Nested Maps or combined keys in java

Question

I need a Map to make a cache in Java for same values that I have two String keys. My question it\'s better to make nested Maps (one for each key) or make some type of custom

Answer 1

You can either make nested maps or use a custom class defining hashCode().

It's usually not a good idea to concatenate the keys, you can end up with collisions, as in the case with keys 1 and 22 and keys 12 and 2. They'd map to the same value 122.

If you'll always use both keys, using a single Map will always be a little more efficient, and you can always define your own adapter to the map that will take two arguments:

public class MyCache { 

    private Map cache = new HashMap();

    public Object getObject(Object key1, Object key2){
        return cache.get(new MyKey(key1, key2));
    }
    public void putObject(Object key1, Object key2, Object value){
        cache.put(new MyKey(key1, key2), value);
    }
}

Remember to define equals() and hashCode() in your custom key class (add checks for nullity if needed).

public int hashCode() {
    int result = 17;
    result = 37 * result + keyA.hashCode();
    result = 37 * result + keyB.hashCode();
    return result;
}

public boolean equals(Object another) { 
    return another.keyA.equals(keyA) && another.keyB.equals(keyB);
}