What Is an Exception

WhatIs an Exception?

The term exception is shorthand for the phrase "exceptional event."

Definition: An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.

When an error occurs within a method, the method creates an object and hands it off to the runtime system. The object, called an exception object, contains information about the error, including its type and the state of the program when the error occurred. Creating an exception object and handing it to the runtime system is called throwing an exception.

After a method throws an exception, the runtime system attempts to find something to handle it. The set of possible "somethings" to handle the exception is the ordered list of methods that had been called to get to the method where the error occurred. The list of methods is known as the call stack (see the next figure).

The call stack showing three method calls, where the first method called has the exception handler.

Thecall stack.

The runtime system searches the call stack for a method that contains a block of code that can handle the exception. This block of code is called an exception handler. The search begins with the method in which the error occurred and proceeds through the call stack in the reverse order in which the methods were called. When an appropriate handler is found, the runtime system passes the exception to the handler. An exception handler is considered appropriate if the type of the exception object thrown matches the type that can be handled by the handler.

The exception handler chosen is said to catch the exception. If the runtime system exhaustively searches all the methods on the call stack without finding an appropriate exception handler, as shown in the next figure, the runtime system (and, consequently, the program) terminates.

The call stack showing three method calls, where the first method called has the exception handler.

Searching the call stack for the exception handler.

Using exceptions to manage errors has some advantages over traditional error-management techniques. You can learn more in the Advantages of Exceptions section.

The first step in constructing an exception handler is to enclose the code that might throw an exception within a try block. In general, a try block looks like the following:

try {

    code

}

catch and finally blocks . . .

The segment in the example labeled code contains one or more legal lines of code that could throw an exception. (The catch and finally blocks are explained in the next two subsections.)

To construct an exception handler for the writeList method from the ListOfNumbers class, enclose the exception-throwing statements of the writeList method within a try block. There is more than one way to do this. You can put each line of code that might throw an exception within its own try block and provide separate exception handlers for each. Or, you can put all the writeList code within a single try block and associate multiple handlers with it. The following listing uses one try block for the entire method because the code in question is very short.

private List<Integer> list;

private static final int SIZE = 10;

PrintWriter out = null;

try {

    System.out.println("Entered try statement");

    out = new PrintWriter(new FileWriter("OutFile.txt"));

    for (int i = 0; i < SIZE; i++) {

        out.println("Value at: " + i + " = " + list.get(i));

    }

}

catch and finally statements . . .

If an exception occurs within the try block, that exception is handled by an exception handler associated with it. To associate an exception handler with a try block, you must put a catch block after it; the next section, The catch Blocks, shows you how.

The catch Blocks

You associate exception handlers with a try block by providing one or more catch blocks directly after the try block. No code can be between the end of the try block and the beginning of the first catch block.

try {

} catch (ExceptionType name) {

} catch (ExceptionType name) {

}

Each catch block is an exception handler and handles the type of exception indicated by its argument. The argument type, ExceptionType, declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable class. The handler can refer to the exception with name.

The catch block contains code that is executed if and when the exception handler is invoked. The runtime system invokes the exception handler when the handler is the first one in the call stack whose ExceptionType matches the type of the exception thrown. The system considers it a match if the thrown object can legally be assigned to the exception handler's argument.

The following are two exception handlers for the writeList method — one for two types of checked exceptions that can be thrown within the try statement:

try {

} catch (FileNotFoundException e) {

    System.err.println("FileNotFoundException: " + e.getMessage());

    throw new SampleException(e);

} catch (IOException e) {

    System.err.println("Caught IOException: " + e.getMessage());

}

Both handlers print an error message. The second handler does nothing else. By catching any IOException that's not caught by the first handler, it allows the program to continue executing.

The first handler, in addition to printing a message, throws a user-defined exception. (Throwing exceptions is covered in detail later in this chapter in the How to Throw Exceptions section.)

In this example, when the FileNotFoundException is caught it causes a user-defined exception called SampleException to be thrown. You might want to do this if you want your program to handle an exception in this situation in a specific way.

Exception handlers can do more than just print error messages or halt the program. They can do error recovery, prompt the user to make a decision, or propagate the error up to a higher-level handler using chained exceptions, as described in the Chained Exceptions section.

Catching More Than One Type of Exception with One Exception Handler

In Java SE 7 and later, a single catch block can handle more than one type of exception. This feature can reduce code duplication and lessen the temptation to catch an overly broad exception.

In the catch clause, specify the types of exceptions that block can handle, and separate each exception type with a vertical bar (|):

catch (IOException|SQLException ex) {

    logger.log(ex);

    throw ex;

}

Note: If a catch block handles more than one exception type, then the catch parameter is implicitly final. In this example, the catch parameter ex is final and therefore you cannot assign any values to it within the catch block.