Question

I have to develop something like a game of life. For that, I have a class called CellPosition that has x and y fields. In order to efficiently use memory, I would like to use some kind of factory method.

CellPosition.at(int x, int y) which would return an instance of CellPosition. I would like though to cache the objects that have the same x, y pair. I though of a List or a HashMap, but I cannot figure out what to use as a key. A concatenation of x and y in a string is doubtingly a good idea.

On the other hand, is it a good idea to just create an object each time and just redefine the equals() method to compare the objects and throw away any caching?

Answer 1

If you don’t mind using Guava, just:

1. Make CellPosition instances immutable, then
2. Use an Interner<CellPosition> (obtained from Interners), then
3. Move on to solving real problems.

Something like this:

class CellPosition
{
    private static final Interner<CellPosition> CACHE = Interners.newStrongInterner();
    // or .newWeakInterner(), to allow instances to be garbage collected

    private final int x;
    private final int y;

    private CellPosition(int x, int y)
    {
        this.x = x;
        this.y = x;
    }

    public int x() { return x; }
    public int y() { return y; }

    public static CellPosition at(int x, int y)
    {
        return CACHE.intern(new CellPosition(x, y));
    }

    @Override
    public boolean equals(Object other) {/* TODO */}

    @Override
    public int hashCode() {/* TODO */}
}   

You could also use a Guava Cache instead of an Interner, but there’s not much point since you’d have to construct an int-pair key for the cache — which you’re doing anyway for the interner, in fewer LoC.