Java xxList.stream().collect(Collectors.groupingBy(User::getCategory)); List对象集合按对象属性分组、分组汇总、过滤等操作示例 |
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userList.stream().collect(Collectors.groupingBy(User::getCategory));使用详解及扩展。直接上代码(底部附带异常)
@Test
void contextLoads1() {
User user0 = new User();
user0.setId(0);
user0.setNickname("name0");
user0.setUsername("username0");
user0.setCategory("学生");
User user1 = new User();
user1.setId(1);
user1.setNickname("name1");
user1.setUsername("username1");
user1.setCategory("老师");
User user2 = new User();
user2.setId(2);
user2.setNickname("name2");
user2.setUsername("username2");
user2.setCategory("教学主任");
List userList = new ArrayList();
userList.add(user0);
userList.add(user1);
userList.add(user2);
//跟据某个属性分组
Map userMap = userList.stream().collect(Collectors.groupingBy(User::getCategory));
System.out.println(userMap); // 查询结果
// {学生=[User{id=0, nickname=name0, birthday=null, username=username0, password=null, status=null, createTime=null, updateTime=null, remarks=null}],
// 老师=[User{id=1, nickname=name1, birthday=null, username=username1, password=null, status=null, createTime=null, updateTime=null, remarks=null}],
// 教学主任=[User{id=2, nickname=name2, birthday=null, username=username2, password=null, status=null, createTime=null, updateTime=null, remarks=null}]}
//根据某个属性分组,汇总某个属性 Map collect2 = userList.stream().collect(Collectors.groupingBy(User::getCategory,Collectors.summingInt(User::getId))); System.out.println(collect2); // 查询结果 // {学生=0, 老师=1, 教学主任=2}
//根据某个属性添加条件过滤数据
userList = userList.stream().filter(u -> !u.getNickname().equals("name0")).collect(Collectors.toList()); System.out.println(userList); // 查询结果 // [User{id=1, nickname=name1, birthday=null, username=username1, password=null, status=null, createTime=null, updateTime=null, remarks=null}, // User{id=2, nickname=name2, birthday=null, username=username2, password=null, status=null, createTime=null, updateTime=null, remarks=null}]
//判断一组对象里面有没有这个属性值
boolean add = userList.stream().anyMatch(m -> "name2".equals(m.getNickname())); System.out.println(add);
//取出一组对象的某个属性组成一个新集合 List Username=userList.stream().map(User::getUsername).collect(Collectors.toList()); System.out.println(Username); // 查询结果 // [username0, username1, username2]
User实体类 public class User implements Serializable { private static final long serialVersionUID=1L; /** * 主键 */ private Integer id; /** * 昵称 */ private String nickname; /** * 类别 */ private String category; /** * 生日 */ private LocalDate birthday; /** * 用户名 */ private String username; /** * 密码 */ private String password; /** * 状态 */ private String status; /** * 创建时间 */ private LocalDateTime createTime; /** * 更新时间 */ private LocalDateTime updateTime; /** * 备注 */ private String remarks; // 省略set/get }element cannot be mapped to a null key / / 按照 User 分组,此时 getCategory 有空值,就会报错。 // 改为,增加非空过滤。
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