tij312.htm

这也是我们java老师给我们的thinking in java的一些资料

        <font color=#004488>"(Dog|Pug|Cat|Rodent|Gerbil|Hamster)"</font>,
        pets.length),
      <font color=#0000ff>new</font> TestExpression(<font color=#004488>"%% class c10\\."</font> +
        <font color=#004488>"(Pet|Dog|Pug|Cat|Rodent|Gerbil|Hamster) : \\d+"</font>,
        petTypes.length)
    });
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>You can see that the <b>isInstance(&#160;)</b> method has eliminated the need for the <b>instanceof</b> expressions. In addition, this means that you can add new types of pets simply by changing the <b>petTypes</b> array; the rest of the program does not need modification (as it did when using the <b>instanceof</b> expressions). <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap12_1855" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h4>
<a name="Heading9936"></a><b>instanceof</b> vs. <b>Class</b>
equivalence<br></h4>
<p><a name="Index915"></a>When querying for type information, there&#146;s an important difference between either form of <b>instanceof</b> (that is, <b>instanceof</b> or <b>isInstance(&#160;)</b>, which produce equivalent results) and the direct comparison of the <b>Class</b> objects. Here&#146;s an example that demonstrates the difference:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c10:FamilyVsExactType.java</font>
<font color=#009900>// The difference between instanceof and class</font>
<font color=#0000ff>package</font> c10;
<font color=#0000ff>import</font> com.bruceeckel.simpletest.*;

