Introduction

In this chapter, we introduce exception handling. An exception is an indication of a problem that occurs during a program's execution. The name "exception" implies that the problem occurs infrequentlyif the "rule" is that a statement normally executes correctly, then the "exception to the rule" is that a problem occurs. Exception handling enables programmers to create applications that can resolve (or handle) exceptions. In many cases, handling an exception allows a program to continue executing as if no problem had been encountered. A more severe problem could prevent a program from continuing normal execution, instead requiring it to notify the user of the problem before terminating in a controlled manner. The features presented in this chapter enable programmers to write robust and fault-tolerant programs (i.e., programs that are able to deal with problems that may arise and continue executing). The style and details of Java exception handling are based in part on the Andrew Koenig's and Bjarne Stroustrup's paper, "Exception Handling for C++ (revised)."[1]

[1] Koenig, A., and B. Stroustrup. "Exception Handling for C++ (revised)," Proceedings of the Usenix C++ Conference, pp. 149176, San Francisco, April 1990.

Error-Prevention Tip 13.1

Exception handling helps improve a program's fault tolerance.

You have already been briefly introduced to exceptions in earlier chapters. In Chapter 7 you learned that an ArrayIndexOutOfBoundsException occurs when an attempt is made to access an element past the end of an array. Such a problem may occur if there is an "off by one" error in a for statement that manipulates an array. In Chapter 10, we introduced the ClassCastException, which occurs when an attempt is made to cast an object that does not have an is-a relationship with the type specified in the cast operator. Chapter 11 briefly mentioned the NullPointerException, which occurs whenever a null reference is used where an object is expected (for example, when an attempt is made to attach a GUI component to a Container, but the GUI component has not yet been created). You have also used class Scanner throughout this text, which as you will see in this chapter also may cause exceptions.

The chapter begins with an overview of exception-handling concepts, then demonstrates basic exception-handling techniques. We show these techniques in action by handling an exception that occurs when a method attempts to divide an integer by zero. Next, we introduce several classes at the top of Java's class hierarchy for exception handling. As you will see, only classes that extend Throwable (package java.lang) directly or indirectly can be used with exception handling. We then discuss the chained exception feature that was introduced in J2SE 1.4. This feature allows programmers to wrap information about an exception that occurred in another exception object to provide more detailed information about a problem in a program. Next, we discuss additional exception-handling issues, such as how to handle exceptions that occur in a constructor. We introduce preconditions and postconditions, which users of the classes you create understand conditions that must be true when your methods are called and when those methods return. Finally, we present assertions, which programmers use at development time to help debug their code.