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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>
Abstract
classes 
<P>and
methods
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
all the instrument examples, the methods in the base class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
were always &#8220;dummy&#8221; methods. If these methods are ever called,
you&#8217;ve done something wrong. That&#8217;s because the intent of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is to create a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>common
interface
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for all the classes derived from it.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
only reason to establish this common interface<A NAME="Index579"></A><A NAME="Index580"></A>
is so it can be expressed differently for each different subtype. It
establishes a basic form, so you can say what&#8217;s in common with all the
derived classes. Another way of saying this is to call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>abstract
base class
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
<A NAME="Index581"></A><A NAME="Index582"></A><A NAME="Index583"></A>(or
simply an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>abstract
class
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">).
You create an abstract class when you want to manipulate a set of classes
through this common interface. All derived-class methods that match the
signature of the base-class declaration will be called using the dynamic
binding mechanism. (However, as seen in the last section, if the method&#8217;s
name is the same as the base class but the arguments are different,
you&#8217;ve got overloading, which probably isn&#8217;t what you want.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you have an abstract class like 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
objects of that class almost always have no meaning. That is, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is meant to express only the interface, and not a particular implementation, so
creating 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 makes no sense, and you&#8217;ll probably want to prevent the user from
doing it. This can be accomplished by making all the methods in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
print error messages, but this delays the information until run-time and
requires reliable exhaustive testing on the user&#8217;s part. It&#8217;s
always better to catch problems at compile time.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Java
provides a mechanism for doing this called the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>abstract
method
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This is a method that is incomplete; it has only a declaration and no method
body. Here is the syntax for an abstract method declaration:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">abstract
void X();
</FONT></TT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
class containing abstract methods is called an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>abstract
class
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If a class contains one or more abstract methods, the class must be qualified as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(Otherwise, the compiler gives you an error message.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
an abstract class is incomplete, what is the compiler supposed to do when
someone tries to make an object of that class? It cannot safely create an
object of an abstract class, so you get an error message from the compiler.
This way the compiler ensures the purity of the abstract class, and you
don&#8217;t need to worry about misusing it.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you <A NAME="Index584"></A><A NAME="Index585"></A><A NAME="Index586"></A>inherit
from an abstract class and you want to make objects of the new type, you must
provide method definitions for all the abstract methods in the base class. If
you don&#8217;t (and you may choose not to), then the derived class is also
abstract and the compiler will force you to qualify 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>that</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class with the <A NAME="Index587"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It&#8217;s
possible to declare a class as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>without</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
including any 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods. This is useful when you&#8217;ve got a class in which it doesn&#8217;t
make sense to have any 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods, and yet you want to prevent any instances of that class.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">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 can easily be turned into an abstract class. Only some of the methods
will be abstract, since making a class abstract doesn&#8217;t force you to make
all the methods abstract.
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here&#8217;s
what it looks like:
</FONT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here&#8217;s
the orchestra example modified to use 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
classes and methods:
<a name="WIND5_CPP"></a></FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Music4.java</font>
<font color="#009900">// Abstract classes and methods</font>
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">abstract</font> <font color="#0000ff">class</font> Instrument4 {
  <font color="#0000ff">int</font> i; <font color="#009900">// storage allocated for each</font>
  <font color="#0000ff">public</font> <font color="#0000ff">abstract</font> <font color="#0000ff">void</font> play();
  <font color="#0000ff">public</font> String what() {
    <font color="#0000ff">return</font> "Instrument4";
  }
  <font color="#0000ff">public</font> <font color="#0000ff">abstract</font> <font color="#0000ff">void</font> adjust();
}

<font color="#0000ff">class</font> Wind4 <font color="#0000ff">extends</font> Instrument4 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Wind4.play()");
  }
  <font color="#0000ff">public</font> String what() { <font color="#0000ff">return</font> "Wind4"; }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> adjust() {}
}

<font color="#0000ff">class</font> Percussion4 <font color="#0000ff">extends</font> Instrument4 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Percussion4.play()");
  }
  <font color="#0000ff">public</font> String what() { <font color="#0000ff">return</font> "Percussion4"; }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> adjust() {}
}

<font color="#0000ff">class</font> Stringed4 <font color="#0000ff">extends</font> Instrument4 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Stringed4.play()");
  }
  <font color="#0000ff">public</font> String what() { <font color="#0000ff">return</font> "Stringed4"; }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> adjust() {}
}

<font color="#0000ff">class</font> Brass4 <font color="#0000ff">extends</font> Wind4 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Brass4.play()");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> adjust() { 
    System.out.println("Brass4.adjust()");
  }
}

<font color="#0000ff">class</font> Woodwind4 <font color="#0000ff">extends</font> Wind4 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Woodwind4.play()");
  }
  <font color="#0000ff">public</font> String what() { <font color="#0000ff">return</font> "Woodwind4"; }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Music4 {
  <font color="#009900">// Doesn't care about type, so new types</font>
  <font color="#009900">// added to the system still work right:</font>
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> tune(Instrument4 i) {
    <font color="#009900">// ...</font>
    i.play();
  }
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> tuneAll(Instrument4[] e) {
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; e.length; i++)
      tune(e[i]);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Instrument4[] orchestra = <font color="#0000ff">new</font> Instrument4[5];
    <font color="#0000ff">int</font> i = 0;
    <font color="#009900">// Upcasting during addition to the array:</font>
    orchestra[i++] = <font color="#0000ff">new</font> Wind4();
    orchestra[i++] = <font color="#0000ff">new</font> Percussion4();
    orchestra[i++] = <font color="#0000ff">new</font> Stringed4();
    orchestra[i++] = <font color="#0000ff">new</font> Brass4();
    orchestra[i++] = <font color="#0000ff">new</font> Woodwind4();
    tuneAll(orchestra);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that there&#8217;s really no change except in the base class.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It&#8217;s
helpful to create 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">classes
and methods because they make the abstractness of a class explicit and tell
both the user and the compiler how it was intended to be used.
</FONT><a name="_Toc375545335"></a><a name="_Toc408018538"></a><P></DIV>

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