Handling Errors using Exceptions

The finally Block

The final step in setting up an exception handler is providing a mechanism for cleaning up the state of the method before (possibly) allowing control to be passed to a different part of the program. You do this by enclosing the cleanup code within a finally block.

The try block of the writeList() method that you've been working with opens a PrintStream. The program should close that stream before allowing control to pass out of the writeList() method. This poses a somewhat complicated problem because writeList()'s try block has three different exit possibilities:

1. The new FileOutputStream statement failed and threw an IOException.
2. The victor.elementAt(i) statement failed and threw an ArrayIndexOutOfBoundsException.
3. Everything succeeded and the try block exited normally.

The runtime system always executes the statements within the finally block regardless of what happens within the try block. That is, regardless of whether control exits the writeList() method's try block due to scenario 1, 2, or 3 listed above, the code within the finally block will be executed.

This is the finally block for the writeList() method which cleans up and closes the PrintStream.

finally {
    if (pStr != null) { 
        System.err.println("Closing PrintStream");
        pStr.close(); 
    } else { 
        System.err.println("PrintStream not open");
    } 
} 

Is the finally Statement Really Necessary?

At first the need for a finally statement may not be immediately apparent. Programmers often ask "Is the finally statement really necessary or is it just sugar for my Java?" In particular, C++ programmers doubt the need for a finally statement because C++ doesn't have one.

The need for a finally statement is not apparent until you consider the following: how would the PrintStream in the writeList() method get closed if you didn't provide an exception handler for the ArrayIndexOutOfBoundsException and an ArrayIndexOutOfBoundsException occurred? (It would be easy and legal to omit an exception handler for ArrayIndexOutOfBoundsException because it's a runtime exception and the compiler won't alert you that the writeList() contains a method call that might throw one.) The answer is that the PrintStream would not get closed if an ArrayIndexOutOfBoundsException occurred and writeList() did not provide a handler for it--unless the writeList() provided a finally statement.

There are other benefits to using the finally statement as well. In the writeList() example it is possible to provide for cleanup without the intervention of a finally statement. For example, you could put the code to close the PrintStream at the end of the try block and again within exception handler for ArrayIndexOutOfBoundsException as shown here:

try {
    . . .
    pStr.close();	// don't do this; it duplicates code 
} catch (ArrayIndexOutOfBoundsException e) {
    pStr.close();	// don't do this; it duplicates code 
    System.err.println("Caught ArrayIndexOutOfBoundsException: " + e.getMessage());
} catch (IOException e) {
    System.err.println("Caught IOException: " + e.getMessage());
}

However, this duplicates code which makes the code hard to read and prone to errors if you modify the code later. For example, if you add code to the try block that may throw a new type of exception, you will have to remember to close the PrintStream within the new exception handler (which if you're anything like me you are bound to forget).

Handling Errors using Exceptions