 3.0, 4.0); // width and height
System.out.println(r);</programlisting></td>
<td valign="top"><screen>(2.0|3.0)</screen><para>Desired:</para><screen><emphasis
role="red">Rectangle at </emphasis>(2.0|3.0)<emphasis
role="red">, width= 3.0, height=4.0</emphasis></screen></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sda_inherit_fig_RectangleOverrideToStringAgain">
<title>Override <methodname>toString()</methodname> in class
<classname>Rectangle</classname>.</title>
<programlisting language="java">public class Rectangle extends Shape {
@Override public String toString() {
return "<emphasis role="red">Rectangle at </emphasis>" + super.toString() <co
linkends="sda_inherit_fig_RectangleOverrideToStringAgain-1"
xml:id="sda_inherit_fig_RectangleOverrideToStringAgain-1-co"/> +
"<emphasis role="red">, width= </emphasis>" + <emphasis role="red">width</emphasis> + "<emphasis
role="red">, height=</emphasis>" + <emphasis role="red">height</emphasis>; <co
linkends="sda_inherit_fig_RectangleOverrideToStringAgain-2"
xml:id="sda_inherit_fig_RectangleOverrideToStringAgain-2-co"/>
} ...</programlisting>
<calloutlist>
<callout arearefs="sda_inherit_fig_RectangleOverrideToStringAgain-1-co"
xml:id="sda_inherit_fig_RectangleOverrideToStringAgain-1">
<para>The <code language="java"
xlink:href="https://docs.oracle.com/javase/tutorial/java/IandI/super.html">super</code>
keyword allows for calling the superclass
<classname>Shape</classname>'s <methodname>toString()</methodname>
method inserting the (2.0|3.0) center coordinate.</para>
</callout>
<callout arearefs="sda_inherit_fig_RectangleOverrideToStringAgain-2-co"
xml:id="sda_inherit_fig_RectangleOverrideToStringAgain-2">
<para>Append class <classname>Rectangle</classname>'s
<quote>own</quote> <code language="java">width</code> and <code
language="java">height</code> attribute values.</para>
</callout>
</calloutlist>
</figure>
<figure xml:id="sda_inherit_fig_rectangleExtendShapa">
<title><classname>Rectangle</classname> extending
<classname>Shape</classname></title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/rectangleRedefineToString.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sda_inherit_fig_CircleComplete">
<title>Implementing
<classname>Circle</classname>.<methodname>toString()</methodname></title>
<programlisting language="java">public class Circle extends Shape {
/**
* Creating a circle of given center and radius
* @param x Center's x component.
* @param y Center's y component.
* @param radius The circle's radius.
*/
public Circle(double x,double y, double radius) {
super(x, y);
this.radius = radius;
}
@Override public String toString() {
return "Circle at " + super.toString() +", radius= " + radius;
}
private double radius;
}</programlisting>
</figure>
<figure xml:id="sda_inherit_fig_shapeToString">
<title><classname>Shape</classname> and
<methodname>toString()</methodname></title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/shapeToString.svg"/>
</imageobject>
</mediaobject>
</figure>
<qandaset defaultlabel="qanda"
xml:id="sd1_quanda_inherit_StringBufferNoEqualMethod">
<title><classname>String</classname> vs.
<classname>StringBuffer</classname></title>
<qandadiv>
<qandaentry>
<question>
<para>Consider two <classname
xlink:href="javaapi://StringBuffer">StringBuffer</classname>
instances:</para>
<informaltable border="1">
<colgroup width="54%"/>
<colgroup width="46%"/>
<tr>
<th>Code</th>
<th>Execution result</th>
</tr>
<tr>
<td valign="top"><programlisting language="java">final <link
xlink:href="javaapi://StringBuffer">StringBuffer</link>
a = new StringBuffer("test"),
b = new StringBuffer("test");
System.out.println(a.equals(b));</programlisting></td>
<td valign="top"><screen>false</screen></td>
</tr>
</informaltable>
<para>Strangely instances of String behave differently:</para>
<informaltable border="1">
<colgroup width="54%"/>
<colgroup width="46%"/>
<tr>
<th>Code</th>
<th>Execution result</th>
</tr>
<tr>
<td valign="top"><programlisting language="java">final String
a = new String("test"),
b = new String("test");
System.out.println(a.equals(b));</programlisting></td>
<td valign="top"><screen>true</screen></td>
</tr>
</informaltable>
<para>Explain this different behaviour.</para>
<tip>
<para>Take a closer look at the <classname
xlink:href="javaapi://String">String</classname> and <classname
xlink:href="javaapi://StringBuffer">StringBuffer</classname>
<xref linkend="glo_API"/> regarding the
<methodname>toString()</methodname> method.</para>
</tip>
</question>
<answer>
<para>The <classname
xlink:href="javaapi://String">String</classname> <xref
linkend="glo_API"/> reveals:</para>
<screen>public boolean equals (Object anObject)
Compares this string to the specified object. The result is true if and only if the argument is
not null and is a String object that represents the same sequence of characters as this object.
