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 key maked with the two Strings?

Access to data on cache will all time accessed with the two keys and I don’t need group it by any of that two keys.

Then if is better combine string key in only one what it’s better?

• Custom class with custom getHash method. But then the problem is what hash function implement?

• Simply concatenate two strings together. For example:

  cache.put(key1+key2, value)

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<MyKey, Object> cache = new HashMap<MyKey, Object>();

    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);
}