Java Pocket Guide

Chapter 7. Exception Handling

An exception is an anomalous condition that alters or interrupts the flow of execution. Java provides built-in exception handling to deal with such conditions. Exception handling should not be part of normal program flow.

The Exception Hierarchy

As shown in Figure 7-1, all exceptions and errors inherit from the class Throwable, which inherits from the class Object.

Figure 7-1. Snapshot of the exception hierarchy

Checked/Unchecked Exceptions and Errors

Exceptions and errors fall into three categories: checked exceptions, unchecked exceptions, and errors.

Checked Exceptions

• Checked exceptions are checked by the compiler at compile time.

• Methods that throw a checked exception must indicate so in the method declaration using the throws clause. This must continue all the way up the calling stack until the exception is handled.

• All checked exceptions must be explicitly caught with a catch block.

• Checked exceptions include exceptions of the type Exception, and all classes that are subtypes of Exception, except for RuntimeException and the subtypes of RuntimeException.

The following is an example of a method that throws a checked exception:

	// Method declaration that throws
	// an IOException
	void readFile(String filename)
	  throws IOException {
	  ...
	}

Unchecked Exceptions

• The compiler does not check unchecked exceptions at compile time.

• Unchecked exceptions occur during runtime ...