Java: Data structure for caching computation result?

Question

I have an expensive computation, the result of which I'd like to cache. Is there some way to make a map with two keys? I'm thinking of something like Map<(Thing1, Thing2), Integer>.

Then I could check:

if (! cache.contains(thing1, thing2)) {
  return computeResult();
}
else {
  return cache.getValue(thing1, thing2);
}

pseudocode. But something along those lines.

Answer 1

You need to create a class that holds Thing1 and Thing2, e.g:

class Things {
    public final Thing1 thing1;
    public final Thing2 thing2;
    public Things(Thing1 thing1, Thing2 thing2) {
      this.thing1 = thing1; 
      this.thing2 = thing2;
    }
    @Override
    public boolean equals(Object obj) { ... }
    @Override
    public int hashCode() { ... };
 }

Then to use it:

Things key = new Things(thing1, thing2);
if (!cache.contains(key) {
    Integer result = computeResult();
    cache.put(key, result);
    return result;
} else {
    return cache.getValue(key);
}

Note you have to implement equals and hashcode to make this code work properly. If you need this code to be thread-safe then have a look at ConcurrentHashMap.