如何使用泛型结合两个包含相同类型的HashMap对象，并控制重复的情况

Question

java-如何使用泛型有效合并两个包含相同类型的Map对象，并控制重复的情况（如果值是Integer则加或乘，如果值是String则Concat）时会发生什么？

Answer 1

Add the elements of one map to the other one at a time using Map.merge :

for (Map.Entry<K, V> entry : map2.entrySet()) {
  map1.merge(entry.getKey(), entry.getValue(), (oldV, newV) -> /* some expression to combine old and new values */);
}

If you are using Java 7 or earlier, the above linked documentation shows how you can implement it without Java 8 features (in the "Implementation Requirements" section).

Answer 2

You can try like below example:

Map<String, Integer> map1 = new HashMap<>();
map1.put("Java", 7);
map1.put("C#", 4);
Map<String, Integer> map2 = new HashMap<>();
map2.put("Java", 3);
map1.put("Scala", 5);

map1.keySet().forEach(e -> {
    map2.computeIfPresent(e, (String key,Integer value)-> value+map1.get(key));
    map2.putIfAbsent(e, map1.get(e));
});

System.out.println(map2);

EDIT: If you want to handle all of your use cases then you can do like below.

Map<String, Object> map1 = new HashMap<>();
map1.put("Java", 7);
map1.put("C#", 4);
Map<String, Object> map2 = new HashMap<>();
map2.put("Java", 3);
map1.put("Scala", 5);

map1.keySet().forEach(e -> {
    map2.computeIfPresent(e, (String key, Object value) -> {
        if (value instanceof Integer)
            return Integer.sum((Integer) value, (Integer) map1.get(key));
        if (value instanceof String) {
            String s1 = (String) value;
            String s2 = (String) map1.get(key);
            return s1.concat(s2);
        }
        return null;
    });
    map2.putIfAbsent(e, map1.get(e));
});

System.out.println(map2);