mergewith.java

    private static class Tuple2<T,U>{
        final T v1;
        final U v2;
        
        Tuple2(T v1, U v2){
            this.v1 = v1;
            this.v2 = v2;
        }
    }
    
    private static <K,V> void merge(Map<K,V> m, K k, V v, Function<Tuple2<V, V>,V> f){
         V oldValue = m.get(k);
         V newValue = (oldValue == null) ? v : f.apply(new Tuple2<V,V>(oldValue, v));
         if (newValue == null)
             m.remove(k);
         else
             m.put(k, newValue);

    }
    
    private static <K,V> HashMap<K, V> mergeWith(Map<K,V> m1, Map<K,V> m2, Function<Tuple2<V,V>,V> vFunction){
        HashMap<K, V> m3 = new HashMap<K,V>(m1);
        for (Map.Entry<K, V> e: m2.entrySet()){
            merge(m3, e.getKey(), e.getValue(), vFunction);
        }

        return m3;
    }