Benchmark for http://stackoverflow.com/questions/12585288/a-fast-way-to-find-unique-values-in-the-list/12585320#12585464

Benchmark.java

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Random;
import java.util.Set;
import java.util.concurrent.TimeUnit;

public class Benchmark {

    public static void main(String[] args) {

        int rounds = 10000;
        int size = 5000;
        List<KVPair> testData = generateRandom(size);

        System.out.println("Starting benchmark of u1");
        long start = System.nanoTime();
        for (int i = 0; i < rounds; i++) {
            Set<String> s = u1(testData);
        }
        long ellapsed = System.nanoTime() - start;
        long ms = TimeUnit.MILLISECONDS.convert(ellapsed, TimeUnit.NANOSECONDS);

        System.out.printf("%d iterations took %s ms\n", rounds, ms);
        System.out.printf("Mean iteration time %.3f ms\n", (1.0 * ms / rounds));

        System.out.println("\nStarting benchmark of u2");
        start = System.nanoTime();
        for (int i = 0; i < rounds; i++) {
            Collection<String> s = u2(testData);
        }
        ellapsed = System.nanoTime() - start;
        ms = TimeUnit.MILLISECONDS.convert(ellapsed, TimeUnit.NANOSECONDS);

        System.out.printf("%d iterations took %s ms\n", rounds, ms);
        System.out.printf("Mean iteration time %.3f ms\n", (1.0 * ms / rounds));

        System.out.println("\nStarting benchmark of u3");
        start = System.nanoTime();
        for (int i = 0; i < rounds; i++) {
            Collection<String> s = u3(testData);
        }
        ellapsed = System.nanoTime() - start;
        ms = TimeUnit.MILLISECONDS.convert(ellapsed, TimeUnit.NANOSECONDS);

        System.out.printf("%d iterations took %s ms\n", rounds, ms);
        System.out.printf("Mean iteration time %.3f ms\n", (1.0 * ms / rounds));

        System.out.println("Done");
    }

    private static Random rand = new Random();

    private static List<KVPair> generateRandom(int size) {
        List<KVPair> list = new ArrayList<KVPair>(size);
        for (int i = 0; i < size; i++) {
            String k = String.valueOf(rand.nextInt());
            //limit value size to [0, 1000) to make sure list contains duplicates
            String v = String.valueOf(rand.nextInt(1000));
            list.add(new KVPair(k, v));
        }
        return list;
    }

    private static Set<String> u1(List<KVPair> pairs) {
        Set<String> undefined = new HashSet<String>();

        for (KVPair pair : pairs) {
            undefined.add(pair.getValue());
        }

        if (undefined.size() == 1) {
            return new HashSet<String>();
        }
        return undefined;
    }

    private static List<String> u2(List<KVPair> pairs) {

        List<String> undefined = new ArrayList<String>();
        for (KVPair pair : pairs) {
            if (!undefined.contains(pair.getValue())) {
                undefined.add(pair.getValue());
            }
        }

        return undefined;
    }

    private static List<String> u3(List<KVPair> pairs) {

        List<String> undefined = new LinkedList<String>();

        Iterator<KVPair> it = pairs.iterator();
        while (it.hasNext()) {
            String value = it.next().getValue();
            if (!undefined.contains(value)) {
                undefined.add(value);
            }
        }
        return undefined;
    }
}

output

Starting benchmark of u1
10000 iterations took 2203 ms
Mean iteration time 0.220 ms

Starting benchmark of u2
10000 iterations took 112173 ms
Mean iteration time 11.217 ms

Starting benchmark of u3
10000 iterations took 127456 ms
Mean iteration time 12.746 ms
Done