Question

I want to implement a HashMap with a key with 2 components. Ex.

Pseudocode:

Key = <Component1, Component2>
CollName<Key, Val> coll = new CollName<Key, Val>;

How can I implement this in Java, considering speed and size of the Collection. Thanks 😀

Answer 1

You need a helper (composite key) class that holds your two keys

public class CompositeKey<... , ...> {
   ... component1;
   ... component2;

   // getter , setter, ...

   // equals

  // hashcode()
}

and then you can use it as key:

CompositeKey cKey = new CompositeKey(1,2);
Map x.put(cKey,val);

It is very important here to implement equals() and hashCode() in a good fashion. Most IDEs can help you here. Important is for the hashCode that it returns a “unique” value in order to prevent hash collisions of the keys (i.e. returning a constant value is the worst case, as all values would end up in the same bucket). Many implementations of hashcode do something along

hashcode = component1.hashCode() + 37* component2.hashCode();

If you want more details, dig out any CS book that talks about hashing algorithms.

If you want to use that for persistence, also have a look at this blog post.