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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>
Creating
your own exceptions
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You&#8217;re
not stuck using the Java <A NAME="Index946"></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><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">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><P></DIV>

<font color="#990000"><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(
      "Throwing MyException from f()");
    <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(
      "Throwing MyException from g()");
    <font color="#0000ff">throw</font> <font color="#0000ff">new</font> MyException("Originated in g()");
  }
  <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">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
inheritance occurs in the creation of the new class:
</FONT><P></DIV>

<font color="#990000"><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><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
key phrase here is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>extends
Exception
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which says &#8220;it&#8217;s everything an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Exception</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is and more.&#8221; The added code is small &#8211; the addition of two
constructors that define the way 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>MyException</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>MyException(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
default constructor. In the second constructor, the base-class constructor with
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
argument is explicitly invoked by using the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>super</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
output of the program is:
</FONT><P></DIV>

<font color="#990000"><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><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see the absence of the detail message in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>MyException</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
thrown from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>f(&#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
process of creating your own exceptions can be taken further. You can add extra
constructors and members:
</FONT><P></DIV>

<font color="#990000"><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(
      "Throwing MyException2 from f()");
    <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(
      "Throwing MyException2 from g()");
    <font color="#0000ff">throw</font> <font color="#0000ff">new</font> MyException2("Originated in g()");
  }
  <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(
      "Throwing MyException2 from h()");
    <font color="#0000ff">throw</font> <font color="#0000ff">new</font> MyException2(
      "Originated in h()", 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("e.val() = " + e.val());
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
data member 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has been added, along with a method that reads that value and an additional
constructor that sets it. The output is:
</FONT><P></DIV>

<font color="#990000"><PRE>Throwing MyException2 from f()
MyException2
        at Inheriting2.f(Inheriting2.java:22)
        at Inheriting2.main(Inheriting2.java:34)
Throwing MyException2 from g()
MyException2: Originated in g()
        at Inheriting2.g(Inheriting2.java:26)
        at Inheriting2.main(Inheriting2.java:39)
Throwing MyException2 from h()
MyException2: Originated in h()
        at Inheriting2.h(Inheriting2.java:30)
        at Inheriting2.main(Inheriting2.java:44)
e.val() = 47 </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Since
an exception is just another kind of object, you can continue this process of
embellishing the power of your exception classes. Keep in mind, however, that
all this dressing up might be lost on the client programmers using your
packages, since they might simply look for the exception to be thrown and
nothing more. (That&#8217;s the way most of the Java library exceptions are
used.) If this is the case, it&#8217;s
<a name="its"></a>
possible to create a new exception type with almost no code at all:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: SimpleException.java</font>
<font color="#0000ff">class</font> SimpleException <font color="#0000ff">extends</font> Exception {
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
relies on the compiler to create the default constructor (which automatically
calls the base-class default constructor). Of course, in this case you
don&#8217;t get a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>SimpleException(String)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
constructor, but in practice that isn&#8217;t used much.
</FONT><a name="_Toc408018600"></a><a name="_Toc375545374"></a><P></DIV>

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