java之简化java流以查找重复属性

我有一个 users list 并且我想找到所有具有重复名称的用户:

var allNames = users 
              .stream() 
              .map(u -> u.getName()).collect(Collectors.toList()); 
 
var duplicateNames = allNames 
                .stream() 
                .filter(i -> Collections.frequency(allNames, i) > 1) 
                .collect(Collectors.toSet()); 

请您参考如下方法:

一种解决方案是

var duplicate = users.stream() 
    .collect(Collectors.toMap(User::getName, u -> false, (x,y) -> true)) 
    .entrySet().stream() 
    .filter(Map.Entry::getValue) 
    .map(Map.Entry::getKey) 
    .collect(Collectors.toSet()); 

var duplicate = users.stream() 
    .collect(Collectors.collectingAndThen( 
        Collectors.toMap(User::getName, u -> false, (x,y) -> true, HashMap::new), 
            m -> { 
                m.values().removeIf(dup -> !dup); 
                return m.keySet(); 
            })); 

HashSet<String> seen = new HashSet<>(), duplicate = new HashSet<>(); 
for(User u: users) 
    if(!seen.add(u.getName())) duplicate.add(u.getName());