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Trail: Essential Java Classes
Lesson: Handling Errors with Exceptions

Unchecked Exceptions — The Controversy

Because the Java programming language does not require methods to catch or to specify runtime exceptions or errors, programmers can be tempted to write code that throws only runtime exceptions or to make all their exception subclasses inherit from RuntimeException. Both of these shortcuts allow programmers to write code without bothering with compiler errors and without bothering to specify or to catch any exceptions. Although this may seem convenient to the programmer, it sidesteps the intent of the catch or specify requirement and can cause problems for programmers using your classes.

Why did the designers decide to force a method to specify all uncaught checked exceptions that can be thrown within its scope? Any exception that can be thrown by a method is part of the methodís public programming interface. Callers of a method must know about the exceptions that a method can throw so they can decide what to do about them. These exceptions are as much a part of that methodís programming interface as its parameters and return value.

Your next question might be, "If itís so good to document a methodís API, including the exceptions it can throw, why not specify runtime exceptions, too?" Runtime exceptions represent problems that are the result of a programming problem, and as such, the API client code cannot reasonably be expected to recover from them or handle them in any way. Such problems include arithmetic exceptions (such as dividing by zero), pointer exceptions (such as trying to access an object through a null reference), and indexing exceptions (such as attempting to access an array element through an index that is too large or too small). Runtime exceptions can occur anywhere in a program and in a typical program can be very numerous. Having to add runtime exceptions in every method declaration would reduce a program's clarity. Thus, the compiler does not require that you catch or specify runtime exceptions (although you can).

One case where it is common practice to throw a RuntimeException is when the user calls a method incorrectly. For example, a method can check if one of its arguments is incorrectly null. If an argument is null, the method might throw a NullPointerException, which is an unchecked exception.

Generally speaking, do not throw a RuntimeException or create a subclass of RuntimeException simply because you donít want to be bothered with specifying the exceptions that your methods can throw.

Here's the bottom line guideline: If a client can reasonably be expected to recover from an exception, make it a checked exception. If a client cannot do anything to recover from the exception, make it an unchecked exception.

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