Most memory-efficient map in java

If you want to map a (String,String,Integer) to a Float, then the best thing to do is to use a Map<MyKey, Float>, where MyKey would be defined like this:

public final class MyKey {
    private final String a;
    private final String b;
    private final Integer c;

    public MyKey(String a, String b, Integer c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    // getters, if needed

    @Override
    public int hashCode() {
        return Objects.hash(a, b, c);
    }

    @Override
    public boolean equals(Object o) {
        if (o == this) {
            return true;
        }
        if (!(o instanceof MyKey)) {
            return false;
        }
        MyKey other = (MyKey) o;
        return Objects.equal(a, o.a)
               && Objects.equal(b, o.b)
               && Objects.equal(c, o.c);
    }
}

You have two kinds of maps here. One which has String keys and Map values. For that I'd probably use Google Guava's ImmutableMap if immutability is ok for you. It will probably not save you a lot of memory, but it might save you some, and perform a bit better than a normal HashMap.

For the other Map with Integer keys and Double values, you should use a specialized Map implementation which stores primitives instead of objects. Take for instance a look at Trove4j's TIntDoubleHashMap. This will save you a lot of memory, as the primitives are stored as primitives instead of Objects.