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diff --git a/Doc/Sd1/inheritance.xml b/Doc/Sd1/inheritance.xml
index b4a694669c46147bf34bc2c4ff8fcbff055e4f5f..41fa9d95a99db65adffe9edd2dffc0a32ba4e8d7 100644
--- a/Doc/Sd1/inheritance.xml
+++ b/Doc/Sd1/inheritance.xml
@@ -690,6 +690,68 @@ true <emphasis role="red">if this object is the same as the obj argument</emphas
</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="https://docs.oracle.com/en/java/javase/12/docs/api/java.base/java/lang/Enum.html#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">
diff --git a/Doc/Sd1/objectsClasses.xml b/Doc/Sd1/objectsClasses.xml
index df4b0c9c783da57ef1375cd6a2eb42c8b00b3412..a34021b5543ead2e33aae3d2da266d76e9dcaa7f 100644
--- a/Doc/Sd1/objectsClasses.xml
+++ b/Doc/Sd1/objectsClasses.xml
@@ -10568,6 +10568,193 @@ class_wrapper_private.Day'</emphasis></screen></td>
linkend="sd1_fig_preventUndesiredDayCreation"><code
language="java">enum</code> external instance creation</link>.</para>
</figure>
+
+ <qandaset defaultlabel="qanda" xml:id="sd1_qanda_enumDirections">
+ <title>Compass directions</title>
+
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>We consider an eight direction compass rose:</para>
+
+ <informalfigure>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="Ref/ObjectsAndClasses/Enum/windrose.svg"/>
+ </imageobject>
+ </mediaobject>
+ </informalfigure>
+
+ <para>Provide an <code language="java">enum</code>
+ <classname>Direction</classname> to be usable in the following
+ manner:</para>
+
+ <informaltable border="0">
+ <tr>
+ <td valign="top"><programlisting language="java">final Direction northWest = Direction.NW;
+
+System.out.println(northWest);</programlisting></td>
+
+ <td valign="top"><screen>north by west (315°)</screen></td>
+ </tr>
+ </informaltable>
+
+ <tip>
+ <para>Provide an appropriate constructor among with suitable
+ «internal» instance attributes and a corresponding <methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/12/docs/api/java.base/java/lang/Object.html#toString()">toString()</methodname>
+ method.</para>
+ </tip>
+ </question>
+
+ <answer>
+ <para>The desired output contains both a given direction's
+ description and degree value. We thus start by:</para>
+
+ <programlisting language="java">public enum Direction {
+ int degree;
+ String fullName;
+}</programlisting>
+
+ <para>For creating a <classname>Direction</classname> instance a
+ constructor accepting both values is being required:</para>
+
+ <programlisting language="java">public enum Direction {
+
+ N(0, "north"); <co linkends="sd1_qanda_enumDirectionsConstruct-1.2"
+ xml:id="sd1_qanda_enumDirectionsConstruct-1.2-co"/>
+
+ Direction(final int degree, final String fullName) { <co
+ linkends="sd1_qanda_enumDirectionsConstruct-2.2"
+ xml:id="sd1_qanda_enumDirectionsConstruct-2.2-co"/>
+ this.degree = degree;
+ this.fullName = fullName;
+ }
+
+ public final int degree; <co linkends="sd1_qanda_enumDirectionsConstruct-3"
+ xml:id="sd1_qanda_enumDirectionsConstruct-3.2-co"/>
+ public final String fullName;
+}</programlisting>
+
+ <calloutlist>
+ <callout arearefs="sd1_qanda_enumDirectionsConstruct-1.2-co"
+ xml:id="sd1_qanda_enumDirectionsConstruct-1.2">
+ <para>For the sake of simplicity we only create the «north»
+ direction here.</para>
+ </callout>
+
+ <callout arearefs="sd1_qanda_enumDirectionsConstruct-2.2-co"
+ xml:id="sd1_qanda_enumDirectionsConstruct-2.2">
+ <para>Constructor accepting:</para>
+
+ <orderedlist>
+ <listitem>
+ <para>A degree value e.g. 90° representing «eastern»
+ direction.</para>
+ </listitem>
+
+ <listitem>
+ <para>An informal name e.g. «east».</para>
+ </listitem>
+ </orderedlist>
+ </callout>
+
+ <callout arearefs="sd1_qanda_enumDirectionsConstruct-3.2-co"
+ xml:id="sd1_qanda_enumDirectionsConstruct-3">
+ <para>Two <code language="java">public</code> attributes
+ representing both a direction's degree value and its
+ informal name.</para>
+ </callout>
+ </calloutlist>
+
+ <para>For creating output texts like <abbrev>e.g.</abbrev>
+ <code>north by west (315°)</code> we need a <methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/12/docs/api/java.base/java/lang/Object.html#toString()">toString()</methodname>
+ method. Also adding the yet missing directions our final result
+ looks like:</para>
+
+ <programlisting language="java">public enum Direction {
+
+ N( 0, "north"),
+ NE( 45, "north by east"),
+ E( 90, "east"),
+ SE(135, "south by east"),
+ S( 180, "south"),
+ SW(225, "south by west"),
+ W( 270, "west"),
+ NW(315, "north by west");
+
+ Direction(final int degree, final String fullName) {
+ this.degree = degree;
+ this.fullName = fullName;
+ }
+
+ @Override
+ public String toString() {
+ return fullName + " (" + degree + "°)";
+ }
+
+ public final int degree;
+ public final String fullName;
+}</programlisting>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
+
+ <qandaset defaultlabel="qanda"
+ xml:id="sd1_qanda_enumDirectionNeighbours">
+ <title>Compass direction neighbours</title>
+
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>We would like to «invert» a given direction
+ <abbrev>e.g.</abbrev> turning <code>N</code> to <code>S</code>
+ and vice versa. Extend <xref
+ linkend="sd1_qanda_enumDirectionNeighbours"/> by adding an
+ <methodname>opposite()</methodname> method to be used
+ like:</para>
+
+ <informaltable border="0">
+ <tr>
+ <td valign="top"><programlisting language="java">final Direction southWest = Direction.SW;
+
+System.out.println("Direction: " + southWest);
+System.out.println(" Opposite: " + southWest.opposite());</programlisting></td>
+
+ <td valign="top"><screen>Direction: south by west (225°)
+ Opposite: north by east (45°)</screen></td>
+ </tr>
+ </informaltable>
+ </question>
+
+ <answer>
+ <para>Using <code language="java">switch</code> allows for a
+ straightforward solution:</para>
+
+ <programlisting language="java">public enum Direction {
+ ...
+ public Direction opposite() {
+ switch (this) {
+ case N: return S;
+ case NE: return SW;
+ case E: return W;
+ case SE: return NW;
+ case S: return N;
+ case SW: return NE;
+ case W: return E;
+ case NW: return SE;
+
+ default: return null; // Unreachable, but required for keeping the compiler happy.
+ }
+ }
+ ...
+}</programlisting>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
</section>
</section>