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Java Coffee Break

Section 9.3
Exceptions and the try...catch Statement

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. 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 method for dealing with errors that can occur while a program is running. The method 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. (More exactly, the thread that throws the exception will crash. In a multithreaded program, it is possible for other threads to continue even after one crashes.)

By the way, 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 the case of an applet, only the current operation -- such as the response to a button -- will be terminated. Parts of the applet might continue to function even when other parts are non-functional because of exceptions.) In many other programming languages, a crashed program will often 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. The object thrown by an exception must be an instance of the standard class java.lang.Throwable or of one of its subclasses. In general, each different type of exception is represented by its own subclass of Throwable. 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 exceptions.

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. You should not try to catch and handle such errors. An example is the 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 usually best just to terminate the program, 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 such as: ArithmeticException, which occurs for example when there is an attempt to divide an integer by zero, ArrayIndexOutOfBoundsException, which occurs when an out-of-bounds index is used in an array, and NullPointerException, which occurs when there is an attempt to use a null reference in a context when an actual object reference is required. 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 I'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.

(Partial class hierarchy for Throwable objects)

To catch exceptions in a Java program, you need a try statement. The idea is that you tell the computer to "try" to execute some commands. If it succeeds, all well and good. But if an exception is thrown during the execution of those commands, you can catch the exception and handle it. For example,

      try {
          double determinant = M[0][0]*M[1][1] - M[0][1]*M[1][0];
          System.out.println("The determinant of M is " + determinant);
      }
      catch ( ArrayIndexOutOfBoundsException e ) {
         System.out.println("M is the wrong size to have a determinant.");
      }

The computer tries to execute the block of statements following the word "try". If no exception occurs during the execution of this block, then the "catch" part of the statement is simply ignored. However, if an ArrayIndexOutOfBoundsException occurs, then the computer jumps immediately to the block of statements labeled "catch (ArrayIndexOutOfBoundsException e)". This block of statements is said to be an exception handler for ArrayIndexOutOfBoundsException. By handling the exception in this way, you prevent it from crashing the program.

You might notice that there is another possible source of error in this try statement. If the value of the variable M is null, then a NullPointerException will be thrown when the attempt is made to reference the array. In the above try statement, NullPointerExceptions are not caught, so they will be processed in the ordinary way (by terminating the program, unless the exception is handled elsewhere). You could catch NullPointerExceptions by adding another catch clause to the try statement:

      try {
          double determinant = M[0][0]*M[1][1] - M[0][1]*M[1][0];
          System.out.println("The determinant of M is " + determinant);
      }
      catch ( ArrayIndexOutOfBoundsException e ) {
         System.out.println("M is the wrong size to have a determinant.");
      }
      catch ( NullPointerException e ) {
         System.out.print("Programming error!  M doesn't exist:  " + );
         System.out.println( e.getMessage() );
      }

This example shows how to use multiple catch clauses in one try block. It also shows what that little "e" is doing in the catch clauses. The e is actually a variable name. (You can use any name you like.) Recall that when an exception occurs, it is actually an object that is thrown. Before executing a catch clause, the computer sets this variable to refer to the exception object that is being caught. This object contains information about the exception. For example, an error message describing the exception can be retrieved using the object's getMessage() method, as is done in the above example. Another useful method in every exception object, e, is e.printStackTrace(). This method will print out the list of subroutines that were being executed when the exception was thrown. This information can help you to track down the part of your program that caused the error.

Note that both ArrayIndexOutOfBoundsException and NullPointerException are subclasses of RuntimeException. It's possible to catch all RuntimeExceptions with a single catch clause. For example:

      try {
          double determinant = M[0][0]*M[1][1] - M[0][1]*M[1][0];
          System.out.println("The determinant of M is " + determinant);
      }
      catch ( RuntimeException e ) {
         System.out.println("Sorry, an error has occurred.");
         e.printStackTrace();
      }

Since any object of type ArrayIndexOutOfBoundsException or of type NullPointerException is also of type RuntimeException, this will catch array index errors and null pointer errors as well as any other type of runtime exception. This shows why exception classes are organized into a class hierarchy. It allows you the option of casting your net narrowly to catch only a specific type of exception. Or you can cast your net widely to catch a wide class of exceptions.

The example I've given here is not particularly realistic. You are not very likely to use exception-handling to guard against null pointers and bad array indices. This is a case where careful programming is better than exception handling: Just be sure that your program assigns a reasonable, non-null value to the array M. You would certainly resent it if the designers of Java forced you to set up a try...catch statement every time you wanted to use an array! This is why handling of potential RuntimeExceptions is not mandatory. There are just too many things that might go wrong! (This also shows that exception-handling does not solve the problem of program robustness. It just gives you a tool that will in many cases let you approach the problem in a more organized way.)

