comparable on a generic class java

1    public class DoubleKey<K extends Comparable<K>, J extends Comparable<J>>
2        implements Comparable<DoubleKey<K, J>> {
3
4    private K key1;
5    private J key2;
6
7    public DoubleKey(K key1, J key2) {
8        this.key1 = key1;
9        this.key2 = key2;
10    }
11
12    public K getFirstKey() {
13        return this.key1;
14    }
15
16    public J getSecondKey() {
17        return this.key2;
18    }
19
20    public int compareTo(DoubleKey<K, J> that) {
21
22        int cmp = this.getFirstKey().compareTo(that.getFirstKey());
23        if (cmp == 0)
24            cmp = this.getSecondKey().compareTo(that.getSecondKey());
25        return cmp;
26    }
27}