Tuesday, May 17, 2011

Is the finally Statement Really Necessary in java?

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 does the PrintWriter in the writeListmethod get closed if you don't provide an exception handler for the ArrayIndexOutOfBoundsException and anArrayIndexOutOfBoundsException occurs? (It's easy and legal to omit an exception handler for ArrayIndexOutOfBoundsExceptionbecause 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 PrintWriter does not get closed if an ArrayIndexOutOfBoundsException occurs and writeList does not provide a handler for it--unless the writeList provides a finally statement.
There are other benefits to using the finally statement. 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 PrintWriter at the end of the try block and again within the exception handler for ArrayIndexOutOfBoundsException, as shown here:

try {
. . .
out.close(); // don't do this; it duplicates code
} catch (ArrayIndexOutOfBoundsException e) {
out.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, making 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 thePrintWriter within the new exception handler (which if you're anything like me, you are bound to forget).

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