Java 8 按键分组映射

Question

我想按键对地图对象进行分组。我尝试使用此代码，但出现编译错误：

Non-static method cannot be referenced from a static context

我的代码：

Map<String, List<A>> getAMap() {        

    return Arrays.stream(SomeArray.values())

            .map(map -> createObject())

            .collect(Collectors.groupingBy(Map.Entry::getKey, 

                  Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

}

private Map<String, A> createObject() 

    final A a = new A(some attributes);

    Map<String, A> map = new LinkedHashMap<>();

    map.put(some key, a);

    .... // add another values. 

    return map;

}

我需要类似的东西

{

"a", {a1, a2, a3},

"b", {a4, a5, a6},

}

Answer 1

看起来您的代码在某些级别上是错误的，并且该错误消息并不是究竟发生了什么。例如createObject()返回 aMap所以你得到 a&nbsp;Stream<Map<...>>，所以显然.collect(Collectors.groupingBy(Map.Entry::getKey...行不通。您需要稍微更改代码才能使其正常工作：Arrays.stream(someArray) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;.flatMap(map&nbsp;->&nbsp;createObject().entrySet().stream()) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;.collect(Collectors.groupingBy(Entry::getKey, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Collectors.mapping(Entry::getValue,&nbsp;Collectors.toList())));