From 693ff6c9e769e4e928a78eed9548a13cbbcc7fde Mon Sep 17 00:00:00 2001
From: "Dr. Martin Goik" <goik@hdm-stuttgart.de>
Date: Tue, 19 Feb 2019 09:38:18 +0100
Subject: [PATCH] Exam exercises to lecture notes
---
Doc/Sd1/coreClasses.xml | 442 +++++++++++++++---
Doc/Sd1/inheritance.xml | 373 ++++++++++++---
.../mi/sd1/shape/model/Rectangle.java | 15 +
.../mi/sd1/shape/model/Shape.java | 44 +-
4 files changed, 717 insertions(+), 157 deletions(-)
diff --git a/Doc/Sd1/coreClasses.xml b/Doc/Sd1/coreClasses.xml
index df418b865..44f0d046f 100644
--- a/Doc/Sd1/coreClasses.xml
+++ b/Doc/Sd1/coreClasses.xml
@@ -113,7 +113,7 @@ equals: true</screen></td>
<itemizedlist>
<listitem>
<para>The <code language="java">==</code> operator acting on primitive
- types compares values of expressions.</para>
+ types compares expression values.</para>
</listitem>
<listitem>
@@ -136,28 +136,83 @@ equals: true</screen></td>
</itemizedlist>
</figure>
- <figure xml:id="sd1_coreclasses_fig_hashPrinciple">
- <title>Hashing principle</title>
+ <section xml:id="sw1_sect_CoreClasses_hashing">
+ <title>Objects, equals() and hash-values</title>
- <informaltable border="0">
- <tr>
- <td valign="top"><mediaobject>
- <imageobject>
- <imagedata fileref="Ref/CoreClasses/Hash/iceCreamPrices.jpg"/>
- </imageobject>
- </mediaobject></td>
-
- <td valign="middle"><quote>I want the 12p one</quote></td>
- </tr>
- </informaltable>
- </figure>
+ <figure xml:id="sd1_coreclasses_fig_hashPrinciple">
+ <title>Hashing principle</title>
- <figure xml:id="sd1_coreclasses_fig_hashExample">
- <title><classname>Rectangle</classname>
- <methodname>equals(...)</methodname> and
- <methodname>hashCode()</methodname></title>
-
- <programlisting language="java">public class Rectangle {
+ <informaltable border="0">
+ <tr>
+ <td valign="top"><mediaobject>
+ <imageobject>
+ <imagedata fileref="Ref/CoreClasses/Hash/iceCreamPrices.jpg"/>
+ </imageobject>
+ </mediaobject></td>
+
+ <td valign="middle"><quote>I want the 12p one</quote></td>
+ </tr>
+ </informaltable>
+ </figure>
+
+ <figure xml:id="sd1_coreclasses_fig_hashQuicklyIdentify">
+ <title>Quickly identify by <quote>simple</quote> value</title>
+
+ <itemizedlist>
+ <listitem>
+ <para>Where is the blond haired guy?</para>
+ </listitem>
+
+ <listitem>
+ <para>I take the pink flower.</para>
+ </listitem>
+
+ <listitem>
+ <para>The 334.50$ cellular phone.</para>
+ </listitem>
+ </itemizedlist>
+ </figure>
+
+ <figure xml:id="sd1_coreclasses_fig_hashBasicIdea">
+ <title>Hashing in Java and <methodname>equals()</methodname></title>
+
+ <para>Method <methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#hashCode()">hashCode()</methodname>:
+ Instance 0 ⇨ o.<methodname>hashCode()</methodname>, of type <code
+ language="java">int</code>.</para>
+
+ <itemizedlist>
+ <listitem>
+ <para>Same value on repeated invocation</para>
+ </listitem>
+
+ <listitem>
+ <para>Objects with identical value with respect to
