From 0a199b4f7ece069fd931d82d75d01fd0a97303e3 Mon Sep 17 00:00:00 2001
From: Martin Goik <goik@hdm-stuttgart.de>
Date: Wed, 10 Jan 2018 11:25:47 +0100
Subject: [PATCH] refining equals()
---
Doc/Sd1/inheritance.xml | 258 +++++++++++++++++++++++++++++-----------
1 file changed, 191 insertions(+), 67 deletions(-)
diff --git a/Doc/Sd1/inheritance.xml b/Doc/Sd1/inheritance.xml
index 6b162c21c..b53d71c42 100644
--- a/Doc/Sd1/inheritance.xml
+++ b/Doc/Sd1/inheritance.xml
@@ -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) &&
+ Math.abs(oRectangle.width- width) +
+ Math.abs(oRectangle.height- height) < 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) &&
- Math.abs(oCircle.radius - radius) < 1.E-15;
+ Math.abs(oCircle.radius - radius) < 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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