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Commit 13bbf199 authored by Goik Martin's avatar Goik Martin
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void methods returning nothing

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......@@ -149,7 +149,7 @@
#1</link> to <link xlink:href="???"><link
xlink:href="http://blogs.kent.ac.uk/mik/2012/07/joc-33/">Joc
#33</link></link>. The order of items is somewhat
confused.</para>
confusing.</para>
</listitem>
<listitem>
......@@ -173,7 +173,8 @@
</section>
<section xml:id="sd1StartGreenfoot">
<title>Working with <productname>Greenfoot</productname></title>
<title><abbrev>GF</abbrev> book 1. : Getting to know
<productname>Greenfoot</productname></title>
<para>Quote from <xref linkend="bibKoelling2010"/>, p. 3:</para>
......@@ -183,59 +184,71 @@
open on your screen and the book open on your desk.</para>
</blockquote>
<itemizedlist>
<listitem>
<para>Compilation: .java --&gt; .class</para>
</listitem>
<listitem>
<para>Classes and objects</para>
</listitem>
<listitem>
<para/>
</listitem>
</itemizedlist>
<qandaset>
<title>Automatic initialization on startup</title>
<qandadiv>
<qandaentry>
<question>
<para>The <quote>leaves and wombats</quote> scenario does not
supply leaves and wombats automatically when starting the game.
Open the <classname>WombatWorld</classname> class (right click
<quote>Open editor</quote>). You will discover a method being
declared as <methodname>public void
populate()</methodname>.</para>
<para>The name suggests that this method may populate the game's
world with wombats and leaves on startup. Modify the code to
achieve this behaviour. Hint: Whenever a new instance of
<classname>WombatWorld</classname> is being created the method
<methodname>public WombatWorld()</methodname> is being
executed.</para>
<para>We will later discover that this is a so called
constructor. As the name suggests a constructor is being
executed whenever a new instance of a class is being
created.</para>
</question>
<answer>
<para>To achieve the desired behaviour we have to call the
method <methodname>populate()</methodname> <coref
linkend="sw1CallPopulate"/> from within the constructor's
code:</para>
<programlisting>public class WombatWorld {
public WombatWorld() {
<section xml:id="sw1ObjClass">
<title>GF 1.1: Classes an objects</title>
<itemizedlist>
<listitem>
<para>Compilation: .java --&gt; .class</para>
</listitem>
<listitem>
<para>Classes and objects</para>
</listitem>
<listitem>
<para>Methods</para>
<figure xml:id="sw1FigMethodVoidVoid">
<title/>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/methVoidVoid.svg"/>
</imageobject>
</mediaobject>
</figure>
</listitem>
</itemizedlist>
<qandaset>
<title>Automatic initialization on startup</title>
<qandadiv>
<qandaentry>
<question>
<para>The <quote>leaves and wombats</quote> scenario does not
create leaves and wombats automatically on startup. Open the
<classname>WombatWorld</classname> class (right click
<quote>Open editor</quote>). You will discover a method being
declared as <methodname>public void
populate()</methodname>.</para>
<para>The name already suggests that this method may be used
to populate the game's world (with wombats and leaves) on
startup. Modify the code to achieve this behaviour. Hint:
Whenever a new instance of <classname>WombatWorld</classname>
is being created the method <methodname>public
WombatWorld()</methodname> is being executed.</para>
<para>We will later explain the meaning of<methodname> public
WombatWorld()</methodname> being a so called constructor. As
the name suggests a constructor is being executed each time a
new instance of a given class is being created.</para>
</question>
<answer>
<para>Calling the method <methodname>populate()</methodname>
<coref linkend="sw1CallPopulate"/> from within the
constructor's code achieves the desired behaviour:</para>
<programlisting>public class WombatWorld {
public WombatWorld() { <emphasis role="bold">// Constructor will be called each time a WombatWorld is being created</emphasis>
super(8, 8, 60);
setBackground("cell.jpg");
setPaintOrder(Wombat.class, Leaf.class);
<emphasis role="bold">populate()</emphasis>; <co
xml:id="sw1CallPopulate"/>
xml:id="sw1CallPopulate"/>
}
/**
* Populate the world with a fixed scenario of wombats and leaves.
......@@ -244,12 +257,27 @@
Wombat w1 = new Wombat();
addObject(w1, 3, 3);
...</programlisting>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<para/>
</section>
<section xml:id="sw1InteractClasss">
<title><abbrev>GF</abbrev> 1.3: Interacting with objects</title>
<para/>
</section>
</section>
</chapter>
<chapter xml:id="ideas">
<title>Ideas</title>
<para/>
</chapter>
<xi:include href="bibliography.xml" xpointer="element(/1)"/>
</book>
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