tij0088.html

学习java的经典书籍

</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Thus,
all you need to do to make objects of your class print out is to override the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method, as shown in the following example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: WorksAnyway.java</font>
<font color="#009900">// In special cases, things just seem</font>
<font color="#009900">// to work correctly.</font>
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">class</font> Mouse {
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> mouseNumber;
  Mouse(<font color="#0000ff">int</font> i) {
    mouseNumber = i;
  }
  <font color="#009900">// Magic method:</font>
  <font color="#0000ff">public</font> String toString() {
    <font color="#0000ff">return</font> "This is Mouse #" + mouseNumber;
  }
  <font color="#0000ff">void</font> print(String msg) {
    <font color="#0000ff">if</font>(msg != <font color="#0000ff">null</font>) System.out.println(msg);
    System.out.println(
      "Mouse number " + mouseNumber);
  }
}

<font color="#0000ff">class</font> MouseTrap {
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> caughtYa(Object m) {
    Mouse mouse = (Mouse)m; <font color="#009900">// Cast from Object</font>
    mouse.print("Caught one!");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> WorksAnyway {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Vector mice = <font color="#0000ff">new</font> Vector();
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 3; i++)
      mice.addElement(<font color="#0000ff">new</font> Mouse(i));
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; mice.size(); i++) {
      <font color="#009900">// No cast necessary, automatic call</font>
      <font color="#009900">// to Object.toString():</font>
      System.out.println(
        "Free mouse: " + mice.elementAt(i));
      MouseTrap.caughtYa(mice.elementAt(i));
    }
  }
} <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 the redefinition of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Mouse</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
In the second 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>for</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
loop 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 find the statement:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">System.out.println("Free
mouse: " + mice.elementAt(i));
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">After
the &#8216;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;
sign the compiler expects to see a <A NAME="Index753"></A><A NAME="Index754"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>elementAt(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
produces an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so to get the desired 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the compiler implicitly calls 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Unfortunately, you can work this kind of magic only with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">;
it isn&#8217;t available for any other type.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
second approach to hiding the cast has been placed inside 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Mousetrap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>caughtYa(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method accepts not a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Mouse</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
but an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object,</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
which it then casts to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Mouse</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This is quite presumptuous, of course, since by accepting an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
anything could be passed to the method. However, if the cast is incorrect
&#8211; if you passed the wrong type &#8211; you&#8217;ll get an exception at
run-time. This is not as good as compile-time checking but it&#8217;s still
robust. Note that in the use of this method:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">MouseTrap.caughtYa(mice.elementAt(i));</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">no
cast is necessary.
</FONT><P></DIV>
<A NAME="Heading248"></A><H4 ALIGN=LEFT>
Making
a type-conscious Vector
<P><A NAME="Index755"></A><A NAME="Index756"></A></H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
might not want to give up on this issue just yet. A more ironclad solution is
to create a new class using the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
such that it will accept only your type and produce only your type:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: GopherVector.java</font>
<font color="#009900">// A type-conscious Vector</font>
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">class</font> Gopher {
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> gopherNumber;
  Gopher(<font color="#0000ff">int</font> i) {
    gopherNumber = i;
  }
  <font color="#0000ff">void</font> print(String msg) {
    <font color="#0000ff">if</font>(msg != <font color="#0000ff">null</font>) System.out.println(msg);
    System.out.println(
      "Gopher number " + gopherNumber);
  }
}

<font color="#0000ff">class</font> GopherTrap {
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> caughtYa(Gopher g) {
    g.print("Caught one!");
  }
}

<font color="#0000ff">class</font> GopherVector {
  <font color="#0000ff">private</font> Vector v = <font color="#0000ff">new</font> Vector();
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> addElement(Gopher m) {
    v.addElement(m);
  }
  <font color="#0000ff">public</font> Gopher elementAt(<font color="#0000ff">int</font> index) {
    <font color="#0000ff">return</font> (Gopher)v.elementAt(index);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">int</font> size() { <font color="#0000ff">return</font> v.size(); }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    GopherVector gophers = <font color="#0000ff">new</font> GopherVector();
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 3; i++)
      gophers.addElement(<font color="#0000ff">new</font> Gopher(i));
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; gophers.size(); i++)
      GopherTrap.caughtYa(gophers.elementAt(i));
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
is similar to the previous example, except that the new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>GopherVector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class has a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">member
of type 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(inheriting from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
tends to be frustrating, for reasons you&#8217;ll see later), and methods just
like 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
However, it doesn&#8217;t accept and produce generic 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s,
only 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Gopher</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Because
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>GopherVector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
will accept only a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Gopher</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
if you were to say:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">gophers.addElement(new
Pigeon());
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">you
would get an error message 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>at
compile time
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This approach, while more tedious from a coding standpoint, will tell you
immediately if you&#8217;re using a type improperly.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that no cast is necessary when using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>elementAt(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
&#8211; it&#8217;s always a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Gopher</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV>
<A NAME="Heading249"></A><H4 ALIGN=LEFT>
Parameterized
types
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
kind of problem isn&#8217;t isolated &#8211; there are numerous cases in which
you need to create new types based on other types, and in which it is useful to
have specific type information at compile-time. This is the concept of a <A NAME="Index757"></A><A NAME="Index758"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>parameterized
type
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
In C++, this is directly supported by the language in <A NAME="Index759"></A><A NAME="Index760"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>templates</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
At one point, Java had reserved the keyword <A NAME="Index761"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>generic</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to someday support parameterized types, but it&#8217;s uncertain if this will
ever occur.
</FONT><a name="_Toc375545352"></a><a name="_Toc408018568"></a><P></DIV>

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