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Commit a7277273 authored by Goik Martin's avatar Goik Martin
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Exercise on direction representation by enum variable

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......@@ -175,7 +175,8 @@
</listitem>
<listitem>
<para>Read <xref linkend="bibKoelling2010Ger"/> till .</para>
<para>Read <xref linkend="bibKoelling2010Ger"/> till section
1.3.</para>
</listitem>
<listitem>
......@@ -299,8 +300,15 @@
</section>
</chapter>
<chapter xml:id="sd1CrabExample">
<title>Lecture 3 - Wombats and crabs</title>
<chapter xml:id="sd1Wombats">
<title>Lecture 3 - Wombats</title>
<section xml:id="sd1WombatsPrepare">
<title>Preparations</title>
<para>Complete reading chapter 1 of <xref
linkend="bibKoelling2010Ger"/>.</para>
</section>
<section xml:id="sw1ObjClass">
<title>GF 1.1: Classes an objects</title>
......@@ -389,7 +397,7 @@
</listitem>
</itemizedlist>
<qandaset>
<qandaset xml:id="sd1GrennAutoInitStartup">
<title>Automatic initialization on startup</title>
<qandadiv>
......@@ -414,7 +422,7 @@
WombatWorld()</methodname> being a so called
<emphasis>constructor</emphasis>. As the name suggests a
constructor is being executed whenever a new instance of a given
class is being created. </para>
class is being created.</para>
</question>
<answer>
......@@ -443,17 +451,17 @@
</qandadiv>
</qandaset>
<qandaset>
<qandaset xml:id="sd1RepresentDirections">
<title>Representation of directions</title>
<qandadiv>
<qandadiv xml:id="sd1DirParameterization">
<qandaentry>
<question>
<para>Our Wombat class does have a method
<methodname>setDirection(int direction)</methodname>. This
allows for setting the direction a wombat is going to move. Call
it inserting different values by right clicking on a wombat
instance. How do entered values relate to directions of a
this method inserting different values by right clicking on a
wombat instance. How do entered values relate to directions of a
wombat's movement? Complete the following table by adding North,
South, East and West accordingly:</para>
......@@ -566,10 +574,10 @@
</qandadiv>
<qandadiv>
<qandaentry>
<qandaentry xml:id="sd1DirRepresentationProblems">
<question>
<para>Representing directions by integer values ranging from 1
to 4 may be considered error prone. Explain possible
<para>Representing directions by integer values ranging from 0
to 3 may be considered error prone. Explain possible
problems.</para>
<para>Hint: The <productname>Greenfoot</productname>
......@@ -604,17 +612,207 @@
<para>A programmer may be misled to use illegal
directional values. Keeping a wrong set of directions in
mind e.g {1, 2, 3, 4} rather than {0, 1, 2, 3} will result
in errors. These may be hard to trace </para>
in errors. These may be hard to trace</para>
</glossdef>
</glossentry>
</glosslist>
</answer>
</qandaentry>
<qandaentry xml:id="sd1DirRepresentEnum">
<question>
<para>Read the section named VARIABLES WITH A FIXED SET OF
INTEGER VALUES in <xref linkend="bibHorton2011"/> about
<code>enum</code> types in Java. Use an <code>enum</code>
definition to consistently replace the following four
definitions:</para>
<programlisting>public class Wombat extends Actor {
...
<emphasis role="bold">private static final int EAST = 0;
private static final int WEST = 1;
private static final int NORTH = 2;
private static final int SOUTH = 3;</emphasis>
...</programlisting>
<para>Hint: The code given in <link
xlink:href="http://www.javabeat.net/how-to-use-enum-in-switch">How
to use Enum in Switch</link> may serve as a blueprint for this
exercise. </para>
</question>
<answer>
<para>First we define an appropriate enumeration within our
<classname>Wombat</classname> class:</para>
<programlisting>public class Wombat extends Actor {
<emphasis role="bold">enum Direction {
EAST, WEST, NORTH, SOUTH
};</emphasis>
...</programlisting>
<para>This definition defines a new type replacing the int
values in e.g. <code>private static final int EAST = 0</code>.
