unwind to the first frame of a recursive call in Java?

Question

Suppose I have a (very simple) recursive method like this:

public static void myMeth(int n)
{
     // do something

     // now execute the recursive call
     if (n < 0) return;
     else if ( n == SOME_CONST ) throw new UnsupportedOperationException();
     else myMeth(n - 1);
}

(The second condition n == SOME_CONST is just there to make the point that sometimes an exception can occur, sometimes it does not).

Suppose I call myMeth(10), and that the exception does happen after a few recursive calls (say SOME_CONST == 5).

Is there any trick I could do (with try-catch block, that is) to get me back to the first frame of myMeth ?

You could catch and re-throw if it isn't the first call, not sure how efficient that is — Kevin DiTraglia, Jan 16 '14 at 19:42
@Juan a frame == a stack-frame. I think I was being pretty clear there. — One Two Three, Jan 16 '14 at 19:43
@KevinDiTraglia How do I know it's the first call? (I mean, without passing along some counter and stuff like that) — One Two Three, Jan 16 '14 at 19:43
@JuanManuel Well, I'm sorry, but you just have to bind it in the current context (ie., frame can only mean stack-frame here) — One Two Three, Jan 16 '14 at 19:45
@OneTwoThree Lots of ways, nothing super clean that I can think of, but you could add a new paramater that is true on the first call and false for the recursive calls. — Kevin DiTraglia, Jan 16 '14 at 19:47
You could wrap your recursive call in try / catch and let it "back track" from there to the call stack level you want to reach. Maybe by re-throwing the exception if you are not at the right level, yet. — tiguchi, Jan 16 '14 at 19:49
@OneTwoThree and you have to admit that you're not giving to much context here.. — Typo, Jan 16 '14 at 20:03

score 1 · Answer 1 · answered Jan 16 '14 at 19:53

1

This could work, there is probably a cleaner solution out there, but it's a start:

public static void myMeth(int n, boolean firstCall)
{
     // do something

     // now execute the recursive call

     try
     {
         if (n < 0) return;
         else if ( n == SOME_CONST ) throw new UnsupportedOperationException();
         else myMeth(n - 1, false);
     }
     catch(UnsupportedOperationException e)
     {
         if (firstCall)
         {
              //logic
         }
         else
         {
              throw e;
         }
     }
}

answered Jan 16 '14 at 19:53

Kevin DiTraglia

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Hi Kevin, he wants to "go back" like a jmp assembler instruction. Not to handle the exception... – Typo Jan 16 '14 at 20:01
He does want to handle the exception, he just wants to handle it on the first recursive call, this would essentially bubble the exception up to the first call. I doubt you are going to get anything resembling a jmp instruction within java. – Kevin DiTraglia Jan 16 '14 at 20:06
I know, but here's what it's posted and i quote: "...to get me back to the first frame of myMeth...". Get back doesn't mean handle the exception in the first iteration (sorry @OneTwoThree "frame"). – Typo Jan 17 '14 at 00:48

score 0 · Answer 2 · answered Jan 16 '14 at 19:47

0

try{

     myMeth(n);

catch (UnsupportedOperationException e) {
 myMeth(n); //or another number
}

answered Jan 16 '14 at 19:47

Typo

1,875
1
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No, that wouldn't work. Because you're in effect re-excuting `myMeth(n)`, and if the code marked as `do something` is not pure-functional (ie., it modifies the environment), you would end up modifying the env twice. – One Two Three Jan 16 '14 at 19:48

score 0 · Answer 3 · answered Jan 16 '14 at 19:55

Using another static variable to keep track of the first number (10)

    static int SOME_CONST = 5;
    static int keepN;

    public static void myMeth(int n) {
        // do something

        // now execute the recursive call
        try {
            if (n < 0) {
                return;
            } else if (n == SOME_CONST) {
                throw new UnsupportedOperationException();
            } else {
                myMeth(n - 1);
            }
        } catch (UnsupportedOperationException e) {
            if (n == keepN) {
                System.out.println(e);
                System.out.println("YES first frame");
            } else {
                System.out.println("NO");
                throw e;
            }
        }
    }

    public static void main(String[] args) {
        keepN = 10;
        myMeth(10);
    }

score 0 · Answer 4 · answered Jan 16 '14 at 20:12

// depth should be 0 on first call
public static boolean myMeth(int n, int depth)
{
     // do something

     // now execute the recursive call
     if (n < 0) return true;
     else if ( n == SOME_CONST ) return false;
     boolean success = myMeth(n - 1, depth + 1);
     if (depth == 0 && !success) {
         // uh-oh
     }
     return success;
}

Or if you don't care about each individual frame in the recursion, replace depth with a boolean and change to boolean success = myMeth(n - 1, false);

I'm not sure what you're asking when you say you want to get back to the first frame though. Do you want to go back to the beginning of the first method call, so you can repeat the steps in the // do something block? Or are you fine executing right after the recursive call to myMeth?

If you're generating the Exception yourself, I replaced the need for that by using booleans. If not, you can replace it. You could also just throw an exception in the first frame, while still using booleans.

score -1 · Answer 5 · answered Jan 16 '14 at 19:41

-1

Yes, but this kind of trick will miss the whole concept of the recursive and will be hard to read and understand. You shouldn't use recursive if you can't expect the definite number of options it can produce.

Otherwise use another solution.

answered Jan 16 '14 at 19:41

Orel Eraki

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2

Can you actually provide the solution? Handling exceptions in recursive methods seems pretty reasonable to me. – OrangeDog Jan 16 '14 at 19:44
@OrangeDog, handling exception - yes, returning to the first recursive forking - isn't. It can be done, but it will miss greatly the purpose of recursive and the solution as i said will be hard to read and understand. – Orel Eraki Jan 16 '14 at 19:46

unwind to the first frame of a recursive call in Java?

5 Answers5