前言

略

Collectors.toMap

List<User> userList = ...;
Map<Long, User> userMap = userList.stream().collect(Collectors.toMap(User::getUserId, Function.identity()));

假如id存在重复值，则会报错Duplicate key xxx, 解决方案

两个重复id中，取后一个：

Map<Long, User> userMap = userList.stream().collect(Collectors.toMap(User::getUserId, Function.identity(),(oldValue,newValue) -> newValue));

两个重复id中，取前一个：

Map<Long, User> userMap = userList.stream().collect(Collectors.toMap(User::getUserId, Function.identity(),(oldValue,newValue) -> oldValue));

转 id 和 name

Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getUserId, User::getName)); 

name 为 null 的解决方案：

Map<Long, String> map = userList.stream().collect(Collectors.toMap(Student::getUserId, e->e.getName()==null?"":e.getName()));