关于TreeMap自定义排序规则的两种方式

package com.zibo.java.february.third;

import java.util.Set;
import java.util.TreeMap;

public class StuTreeMap {
    public static void main(String[] args) {
        TreeMap<Student,String> treeMap = new TreeMap<>();
        treeMap.put(new Student("大哥",24),"大哥");
        treeMap.put(new Student("二哥",23),"二哥");
        treeMap.put(new Student("三哥",22),"三哥");
        treeMap.put(new Student("四哥",21),"四哥");
        Set<Student> studentSet = treeMap.keySet();
        for (Student student : studentSet) {
            System.out.println(student.toString());
        }
    }
}
// 这里就省略getter和setter方法了
class Student implements Comparable<Student>{

    private String name;
    private int age;

    public Student() {
    }

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

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

    @Override
    public int compareTo(Student o) {
        // 实现Comparable接口在这里也是自定义排序规则
        // 如果什么也不写，直接默认return 0的话，只能存储第一个被put的元素
        // 注意：升序就这么个写法，不要看网上其他的什么相等返回0，相等的话要返回this.age，否则会出问题
        if(age > o.age){
            return 1;
        }else if(age < o.age){
            return -1;
        }
        return this.age;
    }
}

package com.zibo.java.february.third;

import java.util.Comparator;
import java.util.Set;
import java.util.TreeMap;

public class StuTreeMap2 {
    public static void main(String[] args) {
        TreeMap<Student2,String> treeMap = new TreeMap<>(new Comparator<Student2>() {
            @Override
            public int compare(Student2 o1, Student2 o2) {
                // 基本和key实现Comparable接口，重写compareTo方法一致
                // 升序排序就是这么写的
                if(o1.getAge() > o2.getAge()){
                    return 1;
                }else if(o1.getAge() < o2.getAge()){
                    return -1;
                }
                // 相等的时候不能返回0
                return o1.getAge();
            }
        });
        treeMap.put(new Student2("大哥",24),"大哥");
        treeMap.put(new Student2("二哥",23),"二哥");
        treeMap.put(new Student2("三哥",22),"三哥");
        treeMap.put(new Student2("四哥",21),"四哥");
        Set<Student2> studentSet = treeMap.keySet();
        for (Student2 student : studentSet) {
            System.out.println(student.toString());
        }
    }
}

class Student2{

    private String name;
    private int age;

    public Student2() {
    }

    public Student2(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

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

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