Java Reference
In-Depth Information
Catching and throwing an Exception further
Figure 10.1 contains method mod . Its first statement throws an exception if
y = 0 . We wrote the function this way so that we could give our own detail mes-
sage. But there is another way to get the same result, as shown in Fig. 10.9.
We remove the if-statement and place the whole body in a try-statement that
catches ArithmeticException s. Then, in the catch-block, we throw a new
exception with the desired message. The new method body does not rethrow
object ae ; instead, it creates a new object and throws it. This is done because it
is not possible to change the detail message of a throwable object.
But there are cases where rethrowing ae makes sense. For example, one
might catch the exception only to dispose of some resources which is beyond
the scope of this text— and then rethrow the same exception.
This second way of detecting that y is 0 is more in keeping with the excep-
tion-mechanism philosophy. Rather than intersperse lots of tests for errors, which
might double the size of the code, let the exception-handling facilities do that
work. Of course, in this case, this second way yields a longer program, but in
general, using the exception-handling facilities can help.
Get function
mod2 of Fig.
10.8 from a
footnote on les-
son page 10-2.
Checked Exceptions
A Java compiler checks to make sure that certain thrown objects are caught by
your program; if they are not caught, the compiler issues an error message and
refuses to compile the program. For example, consider this silly program:
page 10-5
public class C {
public static void main(String[] args)
{ first(); }
public static void first()
{ throw new Exception(); }
Method first may throw an Exception but does not catch it, and this makes the
public static void main(String[] args) {
try {
System.out.println("try-block 0 ");
throw new ArithmeticException("fake exception 1 ");
} catch (ArithmeticException ae) {
System.out.println("catch-block 0 ");
throw new ArithmeticException("fake exception 2 ");
Figure 10.8:
A catch-block itself can throw an exception
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