合并具有重复键和更新值的两个HashMaps

Question

class MyObject
String name;
int counter;

MyObject(String name) {
this.name = name;
this.counter = 1;
}

// getters and setters...

class Main
HashMap<String, MyObject> map1 = new HashMap<>();
HashMap<String, MyObject> map2 = new HashMap<>();

//some code that will put some instances of MyObject into map1 and map2
// assume that least three instances have the same keys

HashMap<String, MyObject> map3 = new HashMap<>(map1);

map2.forEach((key, value) -> map3.merge(key, value, (v1, v2) -> new MyObject(v1.getName())));

假设这两个对象有相同的键；

MyObject.getCounter() = 3 in map1

MyObject.getCounter() = 5 in map2

The new MyObject.getCounter() should be 8.

Answer 1

您可以按以下方式进行：

• 对两个条目集进行流式传输。
• 收集到地图
• 并在合并方法期间添加它们

创建一些数据

HashMap<String, MyObject> map1 = new HashMap<>();
HashMap<String, MyObject> map2 = new HashMap<>();

map1.put("A", new MyObject(2));
map1.put("B", new MyObject(4));
map1.put("C", new MyObject(10));
map2.put("C", new MyObject(10));
map2.put("B", new MyObject(8));

Map<String, MyObject> result = Stream
        .concat(map1.entrySet().stream(),
                map2.entrySet().stream())
        .collect(Collectors.toMap(Entry::getKey,
                Entry::getValue, (a, b) -> {
                    a.setCounter(a.getCounter() + b.getCounter());
                    return a;
                }, HashMap::new));

result.entrySet().forEach(System.out::println);

A=2
B=12
C=20

        Map<String, MyObject> result = Stream.of(map1,map2)
                .flatMap(map->map.entrySet().stream())
                .collect(...);

    
 class MyObject {
    
    int value;
    public MyObject(int v) {
        this.value = v;
    }
    public int getCounter() {
        return value;
    }
    
    public void setCounter(int a) {
        value = a;
    }
    
    public String toString() {
        return value + "";
    }
}