Question

I have hashmap that has keys like this:’2+4+5′ , ‘653+65+1324+75’.(integer values delimited by a + sign)

What could be a good hashcode and equals method so that keys like ‘2+4+5’, ‘5+4+2’, ‘4+5+2’ … (all the permutation of 2,4,5) should return the same hashcode value and equals should return true.

I am planning to take the integer values in the key, sort them, put them in an ascending order into a string and call that strings hashcode and equals method. Suppose if I have ‘5+2+4’ then I would change that to “245” and call the strings hashcode and equals method. But this is going to be an expensive operation since each time I have to do sorting. And all the methods in the hashmap like put,get… would again be expensive

Are there any other ways of doing this in a log or linear time…

Answer 1

class Combination {
    final int[] parts;

    Combination(String str) {
        String[] strings = str.split("\\+");
        parts = new int[strings.length];
        for (int i = 0; i < parts.length; i++) {
            parts[i] = Integer.parseInt(strings[i]);
        }
        Arrays.sort(parts);
    }

    @Override public int hashCode() {
        return Arrays.hashCode(parts);
    }

    @Override public boolean equals(Object o) {
        return o instanceof Combination && Arrays.equals(parts, ((Combination) o).parts);
    }
}

Test code:

public static void main(String[] args) {
    Set<Combination> set = new HashSet<>();
    set.add(new Combination("1+2+3"));
    set.add(new Combination("1+2+4"));
    System.out.println(set.contains(new Combination("3+2+1"))); // prints "true"
    System.out.println(set.contains(new Combination("4+2+1"))); // prints "true"
    System.out.println(set.contains(new Combination("4+3+1"))); // prints "false"
}