Exception handling - Multimedia Introduction to Programming Using Java - page 316

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.

Activity
10-4.2

Get function
mod2 of Fig.
10.8 from a
footnote on les-
son page 10-2.

10.5 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:

Lesson
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 ");
System.out.println(ae);
throw new ArithmeticException("fake exception 2 ");

}

}
Figure 10.8:

A catch-block itself can throw an exception