+ <methodname>equals()</methodname> must have identical hash
+ values:</para>
+
+ <para><code language="java">true == a.equals(b)</code> ⟹ <code
+ language="java">a.hashCode() == b.hashCode()</code>.</para>
+ </listitem>
+
+ <listitem>
+ <para>Conversely: Two instances differing with respect to
+ <methodname>equals()</methodname> may have identical hash
+ values.</para>
+ </listitem>
+ </itemizedlist>
+
+ <para>Consequence: <methodname>equals()</methodname> and
+ <methodname>hashCode()</methodname> must be <emphasis
+ role="red">redefined simultaneously</emphasis>!</para>
+ </figure>
+
+ <figure xml:id="sd1_coreclasses_fig_hashExample">
+ <title><classname>Rectangle</classname>
+ <methodname>equals(...)</methodname> and
+ <methodname>hashCode()</methodname></title>
+
+ <programlisting language="java">public class Rectangle {
int width, height;
@Override public boolean equals(Object o) {
if (o instanceof Rectangle) {
@@ -171,89 +226,328 @@ equals: true</screen></td>
return width + height;
}
}</programlisting>
- </figure>
+ </figure>
- <figure xml:id="sd1_coreclasses_fig_hashRectangleValues">
- <title><classname>Rectangle</classname> hash values</title>
+ <figure xml:id="sd1_coreclasses_fig_hashRectangleValues">
+ <title><classname>Rectangle</classname> hash values</title>
- <informaltable border="1">
- <tr>
- <th>width</th>
+ <informaltable border="1">
+ <tr>
+ <th>width</th>
- <th>height</th>
+ <th>height</th>
- <th>hash value</th>
- </tr>
+ <th>hash value</th>
+ </tr>
- <tr>
- <td valign="top">1</td>
+ <tr>
+ <td valign="top">1</td>
- <td valign="top">3</td>
+ <td valign="top">3</td>
- <td valign="top">4</td>
- </tr>
+ <td valign="top">4</td>
+ </tr>
- <tr>
- <td valign="top">2</td>
+ <tr>
+ <td valign="top">2</td>
- <td valign="top">2</td>
+ <td valign="top">2</td>
- <td valign="top">4</td>
- </tr>
+ <td valign="top">4</td>
+ </tr>
- <tr>
- <td valign="top">5</td>
+ <tr>
+ <td valign="top">5</td>
- <td valign="top">5</td>
+ <td valign="top">5</td>
- <td valign="top">10</td>
- </tr>
+ <td valign="top">10</td>
+ </tr>
- <tr>
- <td valign="top">2</td>
+ <tr>
+ <td valign="top">2</td>
- <td valign="top">7</td>
+ <td valign="top">7</td>
- <td valign="top">9</td>
- </tr>
+ <td valign="top">9</td>
+ </tr>
- <tr>
- <td valign="top">4</td>
+ <tr>
+ <td valign="top">4</td>
- <td valign="top">9</td>
+ <td valign="top">9</td>
- <td valign="top">13</td>
- </tr>
- </informaltable>
- </figure>
+ <td valign="top">13</td>
+ </tr>
+ </informaltable>
+ </figure>
- <figure xml:id="sd1_coreclasses_fig_hashRectangleBetter">
- <title><classname>Rectangle</classname>
- <methodname>equals(...)</methodname> and
- <methodname>hashCode()</methodname></title>
+ <figure xml:id="sd1_coreclasses_fig_hashRectangleBetter">
+ <title><classname>Better </classname><methodname>hashCode()</methodname>
+ method</title>
- <programlisting language="java">public class Rectangle {
+ <programlisting language="java">public class Rectangle {
int width, height;
...
