DouKaku?

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.Random;


public class Sample177 {
    private static final char[] CHARACTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
    private static final int INITIAL_COUNT = 300;
    private static final int CREATE_COUNT = 3;


    private final Random random_ = new Random();
    private List<String> cache_ = new ArrayList<String>();

    private final String target_;
    private final Comparator<String> comparator_;


    public Sample177(String target) {
        target_ = target;
        comparator_ = new StringDistanceComparator(target);
        init();
    }
    private void init() {
        for (int count = 0; count < INITIAL_COUNT; count++) {
            cache_.add(createString(target_.length()));
        }
        Collections.sort(cache_, comparator_);
    }
    private String createString(int length) {
        StringBuilder builder = new StringBuilder(length);
        for (int index = 0; index < length; index++) {
            builder.append(CHARACTERS[random_.nextInt(CHARACTERS.length)]);
        }
        return builder.toString();
    }

    public String getTop() {
        return cache_.get(0);
    }

    public void nextStep() {
        for (String str: cache_.toArray(new String[0])) {
            for (int index = 0; index < CREATE_COUNT; index++) {
                cache_.add(changeCharacter(str));
            }
        }
        Collections.sort(cache_, comparator_);
        cache_ = cache_.subList(0, INITIAL_COUNT);
    }
    public String changeCharacter(String str) {
        StringBuilder builder = new StringBuilder(str);
        builder.setCharAt(random_.nextInt(str.length()), CHARACTERS[random_.nextInt(CHARACTERS.length)]);
        return builder.toString();
    }


    static class StringDistanceComparator implements Comparator<String> {
        private final String target_;
        public StringDistanceComparator(String target) {
            target_ = target;
        }

        @Override
        public int compare(String o1, String o2) {
            return calcDistance(o1) - calcDistance(o2);
        }
        private int calcDistance(String other) {
            int distance = 0;
            for (int index = 0, len = target_.length(); index < len; index++) {
                distance += (target_.charAt(index) == other.charAt(index))? 0 : 1;
            }
            return distance;
        }
    }


    public static void main(String[] args) {
        String target = "METHINKSITISAWEASEL";
        Sample177 sample = new Sample177(target);

        int index = 1;
        String top = sample.getTop();
        System.out.println(index++ + ":" + top);
        while (!top.equals(target)) {
            sample.nextStep();
            top = sample.getTop();
            System.out.println(index++ + ":" + top);
        }
    }
}