Ivor Horton's Beginning Java®, Java 7 Edition

Chapter 7

Exceptions

WHAT YOU WILL LEARN IN THIS CHAPTER

• What an exception is
• How you handle exceptions in your programs
• The standard exceptions in Java
• How to guarantee that a particular block of code in a method will always be executed
• How to define and use your own types of exceptions
• How to throw exceptions in your programs

Java uses exceptions as a way of signaling problems when you execute a program. Exceptions act as a control mechanism through which a program may be able to recover from an exceptional event. They provide important debug information (through stack traces) that help you figure what went wrong. Problems signaled by exceptions can be, but aren’t always, serious (as I describe later in this chapter). The standard classes use them extensively. Because they arise in your Java programs when things go wrong — and if something can go wrong in your code, sooner or later it will — they are a very basic consideration when you are designing and writing your programs.

The reason I’ve been sidestepping the question of exceptions for the past six chapters is that you first needed to understand classes and inheritance before you could understand what an exception is and appreciate what happens when an exception occurs. Now that you have a good grasp of these topics I can delve into how to use and deal with exceptions in a program.

THE IDEA BEHIND EXCEPTIONS

An exception usually signals an error and is so called because errors in your Java programs are bound to be the ...