chapter09.html

java 是一个很好的网络开发环境。由于它是通过解释的方法

automatically by Java and you don’t need to include them in your exception
specifications. Conveniently enough, they’re all grouped together by
putting them under a single base class called <B>RuntimeException</B>, which is
a perfect example of inheritance: it establishes a family of types that have
some characteristics and behaviors in common. Also, you never need to write an
exception specification saying that a method might throw a
<B>RuntimeException</B>, since that&#8217;s just assumed. Because they indicate
bugs, you virtually never catch a
<A NAME="Index943"></A><A NAME="Index944"></A><B>RuntimeException</B> &#8211;
it&#8217;s dealt with automatically. If you were forced to check for
<B>RuntimeException</B>s your code could get messy. Even though you don&#8217;t
typically catch <B>RuntimeExceptions</B>,<B> </B>in your own packages you might
choose to throw some of the <B>RuntimeException</B>s. </FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">What happens when you don&#8217;t
catch such exceptions? Since the compiler doesn&#8217;t enforce exception
specifications for these, it&#8217;s quite plausible that a
<B>RuntimeException</B> could percolate all the way out to your <B>main(&#160;)
</B>method without being caught. To see what happens in this case, try the
following example:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: NeverCaught.java</font>
<font color=#009900>// Ignoring RuntimeExceptions</font>

<font color=#0000ff>public</font> <font color=#0000ff>class</font> NeverCaught {
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> f() {
    <font color=#0000ff>throw</font> <font color=#0000ff>new</font> RuntimeException(<font color=#004488>"From f()"</font>);
  }
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> g() {
    f();
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    g();
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can already see that a
<B>RuntimeException </B>(or anything inherited from it) is a special case, since
the compiler doesn&#8217;t require an exception specification for these
types.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The output is:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>java.lang.RuntimeException: From f()
        at NeverCaught.f(NeverCaught.java:9)
        at NeverCaught.g(NeverCaught.java:12)
        at NeverCaught.main(NeverCaught.java:15)</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">So the answer is: If a
RuntimeException gets all the way out to <B>main(&#160;)</B> without being
caught, <B>printStackTrace(&#160;)</B> is called for that exception as the
program exits.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Keep in mind that it&#8217;s
possible to ignore only <B>RuntimeException</B>s in your coding, since all other
handling is carefully enforced by the compiler. The reasoning is that a
<B>RuntimeException</B> represents a programming error:</FONT><BR></P></DIV>
<OL>
<LI><FONT FACE="Georgia">	An error you cannot catch (receiving a null
handle handed to your method by a client programmer, for example)
</FONT><LI><FONT FACE="Georgia">	An error that you, as a programmer,
should have checked for in your code (such as
<B>ArrayIndexOutOfBoundsException</B> where you should have paid attention to
the size of the array).
</FONT></OL><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can see what a
tremendous benefit it is to have exceptions in this case, since they help in the
debugging process.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">It&#8217;s interesting to notice
that you cannot classify Java exception handling as a single-purpose tool. Yes,
it is designed to handle those pesky run-time errors that will occur because of
forces outside your code&#8217;s control, but it&#8217;s also essential for
certain types of programming bugs that the compiler cannot
detect.</FONT><A NAME="_Toc375545373"></A><A NAME="_Toc408018599"></A><BR></P></DIV>
<A NAME="Heading292"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
Creating your own exceptions</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You&#8217;re not stuck using the
Java <A NAME="Index945"></A>exceptions. This is important because you&#8217;ll
often need to create your own exceptions to denote a special error that your
library is capable of creating, but which was not foreseen when the Java
hierarchy was created.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">To create your own exception class,
you&#8217;re forced to inherit from an existing type of exception, preferably
one that is close in meaning to your new exception. Inheriting an exception is
quite simple:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: Inheriting.java</font>
<font color=#009900>// Inheriting your own exceptions</font>

