Getting a program to work under  ideal circumstances is usually a lot easier than making the program robust. A robust program can survive unusual or “exceptional” circumstances without crashing. One approach to writing robust programs is to anticipate the problems that might arise and to include tests in the program for each possible problem. For example, a program will crash if it tries to use an array element A[i], when i is not within the declared range of indices for the array A. A robust program must anticipate the possibility of a bad index and guard against it. One way to do this is to write the program in a way that ensures (as a postcondition of the code that precedes the array reference) that the index is in the legal range. Another way is to test whether the index value is legal before using it in the array. This could be done with an if statement:

if (i < 0 || i >= A.length) {
   ...  // Do something to handle the out-of-range index, i
else {
   ...  // Process the array element, A[i]

There are some problems with this approach. It is difficult and sometimes impossible to anticipate all the possible things that might go wrong. It’s not always clear what to do when an error is detected. Furthermore, trying to anticipate all the possible problems can turn what would otherwise be a straightforward program into a messy tangle of if statements.

Java (like its cousin, C++) provides a neater, more structured alternative technique for dealing with errors that can occur while a program is running. The technique is referred to as exception handling. The word “exception” is meant to be more general than “error.” It includes any circumstance that arises as the program is executed which is meant to be treated as an exception to the normal flow of control of the program. An exception might be an error, or it might just be a special case that you would rather not have clutter up your elegant algorithm.

When an exception occurs during the execution of a program, we say that the exception is thrown. When this happens, the normal flow of the program is thrown off-track, and the program is in danger of crashing. However, the crash can be avoided if the exception is caught and handled in some way. An exception can be thrown in one part of a program and caught in a different part. An exception that is not caught will generally cause the program to crash.

Since Java programs are executed by a Java interpreter, having a program crash simply means that it terminates abnormally and prematurely. It doesn’t mean that the Java interpreter will crash. In effect, the interpreter catches any exceptions that are not caught by the program. The interpreter responds by terminating the program. In many other programming languages, a crashed program will sometimes crash the entire system and freeze the computer until it is restarted. With Java, such system crashes should be impossible – which means that when they happen, you have the satisfaction of blaming the system rather than your own program.

When an exception occurs, the thing that is actually “thrown” is an object. This object can carry information (in its instance variables) from the point where the exception occurs to the point where it is caught and handled. This information always includes the subroutine call stack, which is a list of the subroutines that were being executed when the exception was thrown. (Since one subroutine can call another, several subroutines can be active at the same time.) Typically, an exception object also includes an error message describing what happened to cause the exception, and it can contain other data as well.

All exception objects must belong to a subclass of the standard class java.lang.Throwable. In general, each different type of exception is represented by its own subclass of Throwable, and these subclasses are arranged in a fairly complex class hierarchy that shows the relationship among various types of exception. Throwable has two direct subclasses, Error and Exception. These two subclasses in turn have many other predefined subclasses. In addition, a programmer can create new exception classes to represent new types of exception.

Most of the subclasses of the class Error represent serious errors within the Java virtual machine that should ordinarily cause program termination because there is no reasonable way to handle them. In general, you should not try to catch and handle such errors. An example is a ClassFormatError, which occurs when the Java virtual machine finds some kind of illegal data in a file that is supposed to contain a compiled Java class. If that class was being loaded as part of the program, then there is really no way for the program to proceed.

On the other hand, subclasses of the class Exception represent exceptions that are meant to be caught. In many cases, these are exceptions that might naturally be called “errors,” but they are errors in the program or in input data that a programmer can anticipate and possibly respond to in some reasonable way. (However, you should avoid the temptation of saying, “Well, I’ll just put a thing here to catch all the errors that might occur, so my program won’t crash.” If you don’t have a reasonable way to respond to the error, it’s best just to let the program crash, because trying to go on will probably only lead to worse things down the road – in the worst case, a program that gives an incorrect answer without giving you any indication that the answer might be wrong!)

The class Exception has its own subclass, RuntimeException. This class groups together many common exceptions, including all those that have been covered in previous sections. For example, IllegalArgumentException and NullPointerException are subclasses of RuntimeException. A RuntimeException generally indicates a bug in the program, which the programmer should fix. RuntimeExceptions and Errors share the property that a program can simply ignore the possibility that they might occur. (“Ignoring” here means that you are content to let your program crash if the exception occurs.) For example, a program does this every time it uses an array reference like A[i] without making arrangements to catch a possible ArrayIndexOutOfBoundsException. For all other exception classes besides Error, RuntimeException and their subclasses, exception-handling is “mandatory” in a sense that we’ll discuss below.

The following diagram is a class hierarchy showing the class Throwable and just a few of its subclasses. Classes that require mandatory exception-handling are shown in red:

throwableThe class Throwable includes several instance methods that can be used with any exception object. If e is of type Throwable (or one of its subclasses), then e.getMessage() is a function that returns a String that describes the exception. The function e.toString(), which is used by the system whenever it needs a string representation of the object, returns a String that contains the name of the class to which the exception belongs as well as the same string that would be returned by e.getMessage(). And the method e.printStackTrace() writes a stack trace to standard output that tells which subroutines were active when the exception occurred. A stack trace can be very useful when you are trying to determine the cause of the problem. (Note that if an exception is not caught by the program, then the default response to the exception prints the stack trace to standard output.)

Next: The try Statement