tij0065.html

学习java的经典书籍

</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>reusing
the interface.
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(The implementation comes along for free, but that part isn&#8217;t the primary
point.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
seen in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>scrub(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it&#8217;s possible to take a method that&#8217;s been defined in the base
class and modify it. In this case, you might want to call the method from the
base class inside the new version. But inside 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>scrub(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
you cannot simply call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>scrub(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
since that would produce a recursive call, which isn&#8217;t what you want. To
solve this problem Java has the <A NAME="Index444"></A>keyword
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>super</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that refers to the &#8220;<A NAME="Index445"></A>superclass&#8221;
that the current class has been inherited from. Thus the expression 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>super.scrub(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
calls the base-class version of the method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>scrub(&#160;)</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">When
inheriting you&#8217;re not restricted to using the methods of the base class.
You can also add new methods to the derived class exactly the way you put any
method in a class: just define it. The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>extends
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">keyword
suggests that you are going to add new methods to the base-class interface, and
the method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>foam(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is an example of this.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
</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">you
can see that for a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Detergent</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object you can call all the methods that are available in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cleanser</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
as well as in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Detergent
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">(i.e.
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>foam(&#160;))</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><a name="_Toc375545308"></a><a name="_Toc408018511"></a><P></DIV>
<A NAME="Heading183"></A><H3 ALIGN=LEFT>
Initializing
the base class
<P><A NAME="Index446"></A><A NAME="Index447"></A><A NAME="Index448"></A></H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Since
there are now two classes involved &#8211; the base class and the <A NAME="Index449"></A><A NAME="Index450"></A>derived
class &#8211; instead of just one, it can be a bit confusing to try to imagine
the resulting object produced by a derived class. From the outside, it looks
like the new class has the same interface as the base class and maybe some
additional methods and fields. But inheritance doesn&#8217;t just copy the
interface of the base class. When you create an object of the derived class, it
contains within it a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>subobject</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of the base class. This <A NAME="Index451"></A><A NAME="Index452"></A>subobject
is the same as if you had created an object of the base class by itself.
It&#8217;s just that, from the outside, the subobject of the base class is
wrapped within the derived-class object.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Of
course, it&#8217;s essential that the base-class subobject be initialized
correctly and there&#8217;s only one way to guarantee that: perform the
initialization in the constructor, by calling the base-class constructor, which
has all the appropriate knowledge and privileges to perform the base-class
initialization. Java automatically inserts calls to the base-class constructor
in the derived-class constructor. The following example shows this working with
three levels of inheritance:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Cartoon.java</font>
<font color="#009900">// Constructor calls during inheritance</font>

<font color="#0000ff">class</font> Art {
  Art() {
    System.out.println("Art constructor");
  }
}

<font color="#0000ff">class</font> Drawing <font color="#0000ff">extends</font> Art {
  Drawing() {
    System.out.println("Drawing constructor");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Cartoon <font color="#0000ff">extends</font> Drawing {
  Cartoon() {
    System.out.println("Cartoon constructor");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Cartoon x = <font color="#0000ff">new</font> Cartoon();
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
output for this program shows the automatic calls:
</FONT><P></DIV>

<font color="#990000"><PRE>Art constructor
Drawing constructor
Cartoon constructor </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that the construction happens from the base &#8220;outward,&#8221; so
the base class is initialized before the derived-class constructors can access
it.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Even
if you don&#8217;t create a constructor for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cartoon(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the compiler will <A NAME="Index453"></A>synthesize
a default constructor for you that calls the base class constructor.
</FONT><P></DIV>
<A NAME="Heading184"></A><H4 ALIGN=LEFT>
Constructors
with arguments
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
above example has default <A NAME="Index454"></A>constructors;
that is, they don&#8217;t have any arguments. It&#8217;s easy for the compiler
to call these because there&#8217;s no question about what arguments to pass.
If your class doesn&#8217;t have default arguments or if you want to call a
base-class constructor that has an argument you must explicitly write the calls
to the base-class constructor using the <A NAME="Index455"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>super</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword and the appropriate argument list:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Chess.java</font>
<font color="#009900">// Inheritance, constructors and arguments</font>

<font color="#0000ff">class</font> Game {
  Game(<font color="#0000ff">int</font> i) {
    System.out.println("Game constructor");
  }
}

<font color="#0000ff">class</font> BoardGame <font color="#0000ff">extends</font> Game {
  BoardGame(<font color="#0000ff">int</font> i) {
    <font color="#0000ff">super</font>(i);
    System.out.println("BoardGame constructor");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Chess <font color="#0000ff">extends</font> BoardGame {
  Chess() {
    <font color="#0000ff">super</font>(11);
    System.out.println("Chess constructor");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Chess x = <font color="#0000ff">new</font> Chess();
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you don&#8217;t call the base-class constructor in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BoardGame(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the compiler will complain that it can&#8217;t find a constructor of the form 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Game(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
In addition, the call to the base-class constructor 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>must</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
be the first thing you do in the derived-class constructor. (The compiler will
remind you if you get it wrong.)
</FONT><P></DIV>
<A NAME="Heading185"></A><H4 ALIGN=LEFT>
Catching
base constructor exceptions
<P><A NAME="Index456"></A><A NAME="Index457"></A><A NAME="Index458"></A></H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
just noted, the compiler forces you to place the base-class constructor call
first in the body of the derived-class constructor. This means nothing else can
appear before it. As you&#8217;ll see in Chapter 9, this also prevents a
derived-class constructor from catching any exceptions that come from a base
class. This can be inconvenient at times.
</FONT><a name="_Toc305593254"></a><a name="_Toc305628726"></a><a name="_Toc312374019"></a><a name="_Toc375545309"></a><a name="_Toc408018512"></a><P></DIV>

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