Java switch vs for loop performance

Question

I am doing something like this:

The first scenario:

for(int i=0; i<50; i++){
        // execute some other code here

        switch(myCustomInt){
            case 1:
            case 2:
            case 3:
                // execude method1
                break;
            case 4: 
                // execute method2
                break;
        }
    }

The second scenario:

 for(int i=0; i<50; i++){
        // execute some other code here
    }

  switch(myCustomInt){
        case 1:
        case 2:
        case 3:
            for(int i=0; i<50; i++){
                // execute method1
            }
            break;
        case 4:
            for(int i=0; i<50; i++){
                // execute method2
            }
            break;
    }

The Question:

The first scenario runs the for loop only once, but check the switch statement 50 times.

The second scenario runs the for loop twice, but check the switch statement only once.

Which is a better way to execute? I know in today's technology, the difference is most likely negligible. But I am still curious which one is, theoretically, the better way to execute it?

Thank you!

The best way is the one that is most readable and maintainable — Tim, Apr 14 '17 at 12:37
They appear to be exactly the same runtime complexity. I'd prefer example 1, because the second is repetitious - and if you have to change the loop bounds, now you have to do it in two (or more) places. — Elliott Frisch, Apr 14 '17 at 12:38
If `myCustomInt` does not change during the loop, it doesn't have to be checked each time the loop is run. So that would mean that the second example will be faster. However, due to [branch prediction](http://stackoverflow.com/a/11227902/507738), the difference will be totally insignificant. I would do what @TimCastelijns said. — MC Emperor, Apr 14 '17 at 12:50
Thank you for all of your responses! I will go for example 1 which is the more readable one. Much appreciated! — Marcus, Apr 14 '17 at 13:07

score 1 · Accepted Answer · edited May 23 '17 at 12:32

Note that there is a semantic difference in terms of when method1 and method2 are called for (say) i == 42 relative to the "other code" in the for loop: In the first, method1/method2 are called for the i == 42 iteration before "other code" for all of the i == 43 through i == 49 iterations is done. In the second, all of the "othe code" for the 50 loop iterations is done before the first call to either method1 or method2.

Other than that semantic difference, it really doesn't matter. Do what's most readable and maintainable. That probably means the first one, but what's "readable and maintainable" varies from person to person. Elliott's point that the second one repeats the loop header an additional two times is well-taken: It opens the door to a bug caused by changing one of the three and not one or both of the other two.

Performance: It's not going to matter. The first requires that the switch (myCustomInt) be evaluated 49 times more than the second. In theory, that evaluation costs time. In practice, I bet you'd have a really, really hard time measuring a difference, particularly if HotSpot (the Oracle Java runtime) decides this is a hotspot and aggressively optimizes it.

Thanks @T.J.Crowder for your detailed explanation. Indeed the semantic difference is worth noting here. I will go with example 1 which is more readable, and less chance of creating a future bug if I were to change the loop boundaries. Much appreciated! — Marcus, Apr 14 '17 at 13:10

Java switch vs for loop performance

1 Answers1