JAVA中TreeSet集合的两种排序方法:自然排序和比较器排序

TreeSet支持2种排序：自然排序、比较器排序。

由于TreeSet对内部元素的排序的方法有2种：自然顺序排序、比较器排序。所以，如果TreeSet采用自然顺序排序，需要内部的元素（即自定义类型）实现Comparable接口。如果TreeSet采用比较器排序，需要提供比较器对象。

自然排序：被放入TreeSet的数据，必须实现Comparable接口。Comparable接口中只有一个方法compareTo(T t) ，即被放入TreeSet中的对象要实现compareTo方法，TreeSet每放入一个元素，就会拿这个元素和已有元素比较（用compareTo方法比较），最终确定新添加的数据的位置，使TreeSet集合所有元素有序。

比较器排序：这种排序方式更加灵活，不要求容器中的元素实现Comparable接口。而是通过一个实现了Comparator接口的类来对TreeSet中的元素进行排序。Comparator实现类必须要实现 compare(Tt1, T t2)方法。TreeSet中每放入一个元素，就会借助比较器比较新加入的元素和已有元素的大小关系，最终确定新添加的数据的位置，使TreeSet集合所有元素有序。

自然排序

创建一个Teacher类,做自然排序.

public class Teacher implements Comparable<Teacher>{
    private int age;
    private String name;
    private int salary;

    public Teacher() {
    }

    public Teacher(int age, String name, int salary) {
        this.age = age;
        this.name = name;
        this.salary = salary;
    }

    public int getAge() {
        return age;
    }

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

    public String getName() {
        return name;
    }

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

    public int getSalary() {
        return salary;
    }

    public void setSalary(int salary) {
        this.salary = salary;
    }

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

    @Override
    public int compareTo(Teacher o) {
        //年龄升序,如果年龄相等按工资降序
        if (this.age-o.age!=0){
            return this.getAge()-o.getAge();
        }else {
            return o.salary-this.salary;
        }

    }
}

比较器排序

创建一个Teacher1,做比较器排序.

public class Teacher1 {
    private int age;
    private String name;
    private int salary;

    public Teacher1() {
    }


    public Teacher1(int age, String name, int salary) {
        this.age = age;
        this.name = name;
        this.salary = salary;
    }

    public int getAge() {
        return age;
    }

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

    public String getName() {
        return name;
    }

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

    public int getSalary() {
        return salary;
    }

    public void setSalary(int salary) {
        this.salary = salary;
    }

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

}

实现两种排序

创建一个TestTeacher类,用来实现两种排序,进行比较两种排序.

import java.util.Comparator;
import java.util.TreeSet;

public class TestTeacher {
    public static void main(String[] args) {
        TreeSet<Teacher> set=new TreeSet<>();
        Teacher t1=new Teacher(24,"王老师",1500);
        Teacher t2=new Teacher(18,"张老师",3500);
        Teacher t3=new Teacher(24,"赵老师",2500);
        Teacher t4=new Teacher(19,"李老师",4500);

        set.add(t1);
        set.add(t2);
        set.add(t3);
        set.add(t4);
        System.out.println("Comparable的排序结果:");
        System.out.println(set);
        System.out.println("--------------------------------------------");
        Teacher1 tt1=new Teacher1(24,"王老师",1500);
        Teacher1 tt2=new Teacher1(18,"张老师",3500);
        Teacher1 tt3=new Teacher1(24,"赵老师",2500);
        Teacher1 tt4=new Teacher1(19,"李老师",4500);

        TreeSet<Teacher1> set1=new TreeSet<>(new Comparator<Teacher1>() {
            @Override
            public int compare(Teacher1 o1, Teacher1 o2) {
//如果两个对象中属性数值相等(age相等或工资相等...),那么后一个对象就添加不上,所以要判断一下相等的情况;
                if (o1.getAge()-o2.getAge()!=0){
                    return o1.getAge()-o2.getAge();
                }else {
                    return o2.getSalary()-o1.getSalary();
                }

            }
        });
        set1.add(tt1);
        set1.add(tt2);
        set1.add(tt3);
        set1.add(tt4);
        System.out.println("Comparator的排序结果:");
        System.out.println(set1);
    }
}