//按id属性为map的key值
Map<Integer, User> userMap = list.stream().collect(Collectors.toMap(User::getId, user -> user));
注意：这里属性值必须不能重复，不然会报错
举例：
public class HelloWorld {
public static void main(String[] args) {
Random random = new Random();
List<User> list = new ArrayList<>();
for(int i=1;i<=10;i++) {
String group = (random.nextInt(3) + 1) + "组";//1-3组随机
User u = new User(i, "用户-" + i, group);
list.add(u);
}
//按id属性分组
Map<Integer, User> userMap = list.stream().collect(Collectors.toMap(User::getId, user -> user));
System.out.println("list-map 后：" + userMap);
}
private static class User{
Integer id;
String name;
String group;
public User(Integer id, String name, String group) {
this.id = id;
this.name = name;
this.group = group;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getGroup() {
return group;
}
public void setGroup(String group) {
this.group = group;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", group='" + group + '\'' +
'}';
}
}
}
执行结果：
list-map 后：{
1=User{
id=1,
name='用户-1',
group='1组'
},
2=User{
id=2,
name='用户-2',
group='3组'
},
3=User{
id=3,
name='用户-3',
group='3组'
},
4=User{
id=4,
name='用户-4',
group='3组'
},
5=User{
id=5,
name='用户-5',
group='3组'
},
6=User{
id=6,
name='用户-6',
group='3组'
},
7=User{
id=7,
name='用户-7',
group='1组'
},
8=User{
id=8,
name='用户-8',
group='2组'
},
9=User{
id=9,
name='用户-9',
group='1组'
},
10=User{
id=10,
name='用户-10',
group='1组'
}
}