Overrides: equals in class Object
Parameters:
anObject - The object to compare this String against
Returns:
true if the given object represents a String equivalent to this string, false otherwise</screen>
<para>In a nutshell: Two instances of String will be compared for
equality character by character. The superclass <classname
xlink:href="javaapi://Object">Object</classname>.<methodname
xlink:href="javaapi://Object#equals(java.lang.Object)">equals()</methodname>
method is being overridden.</para>
<para>In class <classname
xlink:href="javaapi://StringBuffer">StringBuffer</classname> we do
not find any <methodname>equals()</methodname> method. Thus
<classname
xlink:href="javaapi://Object">Object</classname>.<methodname
xlink:href="javaapi://Object#equals(java.lang.Object)">equals()</methodname>
is not being overridden. When comparing two instances of
<classname
xlink:href="javaapi://StringBuffer">StringBuffer</classname>
effectively <classname
xlink:href="javaapi://Object">Object</classname>.<methodname
xlink:href="javaapi://Object#equals(java.lang.Object)">equals()</methodname>
will be executed. Its <xref linkend="glo_API"/> reveals:</para>
<screen>public boolean equals (Object obj)
Indicates whether some other object is "equal to" this one.
...
Parameters:
obj - the reference object with which to compare.
Returns:
true <emphasis role="red">if this object is the same as the obj argument</emphasis>; false otherwise.</screen>
<para>Thus two instances of <classname
xlink:href="javaapi://StringBuffer">StringBuffer</classname> will
be compared for object identity rather than representing the same
string value.</para>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<qandaset defaultlabel="qanda"
xml:id="sd1_quanda_inheritDirectionAlternateImpl">
<title>Alternate implementation of opposite directions</title>
<qandadiv>
<qandaentry>
<question>
<para>Provide a different implementation of your <methodname
xlink:href="https://gitlab.mi.hdm-stuttgart.de/goik/GoikLectures/blob/master/Doc/Sd1/Ref/ObjectsAndClasses/Enum/Compass_2.0/src/main/java/de/hdm_stuttgart/mi/sd1/direction/Direction.java#L28">opposite()</methodname>
from <xref linkend="sd1_qanda_enumDirectionNeighbours"/>. Follow
<uri
xlink:href="https://stackoverflow.com/questions/5678309/illegal-forward-reference-and-enums#answer-49409377">https://stackoverflow.com/questions/5678309/illegal-forward-reference-and-enums#answer-49409377</uri>.
Start by investigating both <methodname
xlink:href="http://www.java2s.com/Tutorial/Java/0040__Data-Type/Usingthebuiltinenumerationmethodsvalues.htm">values()</methodname>
and <methodname
xlink:href="javaapi://Enum#ordinal()">ordinal()</methodname>.</para>
</question>
<answer>
<para>We observe the following ordinal values:</para>
<informaltable border="0">
<tr>
<td valign="top"><programlisting language="java">public enum Direction {
N( 0, "north"), // 0
NE( 45, "north by east"), // 1
E( 90, "east"), // 2
SE(135, "south by east"), // 3
S( 180, "south"), // 4
SW(225, "south by west"), // 5
W( 270, "west"), // 6
NW(315, "north by west"); // 7
...</programlisting></td>
<td valign="top"><programlisting language="none">public Direction opposite() {
switch (this) {
case N: return S; // <emphasis role="red">0 --> 4</emphasis>
case NE: return SW; // <emphasis role="red">1 --> 5</emphasis>
case E: return W; // <emphasis role="red">2 --> 6</emphasis>
case SE: return NW; // <emphasis role="red">3 --> 7</emphasis>
case S: return N; // <emphasis role="red">4 --> 0</emphasis>
case SW: return NE; // <emphasis role="red">5 --> 1</emphasis>
case W: return E; // <emphasis role="red">6 --> 2</emphasis>
case NW: return SE; // <emphasis role="red">7 --> 3</emphasis>
...</programlisting></td>
</tr>
</informaltable>
<para>We are thus left implementing an integer shift <code>(0, 1,
2, 3, 4, 5, 6, 7)</code> to <code>(4, 5, 6, 7, 0, 1, 2,
3)</code>:</para>
<programlisting language="java">public Direction <link
xlink:href="https://gitlab.mi.hdm-stuttgart.de/goik/GoikLectures/blob/master/Doc/Sd1/Ref/ObjectsAndClasses/Enum/Compass_3.0/src/main/java/de/hdm_stuttgart/mi/sd1/direction/Direction.java#L28">opposite()</link> {
return values()[ (ordinal() + 4) % 8];
}</programlisting>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1_inherit_sect_final">
<title><code language="java">final</code> methods</title>
<figure xml:id="sda_inherit_fig_moveRectangle">
<title>Moving <classname>Shape</classname> instances</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/rectangleTranslate.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sda_inherit_fig_shapeMove">
<title>Implementing <classname>Shape</classname> movements</title>
<programlisting language="java">public class Shape {
/**
* Move by a given translation vector
* @param xTrans Translation's x component
* @param yTrans Translation's y component
*/
public void move(final int xTrans, final int yTrans) {
x += xTrans;
y += yTrans;
} ...</programlisting>
</figure>
<figure xml:id="sda_inherit_fig_shapeMoveProblemSubclass">
<title>Fools are everywhere!</title>
<programlisting language="java">public class Rectangle extends Shape {
@Override public void move(int xTrans, int yTrans) {
// I'm so dumb!
