Java tutorial
//package com.java2s; import java.util.HashMap; import java.util.Map; public class Main { public static <K> Map<K, Integer> uniomCounterMap(Map<? extends K, Integer> first, Map<? extends K, Integer> second) { Map<K, Integer> ret = new HashMap<K, Integer>(first); for (K key : second.keySet()) { if (ret.containsKey(key)) { ret.put(key, ret.get(key) + second.get(key)); } else { ret.put(key, second.get(key)); } } return ret; } }