如何使用java流过滤map的map

Question

如何使用java流过滤映射的映射。我可以用双环来做，但我认为这样做效率不高。

public class MapUsage {
static Map<Integer, Map<Integer, String>> parentMap = new HashMap<>();
static Map<String, String> userRole = new HashMap<>();
public static void main(String ... args){
initializeData();
displayMapForUser("user1", parentMap);
//        printMap(parentMap);
}

private static void displayMapForUser(String user1, Map<Integer, Map<Integer, String>> parentMap) {
Integer role = new Integer(userRole.get(user1));
Map<Integer, Map<Integer, String>> userMap = new HashMap<>();
Map<Integer, String> childMap = new HashMap<>();
for(Map.Entry<Integer, Map<Integer, String >> entry : parentMap.entrySet()){
for(Map.Entry<Integer, String > entry1: entry.getValue().entrySet()){
if(entry1.getKey().equals(role))
childMap.put(entry1.getKey(), entry1.getValue());
}
userMap.put(entry.getKey(), childMap);
}
printMap(userMap);
}
private static void printMap(Map<Integer, Map<Integer, String>> parentMap) {
for(Map.Entry<Integer, Map<Integer,String> > entry: parentMap.entrySet()){
System.out.println("key: "+entry.getKey());
System.out.println("value: "+entry.getValue());
}
}
private static void initializeData() {
Map<Integer, String > childMap1 = new HashMap<>();
Map<Integer, String > childMap2 = new HashMap<>();
userRole.put("user1", "1");
userRole.put("user2", "2");
userRole.put("user3", "3");
userRole.put("user4", "4");
childMap1.put(1, "one");
childMap1.put(2, "two");
childMap1.put(3, "three");
childMap1.put(4, "four");
parentMap.put(1, childMap1);
childMap2.put(1, "one");
childMap2.put(2, "two");
childMap2.put(3, "three");
parentMap.put(2, childMap2);
}
}

输出为：

key: 1
value: {1=one}
key: 2
value: {1=one}

Answer 1

您可以将Collectors.toMap用作：

Map<Integer, Map<Integer, String>> userMap = parentMap.entrySet().stream()
.collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue()
.entrySet().stream()
.filter(en -> en.getKey().equals(role))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue))));

但请注意，您在这里并没有获得显著的性能效率。只是可读性，你可能会把它写成：

Map<Integer, Map<Integer, String>> usersMap = new HashMap<>(parentMap);
usersMap.values().forEach(innerMap -> innerMap.entrySet().removeIf(en -> !en.getKey().equals(role)));