public class Peoper implements Comparable {
 int id;
 int age;
 String name;

 public Peoper(int id, int age, String name) {
  super();
  this.id = id;
  this.age = age;
  this.name = name;
 }

 @Override
 public String toString() {
  return "Peoper [id=" + id + ", age=" + age + ", name=" + name + "]";
 }

 @Override
 public int compareTo(Object o) {
  Peoper p;
  if (o instanceof Peoper) {
   p=(Peoper)o;
  }else {
   return -1;//-1表示传入的参数比我本身要小
  }
  int diff=this.id-p.id;
  if(diff!=0){
   diff=diff/Math.abs(diff);
  }
 return diff;
 }

}

import java.awt.List;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Collection;
import java.util.Date;
import java.util.Iterator;
import java.util.Random;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

import javax.swing.plaf.synth.SynthScrollBarUI;

public class Demo {
 public static void main(String[] args) {
  Set set=new TreeSet();
  Peoper P1=new Peoper(1, 18, "小明");
  Peoper P2=new Peoper(2, 5, "大壮");
  Peoper P3=new Peoper(3, 20, "阿强");
  set.add(P1);
  set.add(P2);
  set.add(P3);
  Iterator it=set.iterator();
  while (it.hasNext()) {
   System.out.println(it.next());
   
  }
 
 }

}

上面是两个class，主要是compareTo方法这里看不懂，在Peoper类里这个方法是重写了吧，我不明白为什么这样重写，第一个if和第二个if的功能是啥？还有我尝试不重写，就是方法体力出值写了return 0，结果在Demo运行的时候就只输出了集合的第一个元素，这为啥？还有就是这个compareTo到底在哪里运行了，Demo里没有调用，Peoper里也没有main方法啊？