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Commit 0a199b4f authored by Goik Martin's avatar Goik Martin
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refining equals()

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Doc/Sd1/inheritance.xml
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......@@ -29,8 +29,9 @@
}</programlisting></td>
<td valign="top"><programlisting language="none">public class Circle {
// Center coordinate <coref linkend="sda_inherit_fig_codeDuplicate-1-co"/>
private <emphasis role="red">double x</emphasis>;
// Center coordinate
private <emphasis role="red">double x</emphasis>; <coref
linkend="sda_inherit_fig_codeDuplicate-1-co"/>
private <emphasis role="red">double y</emphasis>;
private double radius; <co linkends="sda_inherit_fig_codeDuplicate-3"
......@@ -437,15 +438,24 @@ public Rectangle(double x, double y,
<programlisting language="java">public class Rectangle extends Shape {
@Override public void move(int xTrans, int yTrans) {
// I'm so dumb!
...
}</programlisting>
</figure>
<para>Solution: Prohibit overriding <methodname>move()</methodname> in
superclass <code language="java">Shape</code>:</para>
<figure xml:id="sda_inherit_fig_shapeMoveProblemSubclassSolution">
<title>Solution: <emphasis role="red">final</emphasis> prevents
overriding</title>
<programlisting language="none">... public <emphasis role="red">final</emphasis> void move(final int xTrans, final int yTrans) {
x += xTrans;
y += yTrans;
} ...</programlisting>
<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>
......@@ -457,20 +467,20 @@ public Rectangle(double x, double y,
<informaltable border="0">
<tr>
<td valign="top"><programlisting language="java"> /**
<td valign="top"><programlisting language="java">public class Rectangle extents Shape {
/**
* Calculate the area.
* @return The rectangle's area
*/
@Override
public double getArea() {
return width * height;
}</programlisting></td>
<td valign="top"><programlisting language="java"> /**
<td valign="top"><programlisting language="java">public class Circle extents Shape {
/**
* Calculate the area.
* @return The circle's area
*/
@Override
public double getArea() {
return Math.PI * radius * radius;
}</programlisting></td>
......@@ -479,19 +489,22 @@ public Rectangle(double x, double y,
</figure>
<figure xml:id="sda_inherit_fig_shapePolymorphicAreaCall">
<title>Wanted: Polymorphic <methodname>getArea()</methodname>
<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
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
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>);
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>
......@@ -511,28 +524,34 @@ Rectangle at (1.0|-1.0), width= 2.0, height=3.0: <emphasis role="red">area = 6.0
<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 upcasting.</para>
<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 xml:id="sda_inherit_fig_getAreaPolymorphicProblem">
<title>Problem</title>
<title>Problems:</title>
<itemizedlist>
<listitem>
<para>No such <methodname>getArea()</methodname> method in class
<classname>Shape</classname>.</para>
<para>No meaningful <methodname>getArea()</methodname> method in
class <classname>Shape</classname> possible.</para>
</listitem>
<listitem>
<para>No meaningful implementation in
<classname>Shape</classname>.</para>
</listitem>
<listitem>
<para>Meaningful implementations in both subclasses
<classname>Rectangle</classname> and
<classname>Circle</classname>.</para>
<para>Meaningful implementations exist both in subclass<classname>
Rectangle</classname> and <classname>Circle</classname>.</para>
</listitem>
</itemizedlist>
......@@ -595,20 +614,27 @@ Rectangle at (1.0|-1.0), width= 2.0, height=3.0: <emphasis role="red">area = 6.0
<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 any concrete (= non-<code
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 a
parent class doing so.</para>
implementation of <classname>getArea()</classname> or will have an
intermediate parent class doing so.</para>
<para>The <code language="java">abstract</code> keyword requires a
corresponding implementation in any derived non-<code
language="java">abstract</code> subclass.</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"
......@@ -923,7 +949,8 @@ public abstract class Shape <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"
......@@ -935,36 +962,37 @@ public class Rectangle <co linkends="sda_inherit_fig_protectedCreationTime-3"
linkends="sda_inherit_fig_protectedCreationTime-4"
xml:id="sda_inherit_fig_protectedCreationTime-4-co"/>);
return width * height;
} ... }</programlisting>
} ...
}</programlisting>
</figure>
<calloutlist>
<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>
<calloutlist>
<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-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-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>
<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>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_protectedPackagePrivate">
<title><code language="java">protected</code> vs. <quote>package
......@@ -1159,14 +1187,15 @@ public class Rectangle
<tr>
<td valign="top"><programlisting language="java">public static void main(String[] args) {
final Shape[] shapes = {new Circle(1, 1, 2.),
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 instanceof Rectangle) {
System.out.println("Type Rectangle");
System.out.println("Type Rectangle");
} else if (s instanceof Circle) {
System.out.println("Type Circle");
}
......@@ -1179,6 +1208,57 @@ Type Rectangle</screen></td>
</informaltable>
</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>
......@@ -1191,6 +1271,27 @@ Type Rectangle</screen></td>
...</programlisting>
</figure>
<figure xml:id="sd1_inherit_fig_rectangleEquals">
<title>Implementing <classname>Rectangle</classname>.<methodname
xlink:href="https://docs.oracle.com/javase/9/docs/api/java/lang/Object.html#equals-java.lang.Object-">equals()</methodname></title>
<programlisting language="java">public class Rectangle extends Shape {
@Override public boolean equals(Object o) {
if (o instanceof Rectangle){
final Rectangle oRectangle = (Rectangle) o;
return super.equalCenter(oRectangle) &amp;&amp;
Math.abs(oRectangle.width- width) +
Math.abs(oRectangle.height- height) &lt; 1.E-15;
}
return false;
}
...</programlisting>
<remark>if <code language="java">o == null</code> the expression <code
language="java">o instanceof Rectangle</code> evaluates to <code
language="java">false</code>.</remark>
</figure>
<figure xml:id="sd1_inherit_fig_circleEquals">
<title>Implementing <classname>Circle</classname>.<methodname
xlink:href="https://docs.oracle.com/javase/9/docs/api/java/lang/Object.html#equals-java.lang.Object-">equals()</methodname></title>
......@@ -1200,14 +1301,37 @@ Type Rectangle</screen></td>
if (o instanceof Circle){
final Circle oCircle = (Circle) o;
return super.equalCenter(o) &amp;&amp;
Math.abs(oCircle.radius - radius) &lt; 1.E-15;
Math.abs(oCircle.radius - radius) &lt; 1.E-15;
}
return false;
} ...</programlisting>
</figure>
<remark>if <code language="java">o == null</code> the expression <code
language="java">o instanceof Circle</code> evaluates to <code
language="java">false</code>.</remark>
<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>
</chapter>
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