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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>
Upcasting<P><A NAME="Index495"></A></H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
most important aspect of inheritance is not that it provides methods for the
new class. It&#8217;s the relationship expressed between the new class and the
base class. This <A NAME="Index496"></A>relationship
can be summarized by saying &#8220;The new class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>is
a type of
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the existing class.&#8221; 
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
description is not just a fanciful way of explaining inheritance &#8211;
it&#8217;s supported directly by the language. As an example, consider a base
class called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that represents musical instruments and a derived class called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Because inheritance means that all of the methods in the base class are also
available in the derived class, any message you can send to the base class can
also be sent to the derived class. If the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class has a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>play(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method, so will 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
instruments. This means we can accurately say that a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is also a type of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The following example shows how the compiler supports this notion:
<a name="WIND_CPP"></a></FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Wind.java</font>
<font color="#009900">// Inheritance &amp; upcasting</font>
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">class</font> Instrument {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {}
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> tune(Instrument i) {
    <font color="#009900">// ...</font>
    i.play();
  }
}

<font color="#009900">// Wind objects are instruments</font>
<font color="#009900">// because they have the same interface:</font>
<font color="#0000ff">class</font> Wind <font color="#0000ff">extends</font> Instrument {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Wind flute = <font color="#0000ff">new</font> Wind();
    Instrument.tune(flute); <font color="#009900">// Upcasting</font>
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">What&#8217;s
interesting in this example is the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method, which accepts an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle. However, in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is called by giving it a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle. Given that Java is particular about type checking, it seems strange
that a method that accepts one type will readily accept another type, until you
realize that a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is also an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object, and there&#8217;s no method that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
could call for an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that isn&#8217;t also in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Inside 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the code works for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and anything derived from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and the act of converting a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle into an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle is called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>upcasting</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><a name="_Toc312374032"></a><a name="_Toc375545316"></a><a name="_Toc408018519"></a><P></DIV>
<A NAME="Heading194"></A><H3 ALIGN=LEFT>
Why
&#8220;upcasting&#8221;?
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
reason for the term is historical and is based on the way class inheritance
diagrams <A NAME="Index497"></A><A NAME="Index498"></A><A NAME="Index499"></A>have
traditionally been drawn with the root at the top of the page, growing
downward. (Of course, you can draw your diagrams any way you find helpful.) The
inheritance diagram for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind.java</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is then: 
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Casting
from derived to base moves 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>up</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
on the inheritance diagram, so it&#8217;s commonly referred to as upcasting.
Upcasting is always safe because you&#8217;re going from a more specific type
to a more general type. That is, the derived class is a superset of the base
class. It might contain more methods than the base class, but it must contain 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>at
least
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the methods in the base class. The only thing that can occur to the class
interface during the upcast is that it can lose methods, not gain them. This is
why the compiler allows upcasting without any explicit casts or other special
notation.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can also perform the reverse of upcasting, called <A NAME="Index500"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>downcasting</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
but this involves a dilemma that is the subject of Chapter 11.
</FONT><a name="_Toc312374033"></a><P></DIV>
<A NAME="Heading195"></A><H4 ALIGN=LEFT>
Composition
vs. inheritance revisited
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
object-oriented programming, the most likely way that you&#8217;ll create and
use code is by simply packaging data and methods together into a class, and
using objects of that class. Occasionally, you&#8217;ll use existing classes to
build new classes with composition. Even less frequently than that you&#8217;ll
use inheritance. So although inheritance gets a lot of emphasis while learning
OOP, it doesn&#8217;t mean that you should use it everywhere you possibly can.
On the contrary, you should use it sparingly, only when it&#8217;s clear that
inheritance is useful. <A NAME="Index501"></A><A NAME="Index502"></A>One
of the clearest ways to determine whether you should use composition or
inheritance is to ask whether you&#8217;ll ever need to upcast from your new
class to the base class. If you must upcast, then inheritance is necessary, but
if you don&#8217;t need to upcast, then you should look closely at whether you
need inheritance. The next chapter (polymorphism) provides one of the most
compelling reasons for upcasting, but if you remember to ask &#8220;Do I need
to upcast?&#8221;, you&#8217;ll have a good tool for deciding between
composition and inheritance.
</FONT><a name="_Toc375545317"></a><a name="_Toc408018520"></a><a name="_Toc305593260"></a><a name="_Toc305628732"></a><a name="_Toc312374036"></a><P></DIV>

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