Programming and Problem Solving with Java

(^432) | Exceptions and Additional Control Structures

9.1 Exception-Handling Mechanism

In Chapter 8, we defined an exceptionas an unusual situation that is detected while an ap-

plication is running. An exception halts the normal execution of a method. There are three

parts to an exception-handling mechanism: defining the exception, raising or generating the

exception, and handling the exception. We look first at handling exceptions and then at

defining and raising them.

The try-catch-finally Statement

Ever since Chapter 2, when we introduced the BufferedReaderclass, we have found it neces-

sary to forward an IOExceptionto the JVM. We noted that the alternative to forwarding is to

catch an exception.

When an error occurs in a method call, it isn’t always possible for the method itself to

take care of it. For example, suppose we ask the user for a file name in a dialog, get the name

from a field, and then attempt to open the file (prepare it for reading by calling the

FileReaderconstructor). The constructor discovers that the file doesn’t exist and,

therefore, cannot open it. Perhaps the file has been deleted, or maybe the user

just mistyped the name. The constructor has no way of knowing that the proper

response to the error is to ask the user to reenter the name. Because the con-

structor can’t deal with this error appropriately, it doesn’t even try. It passes the

problem on to the method that called it, such as main.

When a call returns with an exception, normal execution ends and the JVM

looks to see whether code is available to take care of the problem. That code is called

an exception handlerand is part of a try-catch-finallystatement.

The syntax template for a try-catch-finallystatement follows:

As the diagram shows, the statement has three main parts. The first part is the keyword

tryand a block (a { }pair enclosing any number of statements). The second part is an op-

tional series of catchclauses, each consisting of the keyword catch, a single parameter dec-

laration enclosed in parentheses, and a block. The last part is also optional and consists of

the keyword finallyand a block.

try

Block

finally

Block

catch (Exception-Type Object-Name)

Block

...

Exception handler A section

of code that is executed when a

particular exception occurs. In

Java, an exception handler ap-

pears within a catchclause of a

try-catch-finallycontrol

structure.