Exception handling - Multimedia Introduction to Programming Using Java - page 314

Example of a realistic try-statement
In Fig. 10.6, we present a function that catches two types of errors: a sub-
script out of range and an attempt to convert a String that does not contain an
integer into an int . Class JLiveWindow , which provides a GUI with some int
fields into which a user is expected to type integers, provides the motivation for
this example. Take a look at the code in that class to see a real use of exception
handling.
The body of the try-statement in Fig. 10.6 converts the value of b[f] —a
String — to an int and returns it. This return statement is enclosed in a try-
block because its execution may cause two kinds of Exception : a subscript out
of range and a NumberFormatException , which may be thrown by method
parseInt . Both of these exceptions are caught by catch clauses. This example
shows that a try-statement may have any number of catch clauses, not just one.
We could have used an if-statement to test whether f was in the subscript
range of array b . Here, catching it using a try-statement leads to a simpler, more
consistent method body since the try-statement has to be used anyway to catch
the second kind of object that could be thrown.

Activities 10-
3.2 and 10.3.3
describe try-
statements in
JLiveWindow
and JLiveRead.
Get the pro-
grams from
page 10-3.

10.4.3 Propagation of a thrown exception

We explain the throwing of an object. The program of Fig. 10.7 contains three
methods. Suppose it is executed, by calling method main . Method main calls
method first , which then calls method second .
Now suppose that an object ob is thrown within method second , signaling
some sort of error, and that the throw does not occur within a try-block. Since
there is no catch clause to catch the object, it is thrown further, to the calling
method . Thus, in this example, it appears that the call on method second throws
ob . And if this call on method second does not appear within a try-block, object
ob is thrown out further, to calling method main . So it looks like method call

Activity 10-3.4
makes this ma-
terial more
accessible.

public class C {
public static void main(String[] args) {
try {
first();
} catch ( ... )
{ ... }

}

public static void first()
{ second(); }

public static void second()
{ ... }

}
Figure 10.7: Propagating a thrown exception