有一静态整形变量X，初始值为0，用JAVA写四个线程，二个对其加1，二个对其减一，X等于0时不进行减操作，X等 ...

题目：

有一静态整形变量X，初始值为0，用JAVA写四个线程，二个对其加1，二个对其减一，X等于0时不进行减操作，X等于5时结束程序。

我的实现方式：

import java.util.Random;

public class StaticThread {
 private static int X=0;
 private static Random rd=new Random();
 public synchronized static void add(){
  if(X!=0){
   StaticThread.class.notifyAll();
  }
  X++;
  System.out.println(Thread.currentThread().getName()+": "+"add x: "+X);
 }
 
 public synchronized static void substract(){
  try{
  if(X==0){
   StaticThread.class.wait();
  }
  }catch(InterruptedException e){
   
  }
  X--;
  System.out.println(Thread.currentThread().getName()+": "+"substract x: "+X);

 }
 
 static class StaticAddThread extends Thread{
  public void run(){
   while(true){
    if(X==5){
     System.out.println("X=5");
     System.exit(0);
    }
    add();
    try{
     Thread.sleep(rd.nextInt(1000));
    }catch(InterruptedException e){
     
    }
   }
   
  }
 }
 
 static class StaticSubstractThread extends Thread{
  public void run(){
   while(true){
    if(X==5){
     System.out.println("X=5");
     System.exit(0);
    }
    substract();
    try{
     Thread.sleep(rd.nextInt(1000));
    }catch(InterruptedException e){
     
    }
   }
 }
 }
 
 public static void main(String[] args){
  StaticAddThread sat1=new StaticAddThread();
  StaticAddThread sat2=new StaticAddThread();
  
  StaticSubstractThread sst1=new StaticSubstractThread();
  StaticSubstractThread sst2=new StaticSubstractThread();
  
  sat1.start();
  sst1.start();
  sat2.start();
  sst2.start();

 }
}