Java Try-Finally Return Design Question

Java try-finally return design question

Technically speaking, the return in the try block won't be ignored if a finally block is defined, only if that finally block also includes a return.

It's a dubious design decision that was probably a mistake in retrospect (much like references being nullable/mutable by default, and, according to some, checked exceptions). In many ways this behaviour is exactly consistent with the colloquial understanding of what finally means - "no matter what happens beforehand in the try block, always run this code." Hence if you return true from a finally block, the overall effect must always to be to return true, no?

In general, this is seldom a good idiom, and you should use finally blocks liberally for cleaning up/closing resources but rarely if ever return a value from them.

try{} finally{} construct with return values

Everything works exactly as expected, no bugs here. When you have doubts, the JLS is your savior:

JLS - 14.20.2. Execution of try-finally and try-catch-finally:

If execution of the try block completes abruptly for any other
reason R, then the finally block is executed, and then there is a
choice:

• If the finally block completes normally, then the try statement
  
  completes abruptly for reason R.

• If the finally block completes abruptly for reason S, then the try
  statement completes abruptly for reason S (and reason R is
  
  discarded).

It overrides the value in the try block.

return inside finally discards all exceptions that can be thrown in try clause.

Using try-finally to execute statements after return

Without the finally you declare indeed an additional Foo variable.

But is it really expensive ? Not as in both cases the Foo object exists in memory. You just added a reference to access to it.

A reference to an object in the scope of a method is really cheap.

You should never worry about that.

Besides, you don't have to use finally statement as a way to improve the performance of the executed code.

Readers of the code will never guess such a thing.

finally serves to :

The finally block always executes when the try block exits. This
ensures that the finally block is executed even if an unexpected
exception occurs.

and

putting cleanup code in a finally block is always a good practice,
even when no exceptions are anticipated.

The first code without the finally statement is much clearer and doesn't have any reading indirection.

So I advise to stick to :

Foo result = array[index];
index = (index + 1) % array.length;
return result;  

Try-catch-finally-return clarification

If the return in the try block is reached, it transfers control to the finally block, and the function eventually returns normally (not a throw).

If an exception occurs, but then the code reaches a return from the catch block, control is transferred to the finally block and the function eventually returns normally (not a throw).

In your example, you have a return in the finally, and so regardless of what happens, the function will return 34, because finally has the final (if you will) word.

Although not covered in your example, this would be true even if you didn't have the catch and if an exception were thrown in the try block and not caught. By doing a return from the finally block, you suppress the exception entirely. Consider:

public class FinallyReturn {
  public static final void main(String[] args) {
    System.out.println(foo(args));
  }

  private static int foo(String[] args) {
    try {
      int n = Integer.parseInt(args[0]);
      return n;
    }
    finally {
      return 42;
    }
  }
}

If you run that without supplying any arguments:

$ java FinallyReturn

...the code in foo throws an ArrayIndexOutOfBoundsException. But because the finally block does a return, that exception gets suppressed.

This is one reason why it's best to avoid using return in finally.

try-catch-finally with return after it

The fact that the finally block is executed does not make the program forget you returned. If everything goes well, the code after the finally block won't be executed.

Here is an example that makes it clear:

public class Main {

    public static void main(String[] args) {
        System.out.println("Normal: " + testNormal());
        System.out.println("Exception: " + testException());
    }

    public static int testNormal() {
        try {
            // no exception
            return 0;
        } catch (Exception e) {
            System.out.println("[normal] Exception caught");
        } finally {
            System.out.println("[normal] Finally");
        }
        System.out.println("[normal] Rest of code");
        return -1;
    }

    public static int testException() {
        try {
            throw new Exception();
        } catch (Exception e) {
            System.out.println("[except] Exception caught");
        } finally {
            System.out.println("[except] Finally");
        }
        System.out.println("[except] Rest of code");
        return -1;
    }

}

Output:

[normal] Finally
Normal: 0
[except] Exception caught
[except] Finally
[except] Rest of code
Exception: -1

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