java8 List根据某个字段去重

矫情吗；* 2021-09-08 00:14 509阅读 0赞

## 1、使用toCollection和TreeSet去重 ##

TreeSet内部使用的是TreeMap，使用指定Comparator比较元素，如果元素相同，则新元素代替旧元素。

List<TalentPlanStudentEntity> studentList = relatePlanStudentList.stream()
                        .collect(Collectors.collectingAndThen(Collectors.toCollection(
                                () -> new TreeSet<>(Comparator.comparing(TalentPlanStudentEntity::getUserId))), ArrayList::new));

## 2、使用Collectors.toMap去重 ##

Collectors.toMap需要使用三个参数的版本，前两个参数一个是keyMapper函数一个是valueMapper函数的，第三个参数BinaryOperator函数接口。BinaryOperator函数接收两个参数，一个oldValue，一个newValue。用于当key重复时的数据处理

List<TalentPlanStudentEntity> studentList = new ArrayList<>(relatePlanStudentList.stream()
                        .collect(Collectors.toMap(TalentPlanStudentEntity::getUserId, Function.identity(), (oldValue, newValue) -> oldValue))
                        .values());

## 例子 ##

List<User> userList = new ArrayList<>();
            
            userList.add(new User("1","李大锤","23","南京"));
            userList.add(new User("2","张无忌","18","西安"));
            userList.add(new User("3","刘德华","26","苏州"));
            userList.add(new User("4","郭靖","33","上海"));
            
            userList.add(new User("1","李大锤","23","南京"));    //id相同，其他数据也相同
            userList.add(new User("3","带头大哥","36","杭州"));  //id相同，其他数据不同
            
            System.out.println(userList);
            //根据userid去重
            userList = userList.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new       TreeSet<>(Comparator.comparing(User :: getUserid))), ArrayList::new));