Paste: Generic inner interface

public class ObjectCache<T> extends HashMap<String, T> {
    private final Factory factory;
    public interface Factory<T> {
        T getObject(String key);
    }

    public ObjectCache(Factory factory) {
        this.factory = factory;
    }
    @Override
    public T get(Object key) {
        if (!(key instanceof String))
            throw new IllegalArgumentException("Key must be a string");

        String search = (String)key;
        if (search.isEmpty())
            return null;

        T obj = super.get(key);
        if (null == obj) {
            obj = factory.getObject(search);
            if (null != obj)
                put(search, obj);
        }
        return obj;
    }
}