<font color=#0000ff>class</font> Base {}
<font color=#0000ff>class</font> Derived <font color=#0000ff>extends</font> Base {}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> FamilyVsExactType {
  <font color=#0000ff>private</font> <font color=#0000ff>static</font> Test monitor = <font color=#0000ff>new</font> Test();
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> test(Object x) {
    System.out.println(<font color=#004488>"Testing x of type "</font> +
      x.getClass());
    System.out.println(<font color=#004488>"x instanceof Base "</font> +
      (x <font color=#0000ff>instanceof</font> Base));
    System.out.println(<font color=#004488>"x instanceof Derived "</font> +
      (x <font color=#0000ff>instanceof</font> Derived));
    System.out.println(<font color=#004488>"Base.isInstance(x) "</font> +
      Base.<font color=#0000ff>class</font>.isInstance(x));
    System.out.println(<font color=#004488>"Derived.isInstance(x) "</font> +
      Derived.<font color=#0000ff>class</font>.isInstance(x));
    System.out.println(<font color=#004488>"x.getClass() == Base.class "</font> +
      (x.getClass() == Base.<font color=#0000ff>class</font>));
    System.out.println(<font color=#004488>"x.getClass() == Derived.class "</font> +
      (x.getClass() == Derived.<font color=#0000ff>class</font>));
    System.out.println(<font color=#004488>"x.getClass().equals(Base.class)) "</font>+
      (x.getClass().equals(Base.<font color=#0000ff>class</font>)));
    System.out.println(
      <font color=#004488>"x.getClass().equals(Derived.class)) "</font> +
      (x.getClass().equals(Derived.<font color=#0000ff>class</font>)));
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    test(<font color=#0000ff>new</font> Base());
    test(<font color=#0000ff>new</font> Derived());
    monitor.expect(<font color=#0000ff>new</font> String[] {
      <font color=#004488>"Testing x of type class c10.Base"</font>,
      <font color=#004488>"x instanceof Base true"</font>,
      <font color=#004488>"x instanceof Derived false"</font>,
      <font color=#004488>"Base.isInstance(x) true"</font>,
      <font color=#004488>"Derived.isInstance(x) false"</font>,
      <font color=#004488>"x.getClass() == Base.class true"</font>,
      <font color=#004488>"x.getClass() == Derived.class false"</font>,
      <font color=#004488>"x.getClass().equals(Base.class)) true"</font>,
      <font color=#004488>"x.getClass().equals(Derived.class)) false"</font>,
      <font color=#004488>"Testing x of type class c10.Derived"</font>,
      <font color=#004488>"x instanceof Base true"</font>,
      <font color=#004488>"x instanceof Derived true"</font>,
      <font color=#004488>"Base.isInstance(x) true"</font>,
      <font color=#004488>"Derived.isInstance(x) true"</font>,
      <font color=#004488>"x.getClass() == Base.class false"</font>,
      <font color=#004488>"x.getClass() == Derived.class true"</font>,
      <font color=#004488>"x.getClass().equals(Base.class)) false"</font>,
      <font color=#004488>"x.getClass().equals(Derived.class)) true"</font>
    });
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>The <b>test(&#160;)</b> method performs type checking with its argument using both forms of <b>instanceof</b>. It then gets the <b>Class</b> reference and uses <b>==</b> and <b>equals(&#160;) </b>to test for equality of the <b>Class</b> objects. Reassuringly, <b>instanceof </b>and <b>isInstance(&#160;)</b> produce exactly the same results, as do <b>equals(&#160;)</b> and <b>==</b>. But the tests themselves draw different conclusions. In keeping with the concept of type, <b>instanceof</b> says &#147;are you this class, or a class derived from this class?&#148; On the other hand, if you compare the actual <b>Class</b> objects using <b>==</b>, there is no concern with inheritance&#151;it&#146;s either the exact type or it isn&#146;t. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap12_1856" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h2>
<a name="_Toc305593313"></a><a name="_Toc305628785"></a><a name="_Toc312374146"></a><a name="_Toc375545409"></a><a name="_Toc375545407"></a><a name="_Toc24775728"></a><a name="Heading9996"></a>RTTI
syntax</h2>
<p>Java performs its RTTI using the <a name="Index916"></a><a name="Index917"></a><b>Class</b> object, even if you&#146;re doing something like a cast. The class <b>Class</b> also has a number of other ways you can use RTTI. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap12_1857" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>First, you must get a reference to the appropriate <b>Class</b> object. One way to do this, as shown in the previous example, is to use a string and the <b>Class.forName(&#160;)</b> method. This is convenient because you don&#146;t need an object of that type in order to get the <b>Class</b> reference. However, if you do already have an object of the type you&#146;re interested in, you can fetch the <b>Class</b> reference by calling a method that&#146;s part of the <b>Object</b> root class:<b> </b><a name="Index918"></a><a name="Index919"></a><b>getClass(&#160;)</b>. This returns the <b>Class</b> reference representing the actual type of the object. <b>Class</b> has many interesting methods demonstrated in the following example: <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]A0493" title="Send BackTalk Comment">Feedback</a></font><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c10:ToyTest.java</font>
<font color=#009900>// Testing class Class.</font>
<font color=#0000ff>import</font> com.bruceeckel.simpletest.*;

<font color=#0000ff>interface</font> HasBatteries {}
<font color=#0000ff>interface</font> Waterproof {}
<font color=#0000ff>interface</font> Shoots {}
<font color=#0000ff>class</font> Toy {
  <font color=#009900>// Comment out the following default constructor</font>
  <font color=#009900>// to see NoSuchMethodError from (*1*)</font>
  Toy() {}
  Toy(<font color=#0000ff>int</font> i) {}
}

