java try catch finally - return [duplicate]

Answer 1

A try..catch..finally is actually compiled as a nested try..catch (of the exception you specify) within a try..catch (of "any"), and the "finally" block is cut & pasted by the compiler to the "end" of each code path. (I put "end" in quotes because I can't remember off the top of my head how it handles multiple exits from a "try" or "catch" block.) You can use the javap command (dis-assembler) to experiment on what byte code is generated by your version of Java to correspond to different try..catch..finally combinations.

Answer 2

The finally block gets always executed as last. So the return in the finally block OVERWRITES the other returns in the try/catch blocks. It's a very bad practice to return or throw an exception from the finally block for this reason.

Nothing to do with stacks: it's a matter of the fact that the finally block gets always executed as last block, so what is returned or thrown in the finally block erases and overwrites any previous exit value (returned object or exception thrown).

Try this:

try {
    throw new NullPointerException();
}
finally {
    return 0;
}

The return in the finally block will overwrite (swallow) the NullPointerException...

Answer 3

A try block is executed before its finally block.

When the return statement is executed, the value to be returned is stored. When the finally block completes, that value is returned.

Note that a is not a value. a is a variable that stores a value. If you change a, you change a, you don't change the value that was stored for the return.

For the technical reason, we go to the Java Language Specification on the try-finally block

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).

A return statement completes abruptly, so if a try with a finally has a return and the finally also has a return, the finally's return supersedes the try's.