list对象排序及分组

最新推荐文章于 2025-02-13 09:30:27 发布

小白、学java

最新推荐文章于 2025-02-13 09:30:27 发布

阅读量578

点赞数

本文链接：https://blog.youkuaiyun.com/weixin_39188687/article/details/100518602

版权

本文详细介绍了Java中如何使用Collections.sort()对自定义类进行排序，包括通过实现Comparable接口和使用Comparator接口。同时，展示了如何利用Java Stream API进行数据分组，包括基于对象属性和基本属性的分组方法。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

一、list对象排序
https://blog.youkuaiyun.com/qq_39366175/article/details/79449178
1.对象：
根据age正序排序实现Comparable 重写compareTo
@Data
@AllArgsConstructor
@NoArgsConstructor
public class User implements Comparable{
private Integer age;

private String name;

private Date createTime;

@Override
public String toString() {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    return "User{" + age + ", " + name  + ", " + sdf.format(createTime) + '}';
}

@Override
public int compareTo(User o) {
    return this.age.compareTo(o.getAge());
}

}
测试：
1.直接测试
@Test
public void sort(){
List list = new ArrayList<>();
Random r = new Random();
for (int i = 0; i < 10 ; i++) {
int x = r.nextInt(11);
User user = new User(10+x, “agc”+i, addDateMinut(new Date(),x));
list.add(user);
}
Collections.sort(list);
for (User user : list) {
System.out.println(user.toString());
}
}
2.重写优先
@Test
public void sort(){
List list = new ArrayList<>();
Random r = new Random();
for (int i = 0; i < 10 ; i++) {
int x = r.nextInt(11);
User user = new User(10+x, “agc”+i, addDateMinut(new Date(),x));
list.add(user);
}
Collections.sort(list, new Comparator() {
@Override
public int compare(User o1, User o2) {
long x = o1.getCreateTime().getTime();
long y = o2.getCreateTime().getTime();
int yy = (x < y) ? -1 : ((x == y) ? 0 : 1);
return yy;
}
});
for (User user : list) {
System.out.println(user.toString());
}
}
二、分组
（1）根据对象分组
1.定义分组对象
@Data
@AllArgsConstructor
@EqualsAndHashCode
public class UserGroup {
private Integer age;

private String name;

}
2.测试
@Test
public void test000(){
List list = new ArrayList<>();
Random r = new Random();
for (int i = 0; i < 10 ; i++) {
int x = r.nextInt(11);
User user = new User(10, “AAA”, addDateMinut(new Date(),x));
list.add(user);
}
for (int i = 0; i < 10 ; i++) {
int x = r.nextInt(11);
User user = new User(11, “BBB”, addDateMinut(new Date(),x));
list.add(user);
}
//分组
Map<UserGroup, List> collect = list.stream().
collect(Collectors.groupingBy(new Function<User, UserGroup>() {
@Override
public UserGroup apply(User user) {
return new UserGroup(user.getAge(),user.getName());
}
}));

    System.out.println(collect);
}

（2）根据基本属性分组
@Test
public void test000(){
List list = new ArrayList<>();
Random r = new Random();
for (int i = 0; i < 10 ; i++) {
int x = r.nextInt(11);
User user = new User(10, “AAA”, addDateMinut(new Date(),x));
list.add(user);
}
for (int i = 0; i < 10 ; i++) {
int x = r.nextInt(11);
User user = new User(11, “BBB”, addDateMinut(new Date(),x));
list.add(user);
}
//分组
Map<Integer, List> collect = list.stream().
collect(Collectors.groupingBy(new Function<User, Integer>() {
@Override
public Integer apply(User user) {
return user.getAge();
}
}));

    System.out.println(collect);
}