...
}</programlisting>
</figure>
<figure xml:id="sda_inherit_fig_shapeMoveProblemSubclassSolution">
<title>Solution: <emphasis role="red">final</emphasis> prevents
overriding</title>
<programlisting language="none">public abstract class Shape {
... public <emphasis role="red">final</emphasis> void move(final int xTrans, final int yTrans) {
x += xTrans;
y += yTrans;
}...</programlisting>
<programlisting language="java">public class Rectangle extends Shape {
// Syntax error: 'move(int, int)' cannot override
// 'move(int, int)' in 'inherit.Shape'; overridden method is final
@Override public void move(int xTrans, int yTrans) {...</programlisting>
</figure>
</section>
<section xml:id="sd1_inherit_sect_abstractMethods">
<title>Abstract methods</title>
<figure xml:id="sda_inherit_fig_subclassImplementGetArea">
<title>Calculating a shape's area</title>
<informaltable border="0">
<tr>
<td valign="top"><programlisting language="java">public class Rectangle extents Shape {
/**
* Calculate the area.
* @return The rectangle's area
*/
public double getArea() {
return width * height;
} ...</programlisting></td>
<td valign="top"><programlisting language="java">public class Circle extents Shape {
/**
* Calculate the area.
* @return The circle's area
*/
public double getArea() {
return PI * radius * radius;
} ...</programlisting></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sda_inherit_fig_shapePolymorphicAreaCall">
<title>Desired: Polymorphic <methodname>getArea()</methodname>
call</title>
<programlisting language="none">final Shape[] shapes <co
linkends="sda_inherit_fig_shapePolymorphicAreaCall-1"
xml:id="sda_inherit_fig_shapePolymorphicAreaCall-1-co"/> = {
new Circle(1, 1, 2.) <co
linkends="sda_inherit_fig_shapePolymorphicAreaCall-2"
xml:id="sda_inherit_fig_shapePolymorphicAreaCall-2-co"/>,
new Rectangle(1, -1, 2., 3.)<coref
linkend="sda_inherit_fig_shapePolymorphicAreaCall-2-co"/>};
for (final Shape s : shapes) {
System.out.println(s.toString() + ": <emphasis role="red">area = " + s.getArea()</emphasis>); <co
linkends="sda_inherit_fig_shapePolymorphicAreaCall-3"
xml:id="sda_inherit_fig_shapePolymorphicAreaCall-3-co"/>
}</programlisting>
<screen>Circle at (1.0|1.0), radius= 2.0: <emphasis role="red">area = 12.566370614359172</emphasis>
Rectangle at (1.0|-1.0), width= 2.0, height=3.0: <emphasis role="red">area = 6.0</emphasis></screen>
<calloutlist role="slideExclude">
<callout arearefs="sda_inherit_fig_shapePolymorphicAreaCall-1-co"
xml:id="sda_inherit_fig_shapePolymorphicAreaCall-1">
<para>An array of <classname>Shape</classname> references.</para>
</callout>
<callout arearefs="sda_inherit_fig_shapePolymorphicAreaCall-2-co"
xml:id="sda_inherit_fig_shapePolymorphicAreaCall-2">
<para>A <classname>Rectangle</classname> <quote>is a</quote>
<classname>Shape</classname> and likewise is
<classname>Circle</classname>. We can thus assign both
<classname>Circle</classname> and <classname>Rectangle</classname>
instances to variables (or array elements) of type
<classname>Shape</classname> by means of upcasting.</para>
</callout>
<callout arearefs="sda_inherit_fig_shapePolymorphicAreaCall-3-co"
xml:id="sda_inherit_fig_shapePolymorphicAreaCall-3">
<para>Polymorphic dispatch: Depending on the object's type the <xref
linkend="glo_Java"/> runtime will automatically choose either
<classname>Rectangle</classname>.<methodname>toString()</methodname>/
<classname>Rectangle</classname>.<methodname>getArea()</methodname>
or <classname>Circle</classname>.<methodname>getArea()</methodname>
/ <classname>Circle</classname>.<methodname>toString()</methodname>/
<classname>Circle</classname>.<methodname>getArea()</methodname>
respectively.</para>
</callout>
</calloutlist>
</figure>
<figure xml:id="sda_inherit_fig_getAreaPolymorphicProblem">
<title>Problems:</title>
<itemizedlist>
<listitem>
<para>No meaningful <methodname>getArea()</methodname> method in
class <classname>Shape</classname> possible.</para>
</listitem>
<listitem>
<para>Meaningful implementations exist both in subclass<classname>
Rectangle</classname> and <classname>Circle</classname>.</para>
</listitem>
</itemizedlist>
<para>Solution: Abstract method <methodname>getArea()</methodname> in
superclass <classname>Shape</classname>.</para>
</figure>
<figure xml:id="sda_inherit_fig_implementAbstractGetArea">
<title><code language="java">abstract</code> method
<methodname>getArea()</methodname></title>
<informaltable border="0">
<colgroup width="25%"/>
<colgroup width="25%"/>
<colgroup width="25%"/>
<colgroup width="25%"/>
<tr>
<td valign="top"/>
<td colspan="2" valign="top"><programlisting language="java">abstract<co
linkends="sda_inherit_fig_implementAbstractGetArea-1"
xml:id="sda_inherit_fig_implementAbstractGetArea-1-co"/> public class Shape {
/**
* Calculate the shape's area.