@Override public int hashCode() {
return width + <emphasis role="red">13 * height</emphasis>;
}
}</programlisting>
- </figure>
+ </figure>
- <figure xml:id="sd1_coreclasses_fig_hashEqualsContract">
- <title><classname>Contract
- </classname><methodname>equals(...)</methodname> and
- <methodname>hashCode()</methodname></title>
+ <qandaset defaultlabel="qanda" xml:id="qanda_sw1TimeperiodHashing">
+ <title>Choosing a <quote>good</quote>
+ <methodname>hashCode()</methodname> method</title>
- <para>If <methodname>a.equals(b)</methodname> ==> <code
- language="java">a.hashCode() == b.hashCode()</code>. <emphasis
- role="red">The reverse does not apply!</emphasis></para>
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>We consider:</para>
- <para>Consequence: <methodname>equals()</methodname> and
- <methodname>hashCode()</methodname> must be <emphasis role="red">redefined
- simultaneously</emphasis>!</para>
- </figure>
+ <programlisting language="java">public class TimePeriod {
+ private final int hours, minutes, seconds;
+ /**
+ * A time period within a day e.g. 4 hours, 23 minutes and 11 seconds.
+ *
+ * @param hours Hours ranging from 0 to 23
+ * @param minutes Minutes ranging from 0 to 59
+ * @param seconds Seconds ranging from 0 to 59
+ */
+ public TimePeriod(final int hours, final int minutes, final int seconds) {
+ this.hours = hours;
+ this.minutes = minutes;
+ this.seconds = seconds;
+ }
+
+ @Override
+ public boolean equals(final Object o) {
+ if (o instanceof TimePeriod) {
+ final TimePeriod other = (TimePeriod) o;
+ return hours == other.hours &&
+ minutes == other.minutes &&
+ seconds == other.seconds;
+ } else {
+ return false;
+ }
+ }
+}</programlisting>
+
+ <para>Which of the two <methodname>hashCode()</methodname>
+ implementations is better with respect to quickly telling whether
+ two <classname>TimePeriod</classname> instances are having the
+ same value or not?</para>
+
+ <informaltable border="1">
+ <tr>
+ <th>Method 1</th>
+
+ <th>Method 2</th>
+ </tr>
+
+ <tr>
+ <td valign="top"><programlisting language="java">public class TimePeriod {
+...
+ @Override
+ public int hashCode() {
+ return seconds + 60 * (minutes + 60 * hours);
+ }
+}</programlisting></td>
+
+ <td valign="top"><programlisting language="java">public class TimePeriod {
+...
+ @Override
+ public int hashCode() {
+ return seconds + minutes + hours;
+ }
+}</programlisting></td>
+ </tr>
+ </informaltable>
+
+ <tip>
+ <para>Excerpt from <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html">Object</classname>.<link
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#hashCode()">hashCode()</link>:</para>
+
+ <blockquote>
+ <para>However, the programmer should be aware that producing
+ distinct integer results for unequal objects may improve the
+ performance of hash tables.</para>
+ </blockquote>
+ </tip>
+ </question>
+
+ <answer>
+ <para>An ideal/perfect <methodname>hashCode()</methodname> method
+ in addition to <xref linkend="sd1_coreclasses_fig_hashBasicIdea"/>
+ will return different values whenever two instances <code>a</code>
+ and <code>b</code> differ in value with respect to the underlying
+ <methodname>equals()</methodname> method:</para>
+
+ <para><code language="java">false == a.equals(b)</code> ⟹ <code
+ language="java">a.hashCode() != b.hashCode()</code>.</para>
+
+ <para>Method 1 serves this purpose: Its returns a given
+ <classname>TimePeriod</classname> instance total amount of seconds
+ equivalent e.g turning (<emphasis role="rd">1</emphasis> hour,
+ <emphasis role="rd">2</emphasis> minutes, <emphasis
+ role="rd">5</emphasis> seconds) into (<emphasis
+ role="rd">1</emphasis> * 60 + <emphasis role="rd">2</emphasis>) *
+ 60 + <emphasis role="rd">5</emphasis> = 3725 seconds. Due its
+ specification a <classname>TimePeriod</classname> instance cannot
+ exceed 24 hours being equivalent to 24 * 60 * 60 = 86400 seconds.
