code: public static <K> NumberMap<K, Integer> newIntegerMap() {
  		return new NumberMap<K, Integer>() {
  			@Override
  			public void add(K key, Integer addend) {
  				put(key, containsKey(key) ? (get(key) + addend) : addend);
  			}
  			@Override
  			public void sub(K key, Integer subtrahend) {
  				put(key, (containsKey(key) ? get(key) : 0) - subtrahend);
  			}
  		};
  	}
docs: Creates a NumberMap for Integers.
  
  @param <K>
  @return NumberMap&lt;K, Integer&gt;