* @return The shape's area
*/
abstract<co linkends="sda_inherit_fig_implementAbstractGetArea-2"
xml:id="sda_inherit_fig_implementAbstractGetArea-2-co"/> public double getArea()<co
linkends="sda_inherit_fig_implementAbstractGetArea-3"
xml:id="sda_inherit_fig_implementAbstractGetArea-3-co"/>; ...</programlisting></td>
<td valign="top"/>
</tr>
<tr>
<td colspan="2" valign="top"><programlisting language="java">public class Rectangle extends Shape {
@Override<co linkends="sda_inherit_fig_implementAbstractGetArea-4"
xml:id="sda_inherit_fig_implementAbstractGetArea-4-co"/>
public double getArea() {
return width * height;
}...</programlisting></td>
<td colspan="2" valign="top"><programlisting language="java">public class Circle ... {
@Override<coref linkend="sda_inherit_fig_implementAbstractGetArea-4-co"/>
public double getArea() {
return Math.PI *
radius * radius;
} ...</programlisting></td>
</tr>
</informaltable>
<calloutlist role="slideExclude">
<callout arearefs="sda_inherit_fig_implementAbstractGetArea-1-co"
xml:id="sda_inherit_fig_implementAbstractGetArea-1">
<para>Superclass <classname>Shape</classname> contains an <code
language="java">abstract</code> method and must thus itself be
declared <code language="java">abstract</code> as well.</para>
<note>
<para>You cannot create instances of <code
language="java">abstract</code> classes. You may however create
instances of derived non-<code language="java">abstract</code>
classes.</para>
</note>
</callout>
<callout arearefs="sda_inherit_fig_implementAbstractGetArea-2-co"
xml:id="sda_inherit_fig_implementAbstractGetArea-2">
<para>Method <classname>getArea()</classname> cannot be implemented
in a meaningful way. Its <code language="java">abstract</code>
modifier is a promise that some concrete (= non-<code
language="java">abstract</code>) subclass will either offer an
implementation of <classname>getArea()</classname> or will have an
intermediate parent class doing so.</para>
<para>In other words: The <code language="java">abstract</code>
keyword requires a corresponding implementation in some derived
non-<code language="java">abstract</code> subclass.</para>
</callout>
<callout arearefs="sda_inherit_fig_implementAbstractGetArea-3-co"
xml:id="sda_inherit_fig_implementAbstractGetArea-3">
<para>An <code language="java">abstract</code> method must not have
an implementing body <code language="java">{...}</code>.</para>
</callout>
<callout arearefs="sda_inherit_fig_implementAbstractGetArea-4-co"
xml:id="sda_inherit_fig_implementAbstractGetArea-4">
<para>Both <classname>Rectangle</classname> and
<classname>Circle</classname> are concrete (=non-<code
language="java">abstract</code>) classes and are thus obliged to
provide an implementation of <methodname>double
getArea()</methodname>.</para>
</callout>
</calloutlist>
</figure>
<figure xml:id="sda_inherit_fig_shapeGetAreaAbstract">
<title><code language="java">abstract</code> method
<methodname>getArea()</methodname></title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/shapeGetAreaAbstract.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sda_inherit_fig_whatIsShapeAnyway">
<title>What's a <quote>shape</quote> anyway?</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/ghost.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sd1_inherit_fig_noAbstractInstances">
<title>No instances of <code language="java">abstract</code>
classes.</title>
<programlisting language="none">final Shape s =
new Shape(1., 2.); // <emphasis role="red">'Shape' is abstract; cannot be instantiated</emphasis></programlisting>
</figure>
<figure xml:id="sd1_inherit_fig_getAreaCompulsory">
<title>Mandatory <methodname>getArea()</methodname>
implementation.</title>
<programlisting language="none">// Error: <emphasis role="red">Class 'Circle' must either be declared abstract or</emphasis>
// <emphasis role="red">implement abstract method 'getArea()' in 'Shape'</emphasis>
public class Circle extends Shape {
public Circle(double x,double y, double radius) {
super(x, y);
this.radius = radius;
}
private double radius;
}</programlisting>
</figure>
<figure xml:id="sd1_inherit_fig_abstractFacts">
<title>Facts about <code language="java">abstract</code> fields, methods
and classes.</title>
<itemizedlist>
<listitem>
<para>A class containing an <code language="java">abstract</code>
method must itself be declared <code
language="java">abstract</code>.</para>
</listitem>
<listitem>
<para><code language="java">abstract</code> classes are allowed to
host non-<code language="java">abstract</code> methods.</para>
</listitem>
<listitem>
<para>A class may be declared <code language="java">abstract</code>
irrespective of purely containing non-<code
language="java">abstract</code> methods.</para>
</listitem>
</itemizedlist>
</figure>
<section xml:id="sw1SectGeometryInherit">
<title>Geometry classes reconsidered</title>
<para>Starting in <xref linkend="sd1_qanda_geometry_Rectangle"/> you
implemented rectangle and circle related classes. You may have observed
translation related parameters to be shape independent.</para>
<para>Our <classname>Rectangle</classname> and
