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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</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
Chapter 6 you saw how an object can be used as its own type or as an object of
its base type. Taking an object handle and treating it as the handle of the
base type is called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>upcasting<A NAME="Index557"></A></I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
because of the way inheritance trees are drawn with the base class at the top.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
also saw a problem arise, which is embodied in the following: (See page 
<A HREF=" PAGE#Running_programs">97</A>
if you have trouble executing this program.)
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Music.java </font>
<font color="#009900">// Inheritance &amp; upcasting</font>
<font color="#0000ff">package</font> c07;

<font color="#0000ff">class</font> Note {
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> value;
  <font color="#0000ff">private</font> Note(<font color="#0000ff">int</font> val) { value = val; }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">final</font> Note
    middleC = <font color="#0000ff">new</font> Note(0), 
    cSharp = <font color="#0000ff">new</font> Note(1),
    cFlat = <font color="#0000ff">new</font> Note(2);
} <font color="#009900">// Etc.</font>

<font color="#0000ff">class</font> Instrument {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play(Note n) {
    System.out.println("Instrument.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="#009900">// Redefine interface method:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play(Note n) {
    System.out.println("Wind.play()");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Music {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> tune(Instrument i) {
    <font color="#009900">// ...</font>
    i.play(Note.middleC);
  }
  <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();
    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">The
method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Music.tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
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,
but also 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">.
In 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you can see this happening as 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 is passed to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
with no cast necessary. This is acceptable; the interface in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
must exist in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
because 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is inherited from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Upcasting from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wind</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
may &#8220;narrow&#8221; that interface, but it cannot make it anything less
than the full interface to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><a name="_Toc375545328"></a><a name="_Toc408018531"></a><P></DIV>
<A NAME="Heading209"></A><H3 ALIGN=LEFT>
Why
upcast?
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
program might seem strange to you. Why should anyone intentionally 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>forget</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the type of an object? This is what happens when you upcast, and it seems like
it could be much more straightforward if 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
simply takes 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 as its argument. This brings up an essential point: If you did that,
you&#8217;d need to write a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for every 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">
in your system. Suppose we follow this reasoning and add 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Stringed</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Brass</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
instruments:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Music2.java </font>
<font color="#009900">// Overloading instead of upcasting</font>

<font color="#0000ff">class</font> Note2 {
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> value;
  <font color="#0000ff">private</font> Note2(<font color="#0000ff">int</font> val) { value = val; }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">final</font> Note2
    middleC = <font color="#0000ff">new</font> Note2(0), 
    cSharp = <font color="#0000ff">new</font> Note2(1),
    cFlat = <font color="#0000ff">new</font> Note2(2);
} <font color="#009900">// Etc.</font>

<font color="#0000ff">class</font> Instrument2 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play(Note2 n) {
    System.out.println("Instrument2.play()");
  }
}

<font color="#0000ff">class</font> Wind2 <font color="#0000ff">extends</font> Instrument2 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play(Note2 n) {
    System.out.println("Wind2.play()");
  }
}

<font color="#0000ff">class</font> Stringed2 <font color="#0000ff">extends</font> Instrument2 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play(Note2 n) {
    System.out.println("Stringed2.play()");
  }
}

<font color="#0000ff">class</font> Brass2 <font color="#0000ff">extends</font> Instrument2 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play(Note2 n) {
    System.out.println("Brass2.play()");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Music2 {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> tune(Wind2 i) {
    i.play(Note2.middleC);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> tune(Stringed2 i) {
    i.play(Note2.middleC);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> tune(Brass2 i) {
    i.play(Note2.middleC);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Wind2 flute = <font color="#0000ff">new</font> Wind2();
    Stringed2 violin = <font color="#0000ff">new</font> Stringed2();
    Brass2 frenchHorn = <font color="#0000ff">new</font> Brass2();
    tune(flute); <font color="#009900">// No upcasting</font>
    tune(violin);
    tune(frenchHorn);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
works, but there&#8217;s a major drawback: You must write type-specific methods
for each new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class you add. This means more programming in the first place, but it also
means that if you want to add a new method like 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>tune(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or a new 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">,
you&#8217;ve got a lot of work to do. Add the fact that the compiler
won&#8217;t give you any error messages if you forget to overload one of your
methods and the whole process of working with types becomes unmanageable.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Wouldn&#8217;t
it be much nicer if you could just write a single method that takes the <A NAME="Index558"></A><A NAME="Index559"></A>base
class as its argument, and not any of the specific derived classes? That is,
wouldn&#8217;t it be nice if you could forget that there are <A NAME="Index560"></A><A NAME="Index561"></A>derived
classes, and write your code to talk only to the base class?
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">That&#8217;s
exactly what polymorphism allows you to do. However, most programmers (who come
from a procedural programming background) have a bit of trouble with the way
polymorphism works.
</FONT><a name="_Toc305593264"></a><a name="_Toc305628736"></a><a name="_Toc312374041"></a><a name="_Toc375545329"></a><a name="_Toc408018532"></a><P></DIV>

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