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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
Exception
matching
<P><A NAME="Index975"></A><A NAME="Index976"></A></H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
an exception is thrown, the exception-handling system looks through the
&#8220;nearest&#8221; handlers in the order they are written. When it finds a
match, the exception is considered handled, and no further searching occurs. 
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Matching
an exception doesn&#8217;t require a perfect match between the exception and
its handler. A derived-class object will match a handler for the base class, as
shown in <A NAME="Index977"></A><A NAME="Index978"></A>this
example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Human.java</font>
<font color="#009900">// Catching Exception Hierarchies</font>

<font color="#0000ff">class</font> Annoyance <font color="#0000ff">extends</font> Exception {}
<font color="#0000ff">class</font> Sneeze <font color="#0000ff">extends</font> Annoyance {}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Human {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#0000ff">try</font> {
      <font color="#0000ff">throw</font> <font color="#0000ff">new</font> Sneeze();
    } <font color="#0000ff">catch</font>(Sneeze s) {
      System.out.println("Caught Sneeze");
    } <font color="#0000ff">catch</font>(Annoyance a) {
      System.out.println("Caught Annoyance");
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><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>Sneeze</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
exception will be caught by the first 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>catch</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
clause that it matches, which is the first one, of course. However, if you
remove the first catch clause:
</FONT><P></DIV>

<font color="#990000"><PRE>    <font color="#0000ff">try</font> {
      <font color="#0000ff">throw</font> <font color="#0000ff">new</font> Sneeze();
    } <font color="#0000ff">catch</font>(Annoyance a) {
      System.out.println("Caught Annoyance");
    } </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
remaining catch clause will still work because it&#8217;s catching the base
class of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Sneeze</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Put another way, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>catch(Annoyance
e)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
will catch a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Annoyance</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>or
any class derived from it
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This is useful because if you decide to add more exceptions to a method, if
they&#8217;re all inherited from the same base class then the client
programmer&#8217;s code will not need changing, assuming they catch the base
class, at the very least.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you try to &#8220;mask&#8221; the derived-class exceptions by putting the
base-class catch clause first, like this:
</FONT><P></DIV>

<font color="#990000"><PRE>    <font color="#0000ff">try</font> {
      <font color="#0000ff">throw</font> <font color="#0000ff">new</font> Sneeze();
    } <font color="#0000ff">catch</font>(Annoyance a) {
      System.out.println("Caught Annoyance");
    } <font color="#0000ff">catch</font>(Sneeze s) {
      System.out.println("Caught Sneeze");
    } </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">the
compiler will give you an error message, since it sees that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Sneeze</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
catch-clause can never be reached.
</FONT><a name="_Toc312374128"></a><a name="_Toc375545378"></a><a name="_Toc408018606"></a><P></DIV>
<A NAME="Heading300"></A><H3 ALIGN=LEFT>
Exception
guidelines
<P><A NAME="Index979"></A></H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Use
exceptions to:
</FONT><P></DIV>
<OL>
<LI><FONT FACE="Carmina Md BT" SIZE=5 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Fix
the problem and call the method (which caused the exception) again.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=5 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Patch
things up and continue without retrying the method.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=5 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Calculate
some alternative result instead of what the method was supposed to produce.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=5 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Do
whatever you can in the current context and rethrow the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>same</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
exception to a higher context.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=5 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Do
whatever you can in the current context and throw a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>different</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
exception to a higher context.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=5 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Terminate
the program.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=5 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Simplify.
If your exception scheme makes things more complicated, then it is painful and
annoying to use.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=5 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Make
your library and program safer. This is a short-term investment (for debugging)
and a long-term investment (for application robustness).
</FONT><a name="_Toc375545379"></a><a name="_Toc408018607"></a></OL>
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