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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<a href="tij_c.html">Contents</a> | <a href="tij0079.html">Prev</a> | <a href="tij0081.html">Next</a>
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<H2 ALIGN=LEFT>
Interfaces</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
<A NAME="Index588"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword takes the abstract
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">concept
one step further. You could think of it as a &#8220;pure&#8221; abstract
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">class.
It allows the creator to establish the form for a class: method names, argument
lists and return types, but no method bodies. An 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
can also contain data members of primitive types, but these are implicitly <A NAME="Index589"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index590"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
An 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
provides only a form, but no <A NAME="Index591"></A>implementation.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">An
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
says: &#8220;This is what all classes that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>implement</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
this particular interface will look like.&#8221; Thus, any code that uses a
particular 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
knows what methods might be called for that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and that&#8217;s all. So the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is used to establish a &#8220;protocol&#8221; between classes. (Some
object-oriented programming languages have a keyword called <A NAME="Index592"></A><A NAME="Index593"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>protocol</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to do the same thing.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">To
create an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
use the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword instead of the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword. Like a class, you can add the <A NAME="Index594"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword before the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">keyword
(but only if that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is defined in a file of the same name) or leave it off to give &#8220;<A NAME="Index595"></A>friendly&#8221;
status.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">To
make a class that conforms to a particular 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(or group of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s)
use the <A NAME="Index596"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>implements</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword. You&#8217;re saying &#8220;The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is what it looks like and here&#8217;s how it 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>works</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.&#8221;
Other than that, it bears a strong resemblance to inheritance. The diagram for
the instrument example shows this:
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Once
you&#8217;ve implemented an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
that implementation becomes an ordinary class that can be extended in the
regular way.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can choose to explicitly declare the method declarations in an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
But they are 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
even if you don&#8217;t say it. So when you 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>implement</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the methods from the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
must be defined as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
Otherwise they would default to &#8220;friendly&#8221; and you&#8217;d be
restricting the accessibility of a method during inheritance, which is not
allowed by the Java compiler.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see this in the modified version of the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
example. Note that every method in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is strictly a declaration, which is the only thing the compiler allows. In
addition, none of the methods in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument5</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are declared as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
but they&#8217;re automatically 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
anyway:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Music5.java</font>
<font color="#009900">// Interfaces</font>
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">interface</font> Instrument5 {
  <font color="#009900">// Compile-time constant:</font>
  <font color="#0000ff">int</font> i = 5; <font color="#009900">// static &amp; final</font>
  <font color="#009900">// Cannot have method definitions:</font>
  <font color="#0000ff">void</font> play(); <font color="#009900">// Automatically public</font>
  String what();
  <font color="#0000ff">void</font> adjust();
}

<font color="#0000ff">class</font> Wind5 <font color="#0000ff">implements</font> Instrument5 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Wind5.play()");
  }
  <font color="#0000ff">public</font> String what() { <font color="#0000ff">return</font> "Wind5"; }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> adjust() {}
}

<font color="#0000ff">class</font> Percussion5 <font color="#0000ff">implements</font> Instrument5 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Percussion5.play()");
  }
  <font color="#0000ff">public</font> String what() { <font color="#0000ff">return</font> "Percussion5"; }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> adjust() {}
}

<font color="#0000ff">class</font> Stringed5 <font color="#0000ff">implements</font> Instrument5 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Stringed5.play()");
  }
  <font color="#0000ff">public</font> String what() { <font color="#0000ff">return</font> "Stringed5"; }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> adjust() {}
}

<font color="#0000ff">class</font> Brass5 <font color="#0000ff">extends</font> Wind5 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Brass5.play()");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> adjust() { 
    System.out.println("Brass5.adjust()");
  }
}

<font color="#0000ff">class</font> Woodwind5 <font color="#0000ff">extends</font> Wind5 {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> play() {
    System.out.println("Woodwind5.play()");
  }
  <font color="#0000ff">public</font> String what() { <font color="#0000ff">return</font> "Woodwind5"; }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Music5 {
  <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(Instrument5 i) {
    <font color="#009900">// ...</font>
    i.play();
  }
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> tuneAll(Instrument5[] 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) {
    Instrument5[] orchestra = <font color="#0000ff">new</font> Instrument5[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> Wind5();
    orchestra[i++] = <font color="#0000ff">new</font> Percussion5();
    orchestra[i++] = <font color="#0000ff">new</font> Stringed5();
    orchestra[i++] = <font color="#0000ff">new</font> Brass5();
    orchestra[i++] = <font color="#0000ff">new</font> Woodwind5();
    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">The
rest of the code works the same. It doesn&#8217;t matter if you are <A NAME="Index597"></A>upcasting
to a &#8220;regular&#8221; class called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument5</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument5</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
or to an <A NAME="Index598"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument5</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The behavior is the same. In fact, you can see in 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 that there isn&#8217;t any evidence about whether 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Instrument5</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is a &#8220;regular&#8221; class, an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class or an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This is the intent: Each approach gives the programmer different control over
the way objects are created and used.
</FONT><a name="_Toc375545336"></a><a name="_Toc408018539"></a><P></DIV>
<A NAME="Heading217"></A><H3 ALIGN=LEFT>
&#8220;Multiple
inheritance&#8221; in Java
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
isn&#8217;t simply a &#8220;more pure&#8221; form of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class. It has a higher purpose than that. Because an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has no implementation at all &#8211; that is, there is no storage associated
with an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8211;</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
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