<classname>Circle</classname> instances are being described by <code
language="java">width</code>, <code language="java">height</code> and
<code>radius</code>. Implementing a drawing application requires
<emphasis>arbitrary</emphasis> objects to be moved from one position to
another e.g.:</para>
<figure xml:id="sd1FigFigureMove">
<title>Moving shapes</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/figureMove.svg"/>
</imageobject>
</mediaobject>
</figure>
<para>We might implement translation (move) related parameters and
methods in both classes Rectangle and Circle independently. But since
the underlying operation is indeed shape independent we choose a
different approach using inheritance in the subsequent exercises.</para>
<qandaset defaultlabel="qanda" xml:id="sd1QandaFigureBaseClass">
<title>Defining a <classname>Shape</classname> class hierarchy</title>
<qandadiv>
<qandaentry>
<question>
<para>Our two geometric primitives circle and rectangle will
need a reference point (x,y) to define
<methodname>move(...)</methodname> operations. We choose the
center of gravity both for rectangles and circles:</para>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/figureCenter.svg"/>
</imageobject>
</mediaobject>
<para>We thus need two additional parameters
<property>x</property> and <property>y</property> representing
an object's position. Consider the following inheritance diagram
and implement the three <xref linkend="glo_Java"/> classes
<classname>Shape</classname>, <classname>Circle</classname> and
<classname>Rectangle</classname>:</para>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/qandaShapeBasic.svg"/>
</imageobject>
</mediaobject>
<para>Create unit tests checking for correct implementation of
the translation example from <xref linkend="sd1FigFigureMove"/>
and the all other methods.</para>
<para>Why is <methodname>move(...)</methodname> being defined
returning an instance of class
<classname>Shape</classname>?</para>
<tip>
<para>Think of multiple / combined operations.</para>
</tip>
</question>
<answer>
<annotation role="make">
<para role="eclipse">P/Sd1/Figure/BaseClass</para>
</annotation>
<para>Defining <methodname>move(...)</methodname> returning an
instance of class <classname>Shape</classname> allows for
operation chaining:</para>
<programlisting language="java">final Circle c = new Circle(1., 2., 3.0);
double p = c.move(1, 5).move(-3, 7).getPrimeter();</programlisting>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<section xml:id="sd1SectFigureScale">
<title>Scaling shapes</title>
<qandaset defaultlabel="qanda" xml:id="sd1QandaFigureScaling">
<qandadiv>
<qandaentry>
<question>
<para>We want shapes to become scalable:</para>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/scale.svg"/>
</imageobject>
</mediaobject>
<para>In the above example the rectangle is being shrunk to
50% of its original size whereas the circle's radius is being
doubled. Add a corresponding
<methodname>scale(...)</methodname> method to the inheritance
hierarchy which allows for operation chaining as well.</para>
<para>Provide appropriate unit tests.</para>
</question>
<answer>
<annotation role="make">
<para role="eclipse">P/Sd1/Figure/Scale</para>
</annotation>
<para>This task is pretty much straightforward. Since scaling
requires specific details (like radius or width and height) a
<methodname>scale()</methodname> method cannot be implemented
on top level of our inheritance hierarchy. We thus start by
defining an abstract method in our class
<classname>Shape</classname>:</para>
<programlisting language="java">/**
*
* @param factor Scale the current shape by this value.
* @return The current object.
*/
public abstract Shape scale(double factor);</programlisting>
<para>This method has to be implemented in our two concrete
classes <classname>Circle</classname> and
<classname>Rectangle</classname>.</para>
<para>Sensible unit tests may be based on the observation
that:</para>
<itemizedlist>
<listitem>
<para>A shape's perimeter grows linear with the scaling
factor.</para>
</listitem>
<listitem>
<para>A shape's area grows linear with the scaling
factor's square.</para>
</listitem>
</itemizedlist>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1SectFigureToString">
<title>Providing <methodname
xlink:href="javaapi://Object#toString--">toString()</methodname>
methods</title>
<qandaset defaultlabel="qanda" xml:id="sd1QandaFigureToString">
<qandadiv>
<qandaentry>
<question>
<para>Consider:</para>
<programlisting language="java">final Circle c = new Circle(-2, -1, 3.5);
final Rectangle r = new Rectangle(3, 1, 1.5, 4.4);
System.out.println(c);
System.out.println(r);</programlisting>
<para>This creates the following output:</para>
<screen>de.hdm_stuttgart.mi.sd1.shape.model.Circle@659e0bfd
de.hdm_stuttgart.mi.sd1.shape.model.Rectangle@2a139a55</screen>
<para>This result is due to the invocation of the <methodname
xlink:href="javaapi://Object#toString--">toString()</methodname>
method being defined in the <classname
xlink:href="javaapi://Object">Object</classname> superclass.
