JAVA基础补漏--Collections工具类排序

Collections在对自定义对象进行排序时，自定义类需要对compareTo（）函数进行重写。

public class Student implements Comparable<Student>{//实现Comparable接口，范型定义为类自己

    private String name;
    private Integer age;

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Student(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public Student() {

    }

    @Override
    public int compareTo(Student o) {

        return this.getAge() - o.getAge();//按age升序
        //return o.getAge() - this.getAge();//按age降序
        //return 0;// 0表示所有数据都相等
    }
}

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Main {

    public static void main(String[] args) {

        Student good = new Student("good", 19);
        Student bad = new Student("bad", 15);
        Student normal = new Student("normal", 21);

        List<Student> list = new ArrayList<>();

        Collections.addAll(list,good,bad,normal);
        System.out.println(list);

        Collections.sort(list);
        System.out.println(list);

    }

}

如果用sort带排序参数的方法，则自定义类可以不用写compareTo()方法。

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class Main {

    public static void main(String[] args) {

        Student good = new Student("good", 19);
        Student bad = new Student("bad", 15);
        Student normal = new Student("normal", 21);

        List<Student> list = new ArrayList<>();

        Collections.addAll(list,good,bad,normal);
        System.out.println(list);

        Collections.sort(list, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {

                return o1.getAge() - o2.getAge();//按age升序
                return o2.getAge() - o1.getAge();//按ag降序
                //return 0;
            }
        });

        System.out.println(list);
    }

}