1、List<T>轉Map<S,List<T>>

        Map<String, List<Entity>> demoMap = demoList.stream()
                .collect(Collectors.groupingBy(Entity::getkey));  // the type of demoList is List<Entity>

Entity例項getkey()方法返回的值則作為map的key，即按該欄位給demoList分類。

2、Map<S,List<T>>轉List<T>

        List<Entity> demoList = refDataMap.entrySet().stream()
                .flatMap(map -> map.getValue().stream())
                .collect(Collectors.toList());  //the type of refDataMap is Map<S,List<Entity>>

       Map<String,Entity> map = stats.stream().collect(Collectors.toMap(Entity::getKey,
                c -> c));
       Map<String,String> map = stats.stream().collect(Collectors.toMap(Entity::getKey,
                Entity::getStringValue));   //the type of stats if List<Entity>