Override this method in <classname>Shape</classname>,
<classname>Circle</classname> and
<classname>Rectangle</classname> accordingly to get:</para>
<screen>Circle (-2.0,-1.0), radius=3.5
Rectangle (3.0,1.0), width=1.5, height=4.4</screen>
<para>Provide appropriate unit tests.</para>
<tip>
<para>You may access
<methodname>Shape.toString()</methodname> from derived
classes by using <code
language="java">super()</code>.</para>
</tip>
</question>
<answer>
<annotation role="make">
<para role="eclipse">P/Sd1/Figure/ToString</para>
</annotation>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
</section>
</section>
<section xml:id="sd1_inherit_sect_protected">
<title><code language="java">protected</code> access</title>
<figure xml:id="sda_inherit_fig_protectedCreationTime">
<title><code language="java">protected</code> access</title>
<programlisting language="none"><emphasis role="red">package model</emphasis>;
public abstract class Shape <co
linkends="sda_inherit_fig_protectedCreationTime-1"
xml:id="sda_inherit_fig_protectedCreationTime-1-co"/>{
final protected long <emphasis role="red">creationTime</emphasis> <co
linkends="sda_inherit_fig_protectedCreationTime-2"
xml:id="sda_inherit_fig_protectedCreationTime-2-co"/>= System.nanoTime();
...
}
------------------------------------------------
<emphasis role="red">package model.sub</emphasis>;
public class Rectangle <co linkends="sda_inherit_fig_protectedCreationTime-3"
xml:id="sda_inherit_fig_protectedCreationTime-3-co"/>extends Shape {
static final Logger log = LogManager.getLogger(Rectangle.class);
@Override public double getArea() {
log.info("Rectangle creation time:" + <emphasis role="red">creationTime</emphasis> <co
linkends="sda_inherit_fig_protectedCreationTime-4"
xml:id="sda_inherit_fig_protectedCreationTime-4-co"/>);
return width * height;
} ...
}</programlisting>
<calloutlist role="slideExclude">
<callout arearefs="sda_inherit_fig_protectedCreationTime-1-co"
xml:id="sda_inherit_fig_protectedCreationTime-1">
<para>Defining superclass <classname>Shape</classname> in package
<code language="java">model</code>.</para>
</callout>
<callout arearefs="sda_inherit_fig_protectedCreationTime-2-co"
xml:id="sda_inherit_fig_protectedCreationTime-2">
<para>Defining a <code language="java">protected</code> instance
attribute <property>creationTime</property> in superclass
<classname>Shape</classname>.</para>
</callout>
<callout arearefs="sda_inherit_fig_protectedCreationTime-3-co"
xml:id="sda_inherit_fig_protectedCreationTime-3">
<para>Deriving class <classname>Rectangle</classname> in different
package <code language="java">model.sub</code> from superclass <code
language="java">Shape</code>.</para>
</callout>
<callout arearefs="sda_inherit_fig_protectedCreationTime-4-co"
xml:id="sda_inherit_fig_protectedCreationTime-4">
<para>Accessing superclass attribute
<property>creationTime</property> across package boundary.</para>
</callout>
</calloutlist>
</figure>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_protectedPackagePrivate">
<title><code language="java">protected</code> vs. <quote>package
private</quote></title>
<qandadiv>
<qandaentry>
<question>
<para>Implement <xref
linkend="sda_inherit_fig_protectedCreationTime"/> in your IDE but
exchanging <classname>Rectangle</classname> by
<classname>Circle</classname> accordingly. Then execute
<classname>Circle</classname>.<methodname>getArea()</methodname>
and watch the logging outcome.</para>
<para>Now apply the following two changes:</para>
<orderedlist>
<listitem>
<para>Remove <code language="java">creationTime</code>'s <code
language="java">protected</code> modifier.</para>
</listitem>
<listitem>
<para>Move class <classname>Circle</classname> to package
<package>model</package>.</para>
</listitem>
</orderedlist>
<para>Are you still able to run your code? Explain the
result.</para>
</question>
<answer>
<para>Implementing class <classname>Circle</classname> is
straightforward:</para>
<programlisting language="none">public class Circle extends Shape {
static final Logger log = LogManager.getLogger(Circle.class);
@Override public double getArea() {
log.info("Circle creation time:" + creationTime );
return Math.PI * radius * radius;
}
private double radius;
}</programlisting>
<para>Executing <code language="java">new
Circle().getArea()</code> results in:</para>
<screen>2018-01-03 08:31:18,811 INFO [main] sup.Circle (Circle.java:11) - Circle creation time:3340216708709</screen>
<para>Removing <code language="java">protected</code> yields a
compile time error:</para>
<programlisting language="none">public class Circle extends Shape {
static final Logger log = LogManager.getLogger(Circle.class);
@Override public double getArea() {
<emphasis role="red">// Error: 'creationTime' is not public in 'testprotect.model.Shape'.
