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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="tij0064.html">Prev</a> | <a href="tij0066.html">Next</a>
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<H2 ALIGN=LEFT>
Inheritance
syntax
<P><A NAME="Index433"></A></H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Inheritance
is such an integral part of Java (and OOP languages in general) that it was
introduced in Chapter 1 and has been used occasionally in chapters before this
one because certain situations required it. In addition, you&#8217;re always
doing inheritance when you create a class, because if you don&#8217;t say
otherwise you inherit from Java&#8217;s standard root class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
syntax for composition is obvious, but to perform inheritance there&#8217;s a
distinctly different form. When you inherit, you say &#8220;This new class is
like that old class.&#8221; You state this in code by giving the name of the
class as usual, but before the opening brace of the class body, put the <A NAME="Index434"></A>keyword
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>extends</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
followed by the name of the <A NAME="Index435"></A><A NAME="Index436"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>base
class
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
When you do this, you automatically get all the data members and methods in the
base class. Here&#8217;s an example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Detergent.java</font>
<font color="#009900">// Inheritance syntax &amp; properties</font>

<font color="#0000ff">class</font> Cleanser {
  <font color="#0000ff">private</font> String s = <font color="#0000ff">new</font> String("Cleanser");
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> append(String a) { s += a; }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> dilute() { append(" dilute()"); }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> apply() { append(" apply()"); }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> scrub() { append(" scrub()"); }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> print() { System.out.println(s); }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Cleanser x = <font color="#0000ff">new</font> Cleanser();
    x.dilute(); x.apply(); x.scrub();
    x.print();
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Detergent <font color="#0000ff">extends</font> Cleanser {
  <font color="#009900">// Change a method:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> scrub() {
    append(" Detergent.scrub()");
    <font color="#0000ff">super</font>.scrub(); <font color="#009900">// Call base-class version</font>
  }
  <font color="#009900">// Add methods to the interface:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> foam() { append(" foam()"); }
  <font color="#009900">// Test the new class:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Detergent x = <font color="#0000ff">new</font> Detergent();
    x.dilute();
    x.apply();
    x.scrub();
    x.foam();
    x.print();
    System.out.println("Testing base <font color="#0000ff">class</font>:");
    Cleanser.main(args);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
demonstrates a number of features. First, in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>append(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
are concatenated to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>s</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
using the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+=</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
operator, which is one of the operators (along with &#8216;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;)
that the Java designers &#8220;overloaded&#8221; to work with <A NAME="Index437"></A><A NAME="Index438"></A><A NAME="Index439"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Second,
both 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Detergent</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
contain a <A NAME="Index440"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method. You can create a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for each one of your classes, and it&#8217;s often recommended to code this way
so that your test code is wrapped in with the class. Even if you have a lot of
classes in a program only the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class invoked on the command line will be called. (And you can have only one 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class per file.) So in this case, when you say 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>java
Detergent
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Detergent.main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
will be called. But you can also say 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>java
Cleanser 
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">to
invoke 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser.main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
even though 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is not a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class. This technique of putting a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in each class allows easy <A NAME="Index441"></A><A NAME="Index442"></A>unit
testing for each class. And you don&#8217;t need to remove the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
when you&#8217;re finished testing; you can leave it in for later testing.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here,
you can see that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Detergent.main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
calls 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser.main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
explicitly.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It&#8217;s
important that all of the methods in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">are
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Remember that if you leave off any access specifier the member defaults to
&#8220;friendly,&#8221; which allows access only to package members. Thus,
within this package, anyone could use those methods if there were no access
specifier. 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Detergent</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
would have no trouble, for example. However, if a class from some other package
were to inherit 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
it could access only 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">members.
So to plan for inheritance, as a general rule make all fields 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">and
all methods 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public.
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">(</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">members
also allow access by derived classes; you&#8217;ll learn about this later.) Of
course, in particular cases you must make adjustments, but this is a useful
guideline.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has a set of methods in its interface: 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>append(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>dilute(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>apply(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>scrub(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>print(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Because 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Detergent</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>derived
from
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(via the <A NAME="Index443"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>extends</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword) it automatically gets all these methods in its interface, even though
you don&#8217;t see them all explicitly defined in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Detergent</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
You can think of inheritance, then, as
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