Fastest way to get all values from a Map where the key starts with a certain expression

Question

Consider you have a map myMap.

Given the expression \"some.string.*\", I have to retrieve all the values from m

Answer 1

I used this code to do a speed trial:

public class KeyFinder {

    private static Random random = new Random();

    private interface Receiver {
        void receive(String value);
    }

    public static void main(String[] args) {
        for (int trials = 0; trials < 10; trials++) {
            doTrial();
        }
    }

    private static void doTrial() {

        final Map map = new HashMap();
        giveRandomElements(new Receiver() {
            public void receive(String value) {
                map.put(value, null);
            }
        }, 10000);

        final Set expressions = new HashSet();
        giveRandomElements(new Receiver() {
            public void receive(String value) {
                expressions.add(value);
            }
        }, 1000);

        int hits = 0;
        long start = System.currentTimeMillis();
        for (String expression : expressions) {
            for (String key : map.keySet()) {
                if (key.startsWith(expression)) {
                    hits++;
                }
            }
        }
        long stop = System.currentTimeMillis();
        System.out.printf("Found %s hits in %s ms\n", hits, stop - start);
    }

    private static void giveRandomElements(Receiver receiver, int count) {
        for (int i = 0; i < count; i++) {
            String value = String.valueOf(random.nextLong());
            receiver.receive(value);
        }

    }
}

The output was:

Found 0 hits in 1649 ms
Found 0 hits in 1626 ms
Found 0 hits in 1389 ms
Found 0 hits in 1396 ms
Found 0 hits in 1417 ms
Found 0 hits in 1388 ms
Found 0 hits in 1377 ms
Found 0 hits in 1395 ms
Found 0 hits in 1399 ms
Found 0 hits in 1357 ms

This counts how many of 10000 random keys start with any one of 1000 random String values (10M checks).

So about 1.4 seconds on a simple dual core laptop; is that too slow for you?