// Cannot be accessed from outside package</emphasis>
log.info("Circle creation time:" + creationTime );
...</programlisting>
<para>Moving class <classname>Circle</classname> to package
<package>model</package> resolves the issue: Having both classes
<classname>Shape</classname> and <classname>Circle</classname> in
a common package <package>model</package> implies <quote>package
private</quote> access. Thus every method in
<classname>Circle</classname> has full access to all fields and
methods of <classname>Shape</classname>.</para>
<note>
<para>In a production environment this may or may not be desired
and is thus a design choice.</para>
</note>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_protectedAccessProblem">
<title><code language="java">protected</code> access involving different
instances</title>
<qandadiv>
<qandaentry>
<question>
<para>We reconsider class <classname>AlphaSub</classname> from
<xref linkend="sd1_qanda_oracleAccessExample"/>:</para>
<programlisting language="java">package package_two;
import package_one.Alpha;
public class AlphaSub
extends Alpha {
void dummy(/* Inherited */) {
int v;
v = attribPublic;
v = attribProtected;
v = <emphasis role="red">attribDefault</emphasis>;
v = <emphasis role="red">attribPrivate</emphasis>;
}
}</programlisting>
<para>If we try to access a different instance's <code
language="java">attribProtected</code> we fail:</para>
<programlisting language="java">package package_two;
import package_one.Alpha;
public class AlphaSub
extends Alpha {
void dummy(final Alpha a) {
int v;
...
v = a.<emphasis role="red">attribProtected</emphasis>;
...
}
}</programlisting>
<para>Why is this access prohibited? Both the calling instance and
the argument <code language="java">a</code> still belong to the
<classname>Alpha</classname> / <classname>AlphaSum</classname>
class hierarchy. Explain the different behavior.</para>
</question>
<answer>
<para><code language="java">protected</code> inherited access
grants an instance's subclass method access to a superclass
attribute or method involving one common instance.</para>
<para>The current use case differs: We have two (presumably
different) instances one trying to access another's <code
language="java">protected</code> attribute. This does not involve
inheritance at all and thus fails.</para>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1_sect_finalClass">
<title><code language="java">final</code> classes</title>
<figure xml:id="sd1_inherit_fig_finalClass">
<title><code language="java">final</code> classes</title>
<programlisting language="java">public final class Shape { ... }
-------------------------
public class Rectangle
extends Shape { // Error: final class cannot be extended
...</programlisting>
</figure>
<figure xml:id="sd1_inherit_fig_finalClassRatio">
<title><code language="java">final</code> classes rationale</title>
<itemizedlist>
<listitem>
<para>Design decision.</para>
</listitem>
<listitem>
<para>Slight performance gain.</para>
</listitem>
</itemizedlist>
<blockquote>
<note>
<para>Prominent Example: <classname
xlink:href="javaapi://String">java.lang.String</classname>.</para>
</note>
</blockquote>
</figure>
</section>
<section xml:id="sd1_inherit_sect_instanceOf">
<title>The <code language="java"
xlink:href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2">instanceof</code>
operator</title>
<figure xml:id="sd1_inherit_fig_defeatPolymorphism">
<title><quote>Defeating</quote> polymorphism</title>
<informaltable border="0">
<colgroup width="68%"/>
<colgroup width="32%"/>
<tr>
<td valign="top"><programlisting language="java">public static void main(String[] args) {
final Shape[] shapes = {
new Circle(1, 1, 2.),
new Rectangle(1, -1, 2., 3.)};
print(shapes);
}
static void print(final Shape[] shapes) {
for (final Shape s : shapes) {
if (s <link xlink:href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2">instanceof</link> Rectangle) {
System.out.println("Type Rectangle");
} else if (s <link
xlink:href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2">instanceof</link> Circle) {
System.out.println("Type Circle");
}
}
}</programlisting></td>
<td valign="top"><screen>Type Circle
Type Rectangle</screen></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sd1_inherit_fig_shapeDefineEqualsExamples">
<title>Defining <methodname>equals(...)</methodname>:
Expectations</title>
<programlisting language="java">Rectangle r1 = new Rectangle(1, 2, 5, 4),
r2 = new Rectangle(1, 2, 1, 7),
r3 = new Rectangle(1, 2, 5, 4);
Circle c = new Circle(-2, 3, 5);
System.out.print(r1.equals("Hi"));//<emphasis role="red">false</emphasis>: Differing classes Rectangle and String.
System.out.print(r1.equals(r2)); //<emphasis role="red">false</emphasis>: Differing width and height.
System.out.print(r3.equals(r1)); //<emphasis role="red">true</emphasis>: Two rectangles having identical
// (x|y), width and height.
System.out.print(r1.equals(c)); //<emphasis role="red">false</emphasis>: Differing classes Rectangle and Circle.
