Throw and catch an exception, or use instanceof?

Question 1

I'd advise using instanceof as it will likely be faster. Throwing an exception is a complicated and expensive operation. JVMs are optimized to be fast in the case when exceptions don't happen. Exceptions should be exceptional.

Note that the throw technique probably won't compile as shown, if your exception type is a checked exception, the compiler will complain that you must catch that type or declare it as thrown (corresponding to an else { ... } clause if you use the instanceof technique), which might or might not be helpful, depending on how you want to handle exceptions that are not one of the specific sub-types.

Question 2

Hate to burst everyone's bubble, but using try/catch is faster. That's not to say it is the "correct" way, but if performance is key then that's the winner. Here are the results from the following program:

Run 1

Sub-run 1: Instanceof : 130 ms
Sub-run 1: Try/catch : 118 ms
Sub-run 2: Instanceof : 96 ms
Sub-run 2: Try/catch : 93 ms
Sub-run 3: Instanceof : 100 ms
Sub-run 3: Try/catch : 99 ms

Run 2

Sub-run 1: Instanceof : 140 ms
Sub-run 1: Try/catch : 111 ms
Sub-run 2: Instanceof : 92 ms
Sub-run 2: Try/catch : 92 ms
Sub-run 3: Instanceof : 105 ms
Sub-run 3: Try/catch : 95 ms

Run 3

Sub-run 1: Instanceof : 140 ms
Sub-run 1: Try/catch : 135 ms
Sub-run 2: Instanceof : 107 ms
Sub-run 2: Try/catch : 88 ms
Sub-run 3: Instanceof : 96 ms
Sub-run 3: Try/catch : 90 ms

Test environment

Java: 1.7.0_45
Mac OSX Mavericks

Discounting warmup sub-runs of each run the instanceof method only achieves at best the performance of try/catch. The average (discounting warm-ups) of the instanceof method is 98 ms and the average of try/catch is 92 ms.

Please note I did not vary the order in which each method was tested. I always tested a block of instanceof then a block of try/catch. I would love to see other results contradicting or confirming these findings.

public class test {

    public static void main (String [] args) throws Exception {
        long start = 0L;
        int who_cares = 0; // Used to prevent compiler optimization
        int tests = 100000;

        for ( int i = 0; i < 3; ++i ) {
            System.out.println("Testing instanceof");
            start = System.currentTimeMillis();
            testInstanceOf(who_cares, tests);
            System.out.println("instanceof completed in "+(System.currentTimeMillis()-start)+" ms "+who_cares);

            System.out.println("Testing try/catch");
            start = System.currentTimeMillis();
            testTryCatch(who_cares, tests);
            System.out.println("try/catch completed in "+(System.currentTimeMillis()-start)+" ms"+who_cares);
        }
    }

    private static int testInstanceOf(int who_cares, int tests) {
        for ( int i = 0; i < tests; ++i ) {
            Exception ex = (new Tester()).getException();
            if ( ex instanceof Ex1 ) {
                who_cares = 1;
            } else if ( ex instanceof Ex2 ) {
                who_cares = 2;
            }
        }
        return who_cares;
    }

    private static int testTryCatch(int who_cares, int tests) {
        for ( int i = 0; i < tests; ++i ) {
            Exception ex = (new Tester()).getException();
            try {
                throw ex;
            } catch ( Ex1 ex1 ) {
                who_cares = 1;
            } catch ( Ex2 ex2 ) {
                who_cares = 2;
            } catch ( Exception e ) {}
        }
        return who_cares;
    }

    private static class Ex1 extends Exception {}

    private static class Ex2 extends Exception {}

    private static java.util.Random rand = new java.util.Random();

    private static class Tester {
        private Exception ex;
        public Tester() {
            if ( rand.nextBoolean() ) {
                ex = new Ex1();
            } else {
                ex = new Ex2();
            }
        }
        public Exception getException() {
            return ex;
        }
    }
}

Question 3

I strongly urge you to actually use a plain object to represent your "constraint". Whether a marking interface (e.g. Message) or a java.lang.String is up to you. Exceptions are not meant to be used as you intend, even if either could be made to work (I would expect the second to be faster, but a premature optimization...).

Question 4

You could also use polymorphism by creating an interface for your custom exceptions that contains the getCustomViolation() method. Then each Custom exception would implement that interface and that method.