地图<X, 地图<Y, Z> 到地图<Y, 地图<X, Z>

Question

我想切换地图的键和其中的地图：

Map<X, Map<Y, Z> -> Map<Y, Map<X, Z>

我试过使用流，但无法创建内部映射或如何分别从原始内部映射访问键和值。

//到目前为止我已经尝试过：

originalMap.entrySet().stream().collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));

Answer 1

有点复杂，但它有效：Map<Y,Map<X,Z>> out =&nbsp; &nbsp; &nbsp; originalMap.entrySet()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.flatMap(e -> e.getValue()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .entrySet()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .map(e2 -> {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Map<X,Z> m = new HashMap<>();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.put(e.getKey(),e2.getValue());&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return new SimpleEntry<>(e2.getKey(),m);}))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(v1,v2)->{v1.putAll(v2);return v1;}));Stream基本上，它将原始的Map条目转换为Stream所需输出的平面条目Map（其中每个内部Map只有一个Entry），并使用具有合并功能的收集器合并与相同外部键对应的toMap内部s Map.例如，使用以下输入 Map 运行此代码：Map<String,Map<Integer,Double>> originalMap = new HashMap<>();Map<Integer,Double> inner1 = new HashMap<>();Map<Integer,Double> inner2 = new HashMap<>();Map<Integer,Double> inner3 = new HashMap<>();originalMap.put("aa",inner1);originalMap.put("bb",inner2);originalMap.put("cc",inner3);inner1.put(10,10.0);inner1.put(20,20.0);inner1.put(30,30.0);inner2.put(10,40.0);inner2.put(20,50.0);inner2.put(30,60.0);inner3.put(10,70.0);inner3.put(20,80.0);inner3.put(30,90.0);Map<Integer,Map<String,Double>> out =&nbsp; &nbsp; originalMap.entrySet()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.flatMap(e -> e.getValue()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .entrySet()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .map(e2 -> {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Map<String,Double> m = new HashMap<>();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; m.put(e.getKey(),e2.getValue());&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return new SimpleEntry<>(e2.getKey(),m);}))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(v1,v2)->{v1.putAll(v2);return v1;}));System.out.println (out);将输出：{20={aa=20.0, bb=50.0, cc=80.0}, 10={aa=10.0, bb=40.0, cc=70.0}, 30={aa=30.0, bb=60.0, cc=90.0}}编辑：如果您使用的是 Java 9 或更高版本，您可以使用Map.of它来稍微简化代码：Map<Y,Map<X,Z>> out =&nbsp; &nbsp; &nbsp; originalMap.entrySet()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.flatMap(e -> e.getValue()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .entrySet()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .map(e2 -> new SimpleEntry<>(e2.getKey(),Map.of(e.getKey(),e2.getValue()))))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(v1,v2)->{v1.putAll(v2);return v1;}));

Answer 2

对我来说，实际上，没有流，这要容易得多；但仍在使用 java-8 功能（示例取自其他答案）：&nbsp; &nbsp; Map<Integer, Map<String, Double>> result = new HashMap<>();&nbsp; &nbsp; originalMap.forEach((key, value) -> {&nbsp; &nbsp; &nbsp; &nbsp; value.forEach((innerKey, innerValue) -> {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Map<String, Double> map = new HashMap<>();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; map.put(key, innerValue);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; result.merge(innerKey, map, (left, right) -> {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; left.putAll(right);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return left;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; });&nbsp; &nbsp; &nbsp; &nbsp; });&nbsp; &nbsp; });&nbsp; &nbsp; System.out.println(result);