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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
Choosing
composition
<P>vs.
inheritance
<P><A NAME="Index479"></A><A NAME="Index480"></A></H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Both
composition and inheritance allow you to place subobjects<A NAME="Index481"></A>
inside your new class. You might wonder about the difference between the two,
and when to choose one over the other.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Composition
is generally used when you want the features of an existing class inside your
new class, but not its interface. That is, you embed an object so that you can
use it to implement features of your new class, but the user of your new class
sees the interface you’ve defined rather than the interface from the
embedded object. For this effect, you embed
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects of existing classes inside your new class. <A NAME="Index482"></A><A NAME="Index483"></A></FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Sometimes
it makes sense to allow the class user to directly access the composition of
your new class; that is, to make the member objects
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The member objects use implementation hiding themselves, so this is a safe
thing to do and when the user knows you’re assembling a bunch of parts,
it makes the interface easier to understand. A
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>car</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is a good example:
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: Car.java</font>
<font color="#009900">// Composition with public objects</font>
<font color="#0000ff">class</font> Engine {
<font color="#0000ff">public</font> <font color="#0000ff">void</font> start() {}
<font color="#0000ff">public</font> <font color="#0000ff">void</font> rev() {}
<font color="#0000ff">public</font> <font color="#0000ff">void</font> stop() {}
}
<font color="#0000ff">class</font> Wheel {
<font color="#0000ff">public</font> <font color="#0000ff">void</font> inflate(<font color="#0000ff">int</font> psi) {}
}
<font color="#0000ff">class</font> Window {
<font color="#0000ff">public</font> <font color="#0000ff">void</font> rollup() {}
<font color="#0000ff">public</font> <font color="#0000ff">void</font> rolldown() {}
}
<font color="#0000ff">class</font> Door {
<font color="#0000ff">public</font> Window window = <font color="#0000ff">new</font> Window();
<font color="#0000ff">public</font> <font color="#0000ff">void</font> open() {}
<font color="#0000ff">public</font> <font color="#0000ff">void</font> close() {}
}
<font color="#0000ff">public</font> <font color="#0000ff">class</font> Car {
<font color="#0000ff">public</font> Engine engine = <font color="#0000ff">new</font> Engine();
<font color="#0000ff">public</font> Wheel[] wheel = <font color="#0000ff">new</font> Wheel[4];
<font color="#0000ff">public</font> Door left = <font color="#0000ff">new</font> Door(),
right = <font color="#0000ff">new</font> Door(); <font color="#009900">// 2-door</font>
Car() {
<font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i < 4; i++)
wheel[i] = <font color="#0000ff">new</font> Wheel();
}
<font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
Car car = <font color="#0000ff">new</font> Car();
car.left.window.rollup();
car.wheel[0].inflate(72);
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Because
the composition of a car is part of the analysis of the problem (and not simply
part of the underlying design), making the members public assists the client
programmer’s understanding of how to use the class and requires less code
complexity for the creator of the class.
</FONT><P></DIV><DIV ALIGN=LEFT><A NAME="Index484"></A><A NAME="Index485"></A><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
you inherit, you take an existing class and make a special version of it. In
general, this means that you’re taking a general-purpose class and
specializing it for a particular need. With a little thought, you’ll see
that it would make no sense to compose a car using a vehicle object – a
car doesn’t contain a vehicle, it
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>is</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
a vehicle. The <A NAME="Index486"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>is-a</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
relationship is expressed with inheritance, and the <A NAME="Index487"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>has-a</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
relationship is expressed with composition.
</FONT><a name="this5"></a><a name="_Toc305593256"></a><a name="_Toc305628728"></a><a name="_Toc312374027"></a><a name="_Toc375545313"></a><a name="_Toc408018516"></a><P></DIV>
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