+ A 4 byte <code language="java">int</code> is therefore safe with
+ respect to overflow issues.</para>
+
+ <para>Conclusion: Whenever two instances of
+ <classname>TimePeriod</classname> differ their method 1
+ <methodname>hashCode()</methodname> values will differ as
+ well.</para>
+
+ <para>This does not hold with respect to method 2. Consider two
+ instances (1 hour, 2 minutes, 3 seconds) vs. (3 hours, 2 minutes,
+ 1 second) returning an identical
+ <methodname>hashCode()</methodname> of 1 + 2 + 3 = 3 + 2 + 1 = 6.
+ Thus method 2 requiring just two additions offers (slightly)
+ better runtime performance at the expense of a higher hash value
+ collision rate. </para>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
+
+ <qandaset defaultlabel="qanda" xml:id="qanda_sw1StringPerfectHash">
+ <title><methodname>String</methodname> and good hashCode()
+ implementations.</title>
+
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>In the previous exercise we found an ideal
+ <methodname>hashCode()</methodname> implementation:</para>
+
+ <programlisting language="java">public class TimePeriod {
+...
+ @Override
+ public int hashCode() {
+ return seconds + 60 * (minutes + 60 * hours);
+ }
+}</programlisting>
+
+ <para>Is this possible for instances of String as well?</para>
+
+ <tip>
+ <para>Consider the possible number of different strings.</para>
+ </tip>
+ </question>
+
+ <answer>
+ <para>It is not possible to construct a perfect
+ <methodname>hashCode()</methodname> method acting on arbitrary
+ strings. A Java <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html">String</classname>
+ consists of individual <code language="java">char</code> elements
+ each requiring two bytes. Considering strings of fixed length we
+ have the following number of different strings:</para>
+
+ <informaltable border="1">
+ <tr>
+ <th>Number of chars</th>
+
+ <th>Number of possible strings</th>
+ </tr>
+
+ <tr>
+ <td valign="top">1</td>
+
+ <td valign="top"><inlineequation>
+ <m:math display="inline">
+ <m:msup>
+ <m:mi>2</m:mi>
+
+ <m:mi>16</m:mi>
+ </m:msup>
+ </m:math>
+ </inlineequation></td>
+ </tr>
+
+ <tr>
+ <td valign="top">2</td>
+
+ <td valign="top"><inlineequation>
+ <m:math display="inline">
+ <m:msup>
+ <m:mi>2</m:mi>
+
+ <m:mrow>
+ <m:mi>2</m:mi>
+
+ <m:mo>×</m:mo>
+
+ <m:mi>16</m:mi>
+ </m:mrow>
+ </m:msup>
+ </m:math>
+ </inlineequation></td>
+ </tr>
+
+ <tr>
+ <td valign="top">3</td>
+
+ <td valign="top"><inlineequation>
+ <m:math display="inline">
+ <m:msup>
+ <m:mi>2</m:mi>
+
+ <m:mrow>
+ <m:mi>3</m:mi>
+
+ <m:mo>×</m:mo>
+
+ <m:mi>16</m:mi>
+ </m:mrow>
+ </m:msup>
+ </m:math>
+ </inlineequation></td>
+ </tr>
+ </informaltable>
+
+ <para>A four byte <code language="java">int</code> only offers
+ <inlineequation>
+ <m:math display="inline">
+ <m:msup>
+ <m:mi>2</m:mi>
+
+ <m:mrow>
+ <m:mi>32</m:mi>
+ </m:mrow>
+ </m:msup>
+ </m:math>
+ </inlineequation> different values. Thus even mapping just one-
+ and two-<xref linkend="glo_unicode"/> character strings exceeds
+ the number of different <code language="java">int</code> values
+ thus requiring different string instances being mapped to
+ identical hash values. Consider for example:</para>
+
+ <informaltable border="1">
+ <tr>
+ <th>Code</th>
+
+ <th>Execution result</th>
+ </tr>
+
+ <tr>
+ <td valign="top"><programlisting language="java">System.out.println("hashcode of AA: " + "Aa".hashCode());
+System.out.println("hashcode of BB: " + "BB".hashCode());</programlisting></td>
+
+ <td valign="top"><screen>hashcode of AA: 2112
+hashcode of BB: 2112</screen></td>
+ </tr>
+ </informaltable>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
+ </section>
<section xml:id="sd1SectVarargsFormat">
<title>Reconsidering System.out.format().</title>
diff --git a/Doc/Sd1/inheritance.xml b/Doc/Sd1/inheritance.xml
index 22f897d61..b4a694669 100644
--- a/Doc/Sd1/inheritance.xml
+++ b/Doc/Sd1/inheritance.xml
@@ -194,11 +194,12 @@ double height;</programlisting></td>
<itemizedlist>
<listitem>
- <para>A rectangle is a shape.</para>
+ <para>A rectangle <emphasis role="red">is a</emphasis>
