Reference types - Data Structures and Problem Solving Using Java - page 48

object. For instance, it is clear that the basic problem is a bad index when an
ArrayIndexOutBoundsException is propagated. Exceptions are used to signal
exceptional occurrences such as errors.

2.5.1 processing exceptions

The code in Figure 2.11 illustrates the use of exceptions. Code that might
result in the propagation of an exception is enclosed in a try block. The try
block extends from lines 13 to 17. Immediately following the try block are
the exception handlers. This part of the code is jumped to only if an excep-
tion is raised; at the point the exception is raised, the try block in which it
came from is considered terminated. Each catch block (this code has only
one) is attempted in order until a matching handler is found. A
NumberFormatException is generated by parseInt if oneLine is not convertible
to an int .

A try block
encloses code that
might generate an
exception.

The code in the catch block—in this case line 18—is executed if the
appropriate exception is matched. Then the catch block and the try / catch
sequence is considered terminated. 1 A meaningful message is printed from
the exception object e . Alternatively, additional processing and more detailed
error messages could be given.

A catch block
processes an
exception.

2.5.2 the finally clause

Some objects that are created in a try block must be cleaned up. For instance,
files that are opened in the try block may need to be closed prior to leaving
the try block. One problem with this is that if an exception object is thrown
during execution of the try block, the cleanup might be omitted because the
exception will cause an immediate break from the try block. Although we can
place the cleanup immediately after the last catch clause, this works only if
the exception is caught by one of the catch clauses. And this may be difficult
to guarantee.
The finally clause that may follow the last catch block (or the try block,
if there are no catch blocks) is used in this situation. The finally clause con-
sists of the keyword finally followed by the finally block. There are three
basic scenarios.

The finally clause
is always executed
prior to completion
of a block, regard-
less of exceptions.

1. Note that both try and catch require a block and not simply a single statement. Thus braces
are not optional. To save space, we often place simple catch clauses on a single line with
their braces, indented two additional spaces, rather than use three lines. Later in the text we
will use this style for one-line methods.