<font color=#0000ff>class</font> MyException <font color=#0000ff>extends</font> Exception {
  <font color=#0000ff>public</font> MyException() {}
  <font color=#0000ff>public</font> MyException(String msg) {
    <font color=#0000ff>super</font>(msg);
  }
}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Inheriting {
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> f() <font color=#0000ff>throws</font> MyException {
    System.out.println(
      <font color=#004488>"Throwing MyException from f()"</font>);
    <font color=#0000ff>throw</font> <font color=#0000ff>new</font> MyException();
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> g() <font color=#0000ff>throws</font> MyException {
    System.out.println(
      <font color=#004488>"Throwing MyException from g()"</font>);
    <font color=#0000ff>throw</font> <font color=#0000ff>new</font> MyException(<font color=#004488>"Originated in g()"</font>);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    <font color=#0000ff>try</font> {
      f();
    } <font color=#0000ff>catch</font>(MyException e) {
      e.printStackTrace();
    }
    <font color=#0000ff>try</font> {
      g();
    } <font color=#0000ff>catch</font>(MyException e) {
      e.printStackTrace();
    }
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The inheritance occurs in the
creation of the new class:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#0000ff>class</font> MyException <font color=#0000ff>extends</font> Exception {
  <font color=#0000ff>public</font> MyException() {}
  <font color=#0000ff>public</font> MyException(String msg) {
    <font color=#0000ff>super</font>(msg);
  }
}</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The key phrase here is <B>extends
Exception</B>, which says &#8220;it&#8217;s everything an <B>Exception</B> is
and more.&#8221; The added code is small &#8211; the addition of two
constructors that define the way <B>MyException</B> is created. Remember that
the compiler automatically calls the base-class default constructor if you
don&#8217;t explicitly call a base-class constructor, as in the
<B>MyException(&#160;)</B> default constructor. In the second constructor, the
base-class constructor with a <B>String</B> argument is explicitly invoked by
using the <B>super</B> keyword.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The output of the program
is:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>Throwing MyException from f()
MyException
        at Inheriting.f(Inheriting.java:16)
        at Inheriting.main(Inheriting.java:24)
Throwing MyException from g()
MyException: Originated in g()
        at Inheriting.g(Inheriting.java:20)
        at Inheriting.main(Inheriting.java:29)</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can see the absence of the
detail message in the <B>MyException</B> thrown from
<B>f(&#160;)</B>.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The process of creating your own
exceptions can be taken further. You can add extra constructors and
members:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: Inheriting2.java</font>
<font color=#009900>// Inheriting your own exceptions</font>

<font color=#0000ff>class</font> MyException2 <font color=#0000ff>extends</font> Exception {
  <font color=#0000ff>public</font> MyException2() {}
  <font color=#0000ff>public</font> MyException2(String msg) {
    <font color=#0000ff>super</font>(msg);
  }
  <font color=#0000ff>public</font> MyException2(String msg, <font color=#0000ff>int</font> x) {
    <font color=#0000ff>super</font>(msg);
    i = x;
  }
  <font color=#0000ff>public</font> <font color=#0000ff>int</font> val() { <font color=#0000ff>return</font> i; }
  <font color=#0000ff>private</font> <font color=#0000ff>int</font> i;
}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Inheriting2 {
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> f() <font color=#0000ff>throws</font> MyException2 {
    System.out.println(
      <font color=#004488>"Throwing MyException2 from f()"</font>);
    <font color=#0000ff>throw</font> <font color=#0000ff>new</font> MyException2();
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> g() <font color=#0000ff>throws</font> MyException2 {
    System.out.println(
      <font color=#004488>"Throwing MyException2 from g()"</font>);
    <font color=#0000ff>throw</font> <font color=#0000ff>new</font> MyException2(<font color=#004488>"Originated in g()"</font>);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> h() <font color=#0000ff>throws</font> MyException2 {
    System.out.println(
      <font color=#004488>"Throwing MyException2 from h()"</font>);
    <font color=#0000ff>throw</font> <font color=#0000ff>new</font> MyException2(
      <font color=#004488>"Originated in h()"</font>, 47);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    <font color=#0000ff>try</font> {
      f();
    } <font color=#0000ff>catch</font>(MyException2 e) {
      e.printStackTrace();
    }
    <font color=#0000ff>try</font> {
      g();
    } <font color=#0000ff>catch</font>(MyException2 e) {
      e.printStackTrace();
    }
    <font color=#0000ff>try</font> {
      h();
    } <font color=#0000ff>catch</font>(MyException2 e) {
      e.printStackTrace();
      System.out.println(<font color=#004488>"e.val() = "</font> + e.val());
    }
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">A data member <B>i</B> has been
added, along with a method that reads that value and an additional constructor
that sets it. The output is:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>Throwing MyException2 from f()
MyException2
        at Inheriting2.f(Inheriting2.java:22)
        at Inheriting2.main(Inheriting2.java:34)
Throwing MyException2 from g()