System.out.print(c.equals(c)); //<emphasis role="red">true</emphasis>: Object equal to itself.</programlisting>
</figure>
<figure xml:id="sd1_inherit_fig_shapeDefineEquals">
<title>Defining <methodname>equals(...)</methodname> of
<classname>Shape</classname> instances</title>
<para>Two <classname>Shape</classname> instances shall be considered
equal if:</para>
<itemizedlist>
<listitem>
<para>Both instances are of common type <abbrev>i.e.</abbrev> either
<classname>Rectangle</classname> or
<classname>Circle</classname>.</para>
</listitem>
<listitem>
<para>Their center coordinates match within a threshold of
<inlineequation>
<m:math display="inline">
<m:msup>
<m:mi>10</m:mi>
<m:mrow>
<m:mo>-</m:mo>
<m:mi>15</m:mi>
</m:mrow>
</m:msup>
</m:math>
</inlineequation>.</para>
</listitem>
<listitem>
<para><code language="java">width</code> and <code
language="java">height</code> or <code language="java">radius</code>
match within a threshold of <inlineequation>
<m:math display="inline">
<m:msup>
<m:mi>10</m:mi>
<m:mrow>
<m:mo>-</m:mo>
<m:mi>15</m:mi>
</m:mrow>
</m:msup>
</m:math>
</inlineequation>.</para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_inherit_fig_equalByCoordinate">
<title>Comparing center coordinates</title>
<programlisting language="java">public abstract class Shape {
private double x, y;
protected boolean equalCenter(final Shape o) {
return Math.abs(o.x - x) + Math.abs(o.y - y) < 1.E-15;
}
...</programlisting>
</figure>
<figure xml:id="sd1_inherit_fig_rectangleEquals">
<title>Implementing <classname>Rectangle</classname>.<methodname
xlink:href="javaapi://Object#equals(java.lang.Object)">equals()</methodname></title>
<programlisting language="java">public class Rectangle extends Shape {
@Override public boolean equals(Object o) {
if (o <link
xlink:href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2">instanceof</link> Rectangle <emphasis
role="red">r</emphasis>) {
final Rectangle oRectangle = (Rectangle) o; // Cast is «legal»
return super.equalCenter(<emphasis role="red">r</emphasis>) &&
Math.abs(<emphasis role="red">r</emphasis>.width- width) +
Math.abs(<emphasis role="red">r</emphasis>.height- height) < 1.E-15;
}
return false;
}
...</programlisting>
<para>For <code language="java">o == null</code> the expression <code
language="java">o <link
xlink:href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2">instanceof</link>
Rectangle</code> evaluates to <code language="java">false</code>.</para>
</figure>
<figure xml:id="sd1_inherit_fig_circleEquals">
<title>Implementing <classname>Circle</classname>.<methodname
xlink:href="javaapi://Object#equals(java.lang.Object)">equals()</methodname></title>
<programlisting language="java">public class Circle extends Shape {
@Override public boolean equals(final Object o) {
if (o <link
xlink:href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2">instanceof</link> Circle <emphasis
role="red">c</emphasis>){
return super.equalCenter(<emphasis role="red">c</emphasis>) &&
Math.abs(<emphasis role="red">c</emphasis>.radius - radius) < 1.E-15;
}
return false;
} ...</programlisting>
</figure>
<figure xml:id="sd1_inherit_fig_testShapeEquals">
<title>Testing equality of <classname>Shape</classname> objects</title>
<informaltable border="0">
<colgroup width="72%"/>
<colgroup width="28%"/>
<tr>
<td valign="top"><programlisting language="java">final Rectangle
r1 = new Rectangle(2, 3, 1,4),
r2 = new Rectangle(2, 3, 2,8),
r3 = new Rectangle(2, 3, 1,4);
final Circle c = new Circle(2,3, 7);
System.out.println("r1.equals(r2): " + r1.equals(r2));
System.out.println("r1.equals(r3): " + r1.equals(r3));
System.out.println("c.equals(r1): " + c.equals(r1));</programlisting></td>
<td valign="top"><screen>r1.equals(r2): false
r1.equals(r3): true
c.equals(r1): false</screen></td>
</tr>
</informaltable>
</figure>
</section>
<section xml:id="sd1_inherit_sect_Override">
<title>The <classname
xlink:href="javaapi://Override">@Override</classname> annotation.</title>
<figure xml:id="sd1_inherit_fig_overrideToString">
<title>Overriding Object.toString()</title>
<programlisting language="java">public class Shape {
double x, y;
...
@Override // Promise: Subsequent method overrides Object.toString();
public String toString() {
return "(" + x + "|" + y + ")";
}
}</programlisting>
</figure>
<figure xml:id="sd1_inherit_fig_NotOverrideToString">
<title>@Override: Easy compile time error detection</title>
<programlisting language="java">public class Shape {
double x, y;
...
@Override <emphasis role="red">// Error: method does not override a method from a supertype</emphasis>
public String toString(int value) {
return "(" + x + "|" + y + ")";
}
}</programlisting>
<para>Explanation: The given method does not override <methodname
xlink:href="javaapi://Object#toString()">Object.toString()</methodname>.</para>
</figure>
</section>
</chapter>