如何“卷积”两个流

Question

I'm a newbie in streams, but really fascinating by functional programming. I would like to "concat" two maps (ie dictonaries) Map<Character, Double> map1 and Map<Character, Double> map2 having the same key-sets (the set of chars) in such a way that results in the third map Map<Character, Double> map3 that have the set of keys the same as the previous two, and the values equal to differences of appropriate values from map1 and map2 . Could you please help? How to obtain the result by means of streams?

Answer 1

This solution might be helpful.

    Map<Character, Double> map1 = new HashMap<>();
    map1.put('A', 23.0);
    map1.put('B', 50.0);

    Map<Character, Double> map2 = new HashMap<>();
    map2.put('A', 15.0);
    map2.put('B', 40.0);

    //method 1 : Result stored in third map
    Map<Character, Double> map3 = new HashMap<>();
    map1.keySet().forEach(key -> map3.put(key, map1.get(key) - map2.get(key)));

    //method 2 : using collectors
    Map<Character, Double> map4 = map1.entrySet().stream()
            .collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue() - map2.get(entry.getKey())));

    //method 3 : Result stored in map1
    map2.forEach((key, val) -> map1.merge(key, val, (val1, val2) -> val1 - val2));

    System.out.println("map1 = " + map1);
    System.out.println("map3 = " + map3);
    System.out.println("map4 = " + map4);

Refer Lambda expression java docs for better understanding.