+ shape.</para>
</listitem>
<listitem>
- <para>A circle is a shape.</para>
+ <para>A circle <emphasis role="red">is a</emphasis> shape.</para>
</listitem>
</itemizedlist>
</listitem>
@@ -267,7 +268,169 @@ public Shape(double x,double y) {
}</programlisting>
</figure>
- <section xml:id="sd1_inherit_sect_inheritToString">
+ <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">h</code></para>
+ </callout>
+
+ <callout arearefs="sda_inherit_fig_RectangleConstructor-2-co">
+ <para><code language="java">eight</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 <co linkends="sda_inherit_fig_ShapeEquals-2"
+ xml:id="sda_inherit_fig_ShapeEquals-2-co"/>) {
+ final Shape other = (Shape) o; <co
+ linkends="sda_inherit_fig_ShapeEquals-3"
+ xml:id="sda_inherit_fig_ShapeEquals-3-co"/>
+ return x == other.x && y == other.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-3-co"
+ xml:id="sda_inherit_fig_ShapeEquals-3">
+ <para>Casting other object to class
+ <classname>Shape</classname>.</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) {
+ final Rectangle other = (Rectangle) o;
+ return super.equals(o) <co linkends="sda_inherit_fig_RectangleEquals-1"
+ xml:id="sda_inherit_fig_RectangleEquals-1-co"/> && width == other.width && height == other.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>
+ </section>
+
+ <section xml:id="sd1_inherit_sect_overrideToString">
<title>Overriding toString()</title>
<figure xml:id="sda_inherit_fig_shapeInstanceLog">
@@ -309,71 +472,6 @@ public Shape(double x,double y) {
</mediaobject>
</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.</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 Rectangle(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 constructor.</para>
-
- <note>
- <para>This must be the first statement.</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>
- subclass.</para>
- </callout>
- </calloutlist>
- </figure>
-
<figure xml:id="sda_inherit_fig_RectangleLogging">
<title>Logging <classname>Rectangle</classname> instances</title>
@@ -409,22 +507,24 @@ public Rectangle(double x, double y,
<calloutlist>
<callout arearefs="sda_inherit_fig_RectangleOverrideToStringAgain-1-co"
xml:id="sda_inherit_fig_RectangleOverrideToStringAgain-1">
- <para>Include center coordinate from superclass: The <code
- language="java">super</code> keyword will call the superclass'
- method <methodname>toString()</methodname>.</para>
+ <para>The <code language="java"
+ xlink:href="https://docs.oracle.com/javase/tutorial/java/IandI/super.html">super</code>
+ keyword allows for calling the <methodname>toString()</methodname>
+ method from superclass <classname>Shape</classname>.</para>
</callout>
<callout arearefs="sda_inherit_fig_RectangleOverrideToStringAgain-2-co"
xml:id="sda_inherit_fig_RectangleOverrideToStringAgain-2">
- <para>Append <quote>own</quote> <code language="java">width</code>
- and <code language="java">height</code> attributes.</para>
+ <para>Append class <classname>Rectangle</classname>'s
+ <quote>own</quote> <code language="java">width</code> and <code
+ language="java">height</code> attributes.</para>
</callout>
</calloutlist>
</figure>
<figure xml:id="sda_inherit_fig_rectangleExtendShapa">
<title><classname>Rectangle</classname> extending
- <classname>Shape´</classname></title>
+ <classname>Shape</classname></title>
<mediaobject>
<imageobject>
@@ -465,6 +565,131 @@ public Rectangle(double x, double y,
</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="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/StringBuffer.html">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="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/StringBuffer.html">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="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html">String</classname>
+ and <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/StringBuffer.html">StringBuffer</classname>
+ <xref linkend="glo_API"/> regarding the
+ <methodname>toString()</methodname> method.</para>
+ </tip>
+ </question>
+
+ <answer>
