Is this a pattern? What would be its name?

I'm learning about patterns and would like to know if this implementation is one of them.

I have two Java classes. The first one is called "Index", and the second one, temporarily, I called it "IndexCache":

public class IndexCache {
    private static Map<String, Index> map = null;

    static {
        map = new HashMap<String, Index>();
    }

    @Override
    public Index getIndexString key) {
        Index index = map.get(key);

        if (index == null) {
            index = new Index("yadayadayada");
            map.put(key, index);
        }

        return index;

    }

}

As you can see, IndexCache keeps a map of the Index objects to avoid to create the same object once and again.

Well, is this a pattern? What would be its name? Or is it an anti-pattern?