In the preceding example, declaring that the subroutine root() can throw an IllegalArgumentException is just a courtesy to potential readers of this routine. This is because handling of IllegalArgumentExceptions is not “mandatory.” A routine can throw an IllegalArgumentException without announcing the possibility. And a program that calls that routine is free either to catch or to ignore the exception, just as a programmer can choose either to catch or to ignore an exception of type NullPointerException.
For those exception classes that require mandatory handling, the situation is different. If a subroutine can throw such an exception, that fact must be announced in a throws clause in the routine definition. Failing to do so is a syntax error that will be reported by the compiler. Exceptions that require mandatory handling are called checked exceptions. The compiler will check that such exceptions are handled by the program.
Suppose that some statement in the body of a subroutine can generate a checked exception, one that requires mandatory handling. The statement could be a throw statement, which throws the exception directly, or it could be a call to a subroutine that can throw the exception. In either case, the exception must be handled.
This can be done in one of two ways: The first way is to place the statement in a try statement that has a catch clause that handles the exception; in this case, the exception is handled within the subroutine, so that any caller of the subroutine will never see the exception.
The second way is to declare that the subroutine can throw the exception. This is done by adding a “throws” clause to the subroutine heading, which alerts any callers to the possibility that an exception might be generated when the subroutine is executed. The caller will, in turn, be forced either to handle the exception in a try statement or to declare the exception in a throws clause in its own header.
Exception-handling is mandatory for any exception class that is not a subclass of either Error or RuntimeException. These checked exceptions generally represent conditions that are outside the control of the programmer. For example, they might represent bad input or an illegal action taken by the user. There is no way to avoid such errors, so a robust program has to be prepared to handle them. The design of Java makes it impossible for programmers to ignore the possibility of such errors.
Among the checked exceptions are several that can occur when using Java’s input/output routines. This means that you can’t even use these routines unless you understand something about exception-handling.