+ <para>The <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html">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="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html">Object</classname>.<methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)">equals()</methodname>
+ method is being overridden.</para>
+
+ <para>In class <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/StringBuffer.html">StringBuffer</classname>
+ we do not find any <methodname>equals()</methodname> method. Thus
+ <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html">Object</classname>.<methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)">equals()</methodname>
+ is not being overridden. When comparing two instances of
+ <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/StringBuffer.html">StringBuffer</classname>
+ effectively <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html">Object</classname>.<methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#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="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/StringBuffer.html">StringBuffer</classname>
+ will be compared for object identity rather than representing the
+ same string value.</para>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
</section>
<section xml:id="sd1_inherit_sect_final">
diff --git a/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Rectangle.java b/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Rectangle.java
index 20b554bb7..f0972879e 100644
--- a/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Rectangle.java
+++ b/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Rectangle.java
@@ -19,6 +19,21 @@ public class Rectangle extends Shape {
setHeight(height);
}
+ @Override
+ public boolean equals(final Object o) {
+ if (o instanceof Rectangle) {
+ final Rectangle other = (Rectangle) o;
+ return super.equals(o) && width == other.width && height == other.height;
+ } else {
+ return false;
+ }
+ }
+
+ @Override
+ public int hashCode() {
+ return super.hashCode () + (int) (width + 13 * height);
+ }
+
@Override
public Shape scale(final double factor) {
width *= factor;
diff --git a/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Shape.java b/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Shape.java
index 4ef5a8671..6657a9f67 100644
--- a/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Shape.java
+++ b/P/Sd1/Figure/ToString/src/main/java/de/hdm_stuttgart/mi/sd1/shape/model/Shape.java
@@ -1,12 +1,11 @@
package de.hdm_stuttgart.mi.sd1.shape.model;
/**
- * @author goik
- *
+ * Base class of geometric primitives like {@link Rectangle} and {@link Circle}
*/
public abstract class Shape {
- private double x,y;
+ private double x, y;
/**
* Creating a shape with given center coordinates
@@ -19,13 +18,27 @@ public abstract class Shape {
setY(y);
}
+ @Override
+ public boolean equals(final Object o) {
+ if (o instanceof Shape) {
+ final Shape other = (Shape) o;
+ return x == other.x && y == other.y;
+ } else {
+ return false;
+ }
+ }
+
+ @Override
+ public int hashCode() {
+ return (int) (x + 13 * y);
+ }
+
@Override
public String toString() {
return "(" + x + ',' + y + ')';
}
/**
- *
* @param factor Scale the current figure by this value.
* @return The current object.
*/
@@ -33,20 +46,33 @@ public abstract class Shape {
/**
* The current shape's area.
+ *
* @return e.g. with * height in case of a rectangle
*/
public abstract double getArea();
/**
* The current shape's perimeter.
- * @return e.g. with 2 * (height + width) in case of a rectangle
+ *
+ * @return e.g. with 2 * (height + width) in case of a rectangle
*/
public abstract double getPerimeter();
- public double getX() { return x;}
- public void setX(final double x) { this.x = x;}
- public double getY() { return y;}
- public void setY(final double y) { this.y = y;}
+ public double getX() {
+ return x;
+ }
+
+ public void setX(final double x) {
+ this.x = x;
+ }
+
+ public double getY() {
+ return y;
+ }
+
+ public void setY(final double y) {
+ this.y = y;
+ }
/**
* Translating a shape by a given vector (x,y).
--
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