<font color=#0000ff>class</font> FancyToy <font color=#0000ff>extends</font> Toy
<font color=#0000ff>implements</font> HasBatteries, Waterproof, Shoots {
  FancyToy() { <font color=#0000ff>super</font>(1); }
}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> ToyTest {
  <font color=#0000ff>private</font> <font color=#0000ff>static</font> Test monitor = <font color=#0000ff>new</font> Test();
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> printInfo(Class cc) {
    System.out.println(<font color=#004488>"Class name: "</font> + cc.getName() +
      <font color=#004488>" is interface? ["</font> + cc.isInterface() + <font color=#004488>"]"</font>);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    Class c = <font color=#0000ff>null</font>;
    <font color=#0000ff>try</font> {
      c = Class.forName(<font color=#004488>"FancyToy"</font>);
    } <font color=#0000ff>catch</font>(ClassNotFoundException e) {
      System.out.println(<font color=#004488>"Can't find FancyToy"</font>);
      System.exit(1);
    }
    printInfo(c);
    Class[] faces = c.getInterfaces();
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; faces.length; i++)
      printInfo(faces[i]);
    Class cy = c.getSuperclass();
    Object o = <font color=#0000ff>null</font>;
    <font color=#0000ff>try</font> {
      <font color=#009900>// Requires default constructor:</font>
      o = cy.newInstance(); <font color=#009900>// (*1*)</font>
    } <font color=#0000ff>catch</font>(InstantiationException e) {
      System.out.println(<font color=#004488>"Cannot instantiate"</font>);
      System.exit(1);
    } <font color=#0000ff>catch</font>(IllegalAccessException e) {
      System.out.println(<font color=#004488>"Cannot access"</font>);
      System.exit(1);
    }
    printInfo(o.getClass());
    monitor.expect(<font color=#0000ff>new</font> String[] {
      <font color=#004488>"Class name: FancyToy is interface? [false]"</font>,
      <font color=#004488>"Class name: HasBatteries is interface? [true]"</font>,
      <font color=#004488>"Class name: Waterproof is interface? [true]"</font>,
      <font color=#004488>"Class name: Shoots is interface? [true]"</font>,
      <font color=#004488>"Class name: Toy is interface? [false]"</font>
    });
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>You can see that <b>class FancyToy</b> is quite complicated, since it inherits from <b>Toy</b> and <b>implements</b> the <b>interface</b>s <b>HasBatteries</b>, <b>Waterproof</b>, and <b>Shoots</b>. In <b>main(&#160;)</b>, a <b>Class</b> reference is created and initialized to the <b>FancyToy</b> <b>Class</b> using <b>forName(&#160;)</b> inside an appropriate <b>try</b> block. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap12_1858" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>The <b>Class.getInterfaces(&#160;)</b> method returns an array of <a name="Index920"></a><a name="Index921"></a><b>Class</b> objects representing the interfaces that are contained in the <b>Class</b> object of interest. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap12_1859" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>If you have a <b>Class</b> object, you can also ask it for its direct base class using <a name="Index922"></a><a name="Index923"></a><b>getSuperclass(&#160;)</b>. This, of course, returns a <b>Class</b> reference that you can further query. This means that at run time, you can discover an object&#146;s entire class hierarchy. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap12_1860" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>The <a name="Index924"></a><a name="Index925"></a><b>newInstance(&#160;)</b> method of <b>Class</b> can, at first, seem like just another way to <b>clone(&#160;)</b> an object. However, you can create a new object with <b>newInstance(&#160;)</b> <i>without</i> an existing object, as seen here, because there is no <b>Toy</b> object&#151;only <b>cy</b>, which is a reference to <b>y</b>&#146;s <b>Class</b> object. This is a way to implement a &#147;virtual constructor,&#148; which allows you to say &#147;I don&#146;t know exactly what type you are, but create yourself properly anyway.&#148; In the preceding example, <b>cy</b> is just a <b>Class</b> reference with no further type information known at compile time. And when you create a new instance, you get back an <b>Object reference</b>. But that reference is pointing to a <b>Toy</b> object. Of course, before you can send any messages other than those accepted by <b>Object</b>, you have to investigate it a bit and do some casting. In addition, the class that&#146;s being created with <b>newInstance(&#160;)</b> must have a default constructor. In the next section, you&#146;ll see how<b> </b>to dynamically create objects of classes using any constructor, with the Java <i>reflection</i> API (Application Programmer Interface). <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap12_1861" title="Send BackTalk Comment">Feedback</a></font