Question

I have an X object whose constructor takes in 4 integers fields. To calculate it’s hash function, I simple throw them in an array and use Arrays.hashCode.

Currently the constructor is private and I have a static creator method. I’d like to memoize construction so that whenever the creator method is called with 4 integer parameters that have been called before, I can return the same object as last time. [Ideally without having to create another X object to compare with.]

Originally I tried a hashSet but that required me to create a new X to check if my hashSet.contains the equal object… nevermind the fact that I can’t ‘get’ out of a hashSet.

My next idea is to use a HashTable which maps:
the hashCode of the int array of the 4 fields –> object. I’m not sure why, but that doesn’t feel right. It feels like I’m doing too much work, isn’t the point of a hashCode to be a sort of mapping to a bunch of objects which calculate to the same hashCode?

I appreciate your advice.

Answer 1

Not sure how you plan to use the Hashtable but I think below would do your job:

    private static Hashtable<Integer, MyObject> objectInstances = 
             new Hashtable<Integer, MyObject>();


    public static MyObject instance(int i1, int i2, int i3, int i4){
         int hashKey = Arrays.hashCode(new int[]{i1, i2,i3,i4});
         //get the object from hashtable
         MyObject myObject = objectInstances.get(hashKey);

         //if object was not already created, create now and put in the hashtable
         if(myObject == null){
           myObject = new MyObject(i1,i2,i3,i4);
           objectInstances.put(hashKey, myObject);
         }
        return myObject;
    }