The syntax of a try statement is a little more complicated than I've indicated so far. The syntax can be described as

       try {
          statements
       }
       optional-catch-clauses
       optional-finally-clause

Note that this is a case where a block of statements, enclosed between { and }, is required. You need the { and } even if they enclose just one statement. The try statement can include zero or more catch clauses and, optionally, a finally clause. (The try statement must include either a finally clause or at least one catch clause.) The syntax for a catch clause is

       catch ( exception-class-name variable-name ) {
          statements
       }

and the syntax for a finally clause is

       finally {
          statements
       }

The semantics of the finally clause is that the block of statements in the finally clause is guaranteed to be executed as the last step in the execution of the try statement, whether or not any exception occurs and whether or not any exception that does occur is caught and handled. The finally clause is meant for doing essential cleanup that under no circumstances should be omitted.

There are times when it makes sense for a program to deliberately throw an exception. This is the case when the program discovers some sort of exceptional or error condition, but there is no reasonable way to handle the error at the point where the problem is discovered. The program can throw an exception in the hope that some other part of the program will catch and handle the exception.

To throw an exception, use a throw statement. The syntax of the throw statement is

       throw exception-object ;

The exception-object must be an object belonging to one of the subclasses of Throwable. Usually, it will in fact belong to one of the subclasses of Exception. In most cases, it will be a newly constructed object created with the new operator. For example:

        throw new ArithmeticException("Division by zero");

The parameter in the constructor becomes the error message in the exception object. (You might find this example a bit odd, because you might expect to system itself to throw an ArithmeticException when an attempt is made to divide by zero. So why should a programmer bother to throw the exception? The answer is a little surprising: If the numbers that are being divided are of type int, then division by zero will indeed throw an ArithmeticException. However, no arithmetic operations with floating-point numbers will ever produce an exception. Instead, the special value Double.NaN is used to represent the result of an illegal operation.)

An exception can be thrown either by the system or by a throw statement. The exception is processed in exactly the same way in either case. Suppose that the exception is thrown inside a try statement. If that try statement has a catch clause that handles that type of exception, then the computer jumps to the catch clause and executes it. The exception has been handled. After handling the exception, the computer executes the finally clause of the try statement, if there is one. It then continues normally with the rest of the program which follows try statement. If the exception is not immediately caught and handled, the processing of the exception will continue.

When an exception is thrown during the execution of a subroutine and the exception is not handled in the same subroutine, then that subroutine is terminated (after the execution of any pending finally clauses). Then the routine that called that subroutine gets a chance to handle the exception. That is, if the subroutine was called inside a try statement that has an appropriate catch clause, then that catch clause will be executed and the program will continue on normally from there. Again, if that routine does not handle the exception, then it also is terminated and the routine that called it gets the next shot at the exception. The exception will crash the program only if it passes up through the entire chain of subroutine calls without being handled.

A subroutine that might generate an exception can announce this fact by adding the clause "throws exception-class-name" to the header of the routine. For example:

     static double root(double A, double B, double C) 
                                 throws IllegalArgumentException {
            // Returns the larger of the two roots of
            // the quadratic equation A*x*x + B*x + C = 0.
            // (Throws an exception if A == 0 or B*B-4*A*C < 0.)
         if (A == 0) {
           throw new IllegalArgumentException("A can't be zero.");
         }
         else {
            double disc = B*B - 4*A*C;
            if (disc < 0)
               throw new IllegalArgumentException("Discriminant < zero.");
            return  (-B + Math.sqrt(disc)) / (2*A);
         }
     }

As discussed in the previous section, The computation in this subroutine has the preconditions that A != 0 and B*B-4*A*C >= 0. The subroutine throws an exception of type IllegalArgumentException when either of these preconditions is violated. When an illegal condition is found in a subroutine, throwing an exception is often a reasonable response. If the program that called the subroutine knows some good way to handle the error, it can catch the exception. If not, the program will crash -- and the programmer will know that the program needs to be fixed.

Mandatory Exception Handling

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.

On the other hand, suppose that some statement in a program can generate an exception 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. One possibility is to place the statement is a try statement that has a catch clause that handles the exception. The other possibility is to declare that the subroutine that contains the statement can throw the exception. This is done by adding a "throws" clause to the subroutine heading. If the throws clause is used, then any other routine that calls the subroutine will be responsible for handling the exception. If you don't handle the possible exception in one of these two ways, it will be considered a syntax error, and the compiler will not accept your program.

Exception-handling is mandatory for any exception class that is not a subclass of either Error or RuntimeException. Exceptions that require mandatory handling 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. A robust program has to be prepared to handle such conditions. The design of Java makes it impossible for programmers to ignore such conditions.

Among the exceptions that require mandatory handling 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. The next chapter deals with input/output and uses exception-handling extensively.

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