import com.google.common.base.Function;import com.google.common.collect.Maps;import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;public class ListToMap {  public static void main(String[] args) {    List<User> userList = new ArrayList<>();    User user1 = new User();    user1.setId(1L);    user1.setAge("12");    User user2 = new User();    user2.setId(2L);    user2.setAge("13");    userList.add(user1);    userList.add(user2);    Map<Long, User> maps = new HashMap<>();    for (User user : userList) {      maps.put(user.getId(), user);    }    System.out.println(maps);  }  public static class User {    private Long id;    private String age;    public Long getId() {      return id;    }    public void setId(Long id) {      this.id = id;    }    public String getAge() {      return age;    }    public void setAge(String age) {      this.age = age;    }    @Override    public String toString() {      return "User{" +          "id=" + id +          ", age='" + age + '/'' +          '}';    }  }}

Map<Long, User> maps = Maps.uniqueIndex(userList, new Function<User, Long>() {      @Override      public Long apply(User user) {        return user.getId();      }});

Map<Long, User> maps = userList.stream().collect(Collectors.toMap(User::getId, user -> user));