ClobberTable for Syntax Tables

ClobberTable.java

class ClobberTable<K, T> {
	ClobberTable parent;
	HashTable<K, T> my;
        ArrayList<K> whites;

	public ClobberTable() {
		parent = null;
		my = new HashTable<K, T>();
                whites = new ArrayList<K>();
	}

	public void put(K a, T b) {
		my.put(a,b);
                if(whites.contains(a)) whites.remove(a);
	}

	public T get(K a) {
                if(whites.contains(a)) return null;
		if(my.hasKey(a)) { return my.get(a); }
		if(parent != null && parent.hasKey(a)) return parent.get(a);
		return null;
	}

        public void remove(K a) {
                if(my.contains(a)) {
                    my.remove(a);
                }
                else if (parent.contains(a)) {
                    whites.add(a);
                }
        }

	public boolean hasKey() {
		if(my.hasKey(a) || (parent != null && parent.hasKey(a))) return true;
	}

	public ClobberTable<K, T> fork() {
		ClobberTable child = new ClobberTable<K, T>();
		child.parent = this;
		return child;
	}
}

Pretty neat.