📄 tij0121.html

📁 学习java的经典书籍
💻 HTML
字号:
<html><body>

<table width="100%"><tr>
<td>
<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
</td>
<td align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0120.html">Prev</a> | <a href="tij0122.html">Next</a>
</td>
</tr></table>
<hr>

<H2 ALIGN=LEFT>
RTTI
syntax
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Java
performs its <A NAME="Index1398"></A><A NAME="Index1399"></A>RTTI
using the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object, even if you&#8217;re doing something like a cast. The class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
also has a number of other ways you can use RTTI.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">First,
you must get a handle to the appropriate 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. One way to do this, as shown in the previous example, is to use a
string and the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class.forName(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method. This is convenient because you don&#8217;t need an object of that type
in order to get the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle. However, if you do already have an object of the type you&#8217;re
interested in, you can fetch the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle by calling a method that&#8217;s part of the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
root class:
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
<A NAME="Index1400"></A><A NAME="Index1401"></A>getClass(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This returns the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle representing the actual type of the object. 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has several interesting and sometimes useful methods, demonstrated in the
following example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: ToyTest.java</font>
<font color="#009900">// Testing class Class</font>

<font color="#0000ff">interface</font> HasBatteries {}
<font color="#0000ff">interface</font> Waterproof {}
<font color="#0000ff">interface</font> ShootsThings {}
<font color="#0000ff">class</font> Toy {
  <font color="#009900">// Comment out the following default</font>
  <font color="#009900">// constructor to see </font>
  <font color="#009900">// 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, ShootsThings {
  FancyToy() { <font color="#0000ff">super</font>(1); }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> ToyTest {
  <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("FancyToy");
    } <font color="#0000ff">catch</font>(ClassNotFoundException e) {}
    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) {}
      <font color="#0000ff">catch</font>(IllegalAccessException e) {}
    printInfo(o.getClass());
  }
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> printInfo(Class cc) {
    System.out.println(
      "Class name: " + cc.getName() +
      " is <font color="#0000ff">interface</font>? [" +
      cc.isInterface() + "]");
  }
} <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 that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>class
FancyToy
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is quite complicated, since it inherits from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Toy</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>implements</B></FONT><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">s
of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>HasBatteries</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Waterproof</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ShootsThings</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
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">,
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle is created and initialized to the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>FancyToy</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>forName(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
inside an appropriate 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
block.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class.getInterfaces(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
<A NAME="Index1402"></A><A NAME="Index1403"></A>method
returns an array of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects representing the interfaces that are contained in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object of interest.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you have a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object you can also ask it for its direct base class using <A NAME="Index1404"></A><A NAME="Index1405"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getSuperclass(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This, of course, returns a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle that you can further query. This means that, at run time, you can
discover an object&#8217;s entire class hierarchy.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
<A NAME="Index1406"></A><A NAME="Index1407"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>newInstance(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
can, at first, seem like just another way to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>clone(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
an object. However, you can create a new object with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>newInstance(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>without</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
an existing object, as seen here, because there is no 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Toy</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object, only 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>cy</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which is a handle to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>y</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. This is a way to implement a &#8220;virtual constructor,&#8221; which
allows you to say &#8220;I don&#8217;t know exactly what type you are, but
create yourself properly anyway.&#8221; In the example above, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>cy</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is just a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle with no further type information known at compile time. And when you
create a new instance, you get back an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle. But that handle is pointing to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Toy</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. Of course, before you can send any messages other than those accepted by 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you have to investigate it a bit and do some casting. In addition, the class
that&#8217;s being created with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>newInstance(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
must have a default constructor. There&#8217;s no way to use 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>newInstance(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">to
create objects that have non-default constructors, so this can be a bit
limiting in Java 1. However, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>reflection</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
API in Java 1.1<A NAME="Index1408"></A>
(discussed in the next section) allows you to dynamically use any constructor
in a class.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
final method in the listing is <A NAME="Index1409"></A><A NAME="Index1410"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>printInfo(&#160;),</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
which takes a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle and gets its name with <A NAME="Index1411"></A><A NAME="Index1412"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getName(&#160;),</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and finds out whether it&#8217;s an interface with <A NAME="Index1413"></A><A NAME="Index1414"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>isInterface(&#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">The
output from this program is:
</FONT><P></DIV>

<font color="#990000"><PRE>Class name: FancyToy is <font color="#0000ff">interface</font>? [<font color="#0000ff">false</font>]
Class name: HasBatteries is <font color="#0000ff">interface</font>? [<font color="#0000ff">true</font>]
Class name: Waterproof is <font color="#0000ff">interface</font>? [<font color="#0000ff">true</font>]
Class name: ShootsThings is <font color="#0000ff">interface</font>? [<font color="#0000ff">true</font>]
Class name: Toy is <font color="#0000ff">interface</font>? [<font color="#0000ff">false</font>] </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Thus,
with the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object you can find out just about everything you want to know about an object.
</FONT><a name="_Toc375545410"></a><a name="_Toc408018649"></a><a name="_Toc312374147"></a><P></DIV>

<div align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0120.html">Prev</a> | <a href="tij0122.html">Next</a>
</div>
</body></html>
⌨️ 快捷键说明

复制代码 Ctrl + C
搜索代码 Ctrl + F
全屏模式 F11
切换主题 Ctrl + Shift + D
显示快捷键 ?
增大字号 Ctrl + =
减小字号 Ctrl + -