java下map合併

java8下map合併可以有以下幾種方式：

map爲待合併集合，map2爲被合併集合（將map2中元素合併到map中）

1. map.merge()

   map2.forEach((key, value) -> {
         map.merge(key, value, (origin, newVlue) -> {
           origin.putAll(newVlue);
           return origin;
         });
       });

    

2. Stream.concat()

   Stream
           .concat(map.entrySet().stream(), map2.entrySet().stream())
           .collect(Collectors.toMap(
               Map.Entry::getKey, Map.Entry::getValue,
               (origin, newValue) -> {
                 origin.putAll(newValue);
                 return origin;
               }));

    

3. Stream.of()

   Stream
           .of(map, map2)
           .flatMap(m -> m.entrySet().stream())
           .collect(Collectors.toMap(
               Map.Entry::getKey, Map.Entry::getValue,
               (origin, newValue) -> {
                 origin.putAll(newValue);
                 return origin;
               }));

    

4. Simple Streaming

   map2.entrySet()
           .stream()
           .collect(Collectors.toMap(
               Map.Entry::getKey,
               Map.Entry::getValue,
               (origin, newValue) -> {
                 origin.putAll(newValue);
                 return origin;
               }, () -> map // 彙總者
           ));

    

參考：https://www.baeldung.com/java-merge-maps

源碼：https://github.com/eugenp/tutorials/tree/master/java-collections-maps-2