We may use this newly defined type to redefine our
<code>direction</code> attribute:</para>
<programlisting>public class Wombat extends Actor {
...
<emphasis role="bold">private Direction direction;</emphasis> // used to be "private int direction"
...</programlisting>
<para>Our constructor <classname>Wombat()</classname> no
reads:</para>
<programlisting> public Wombat() {
<emphasis role="bold">setDirection(Direction.EAST)</emphasis>; // improved readability, cf. setDirection(0)
leavesEaten = 0;
}</programlisting>
<para>The <code>setDirection</code> method may be re-written
as:</para>
<programlisting> public void setDirection(Direction direction) {
this.direction = direction;
switch (direction) {
case SOUTH:
setRotation(90);
break;
case EAST:
setRotation(0);
break;
case NORTH:
setRotation(270);
break;
case WEST:
setRotation(180);
break;
}
}</programlisting>
<para>This way we got rid of the if clause being present in the
original code:</para>
<programlisting>public void setDirection(int direction)
{
<emphasis role="bold">if ((direction &gt;= 0) &amp;&amp; (direction &lt;= 3)) {
this.direction = direction;
}</emphasis>
switch(direction) {
case SOUTH :...</programlisting>
<para>This check is no longer required since our variable
direction no longer needs to be checked belonging to an interval
{0,1,2,3}. Re-writing the remaining methods is
straightforward:</para>
<programlisting> /**
* Move one cell forward in the current direction.
*/
public void move() {
if (!canMove()) {
return;
}
switch (direction) {
case SOUTH:
setLocation(getX(), getY() + 1);
break;
case EAST:
setLocation(getX() + 1, getY());
break;
case NORTH:
setLocation(getX(), getY() - 1);
break;
case WEST:
setLocation(getX() - 1, getY());
break;
}
}
/**
* Test if we can move forward. Return true if we can, false otherwise.
*/
public boolean canMove() {
World myWorld = getWorld();
int x = getX();
int y = getY();
switch (direction) {
case SOUTH:
y++;
break;
case EAST:
x++;
break;
case NORTH:
y--;
break;
case WEST:
x--;
break;
}
// test for outside border
if (x &gt;= myWorld.getWidth() || y &gt;= myWorld.getHeight()) {
return false;
} else if (x &lt; 0 || y &lt; 0) {
return false;
}
return true;
}
/**
* Turns towards the left.
*/
public void turnLeft() {
switch (direction) {
case SOUTH:
setDirection(Direction.EAST);
break;
case EAST:
setDirection(Direction.NORTH);
break;
case NORTH:
setDirection(Direction.WEST);
break;
case WEST:
setDirection(Direction.SOUTH);
break;
}
}</programlisting>
<para>Calling the <code>setDirection(...)</code> method from the
<productname>Greenfoot</productname> GUI (right click on a
wombat) now requires to fill in a value like
<quote>Wombat.Direction.WEST</quote> rather than simply
<quote><code>1</code></quote>.</para>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
</chapter>
<chapter xml:id="sd1Crabs">
<title>Lecture 4 - A crabs' world</title>
<section xml:id="sd1CrabsPrepare">
<title>Preparations</title>
<itemizedlist>
<listitem>
<para>Read chapter 2 of <xref linkend="bibKoelling2010Ger"/>.</para>
</listitem>
<listitem>
<para>Read Chapter 3 from <xref linkend="bibHorton2011"/> till
<quote>THE CONDITIONAL OPERATOR</quote>.</para>
</listitem>
</itemizedlist>
</section>
<section xml:id="sd1EnhanceCrabMove">
<section xml:id="sd1CrabsEnhanceMove">
<title>Enhancing crabs' movements</title>
<qandaset xml:id="sd1QandCrabStatesIf">
......
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