Handling Errors with Exceptions

Java's Catch or Specify Requirement

As stated previously, Java requires that a method either catch or specify all checked exceptions that can be thrown within the scope of the method. This requirement has several components that need further description: "catch", "specify," "checked exceptions," and "exceptions that can be thrown within the scope of the method."

Catch

A method can catch an exception by providing an exception handler for that type of exception. The next page, Dealing with Exceptions, introduces an example program, talks about catching exceptions, and shows you how to write an exception handler for the example program.

Specify

If a method chooses not to catch an exception, the method must specify that it can throw that exception. Why did the Java designers make this requirement? Because any exception that can be thrown by a method is really part of the method's public programming interface: callers of a method must know about the exceptions that a method can throw in order to intelligently and consciously decide what to do about those exceptions. Thus, in the method signature you specify the exceptions that the method can throw.

The next page, Dealing with Exceptions, talks about specifying exceptions that a method throws and shows you how to do it.

Checked Exceptions

Java has different types of exceptions, including I/O Exceptions, runtime exceptions, and exceptions of your own creation, to name a few. Of interest to us in this discussion are runtime exceptions. Runtime exceptions are those exceptions that occur within the Java runtime system. This includes arithmetic exceptions (such as when 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. The cost of checking for runtime exceptions often exceeds the benefit of catching or specifying them. Thus the compiler does not require that you catch or specify runtime exceptions, although you can. Checked exceptions are exceptions that are not runtime exceptions and are checked by the compiler; the compiler checks that these exceptions are caught or specified.

Some consider this a loophole in Java's exception handling mechanism, and programmers are tempted to make all exceptions runtime exceptions. In general, this is not recommended. Runtime Exceptions--The Controversy contains a thorough discussion about when and how to use runtime exceptions.

Exceptions that can be thrown within the scope of the method

The statement "exceptions that can be thrown within the scope of the method" may seem obvious at first: just look for the throw statement. However, this statement includes more than just the exceptions that can be thrown directly by the method: the key is in the phrase within the scope of. This phrase includes any exception that can be thrown while the flow of control remains within the method. This statement includes both

• Exceptions that are thrown directly by the method with Java's throw statement.
• Exceptions that are thrown indirectly by the method through calls to other methods

Handling Errors with Exceptions