Multi-threaded, worship god , lock

  public class Demo{
  	public static int rocket = 20;
  	public static void main(String[] args){
  		MyThread mt1 = new MyThread();
  		Thread t1 = new Thread(mt1);
  		MyThread2 t2 = new MyThread2();
  		t1.start();
  		t2.start();
  	}
  }
  class MyThread implements Runnable{
  	public  void run(){
  	synchronized("11"){
  		while(Demo.rocket>0){
  		 
  			if(Demo.rocket==1){
  				try{
  					Thread.sleep(1000);
  				}catch(Exception e){
  					e.printStackTrace();
  				}
  			}
  			Demo.rocket--;
  			System.out.println("1111卖票，剩余:"+Demo.rocket+"张票");
  		}
  	}
  	}
  }
  class MyThread2 extends Thread{
  	public   void run(){
  	synchronized("11111"){
  		while(Demo.rocket>0){ 
  		 
  			Demo.rocket--;
  			System.out.println("22222222卖票，剩余 :"+Demo.rocket+"张票........");
  		}
  	}
  	}
  }
   
  
  

Results:
1111 ticket , remaining : 19 helpful
22222222 sell tickets remaining : 18 helpful .....
1111 ticket , remaining : 17 helpful
22222222 sell tickets remaining : 16 helpful .....
1111 ticket , remaining : 15 helpful
1111 ticket , remaining : 13 helpful
22222222 sell tickets remaining : 14 helpful .....
1111 ticket , remaining : 12 tickets
22222222 sell tickets remaining : 11 helpful .....
22222222 sell tickets remaining : 9 helpful ......
22222222 sell tickets remaining : 8 helpful ......
1111 ticket , remaining : 10 helpful
1111 ticket , remaining : 6 tickets
1111 selling tickets remaining : 5 helpful
22222222 sell tickets remaining : 7 helpful ......
22222222 sell tickets remaining : 3 helpful ......
1111 ticket , remaining : 4 helpful
1111 selling tickets remaining : 1 tickets
22222222 sell tickets remaining : 2 tickets ......
22222222 sell tickets remaining : 0 tickets ......
1111 ticket , remaining : -1 tickets
why results have -1 ah ? How to solve ?
------ Solution ---------------------------------------- ----
-1 is the thread synchronization phenomena have caused , when the judge while condition is certainly satisfied , into the implementation of the time , another thread will Demo.rocket execution result is 0 , so ......
------ Solution ------------------------------------------ -
to lock this object demo

package com.xiehou.th;
  
  public class Demo {
  	
  	public static int rocket = 20;
  
  	public static void main(String[] args) {
  		
  		Demo demo=new Demo();
  		
  		MyThread mt1 = new MyThread(demo);
  		Thread t1 = new Thread(mt1);
  		MyThread2 t2 = new MyThread2(demo);
  		t1.start();
  		t2.start();
  	}
  }
  
  class MyThread implements Runnable {
  	
  	public Demo demo;
  	
  	public MyThread(Demo demo){
  		this.demo=demo;
  	}
  	
  	public void run() {
  		synchronized (demo) {
  			while (Demo.rocket > 0) {
  
  				if (Demo.rocket == 1) {
  					try {
  						Thread.sleep(1000);
  					} catch (Exception e) {
  						e.printStackTrace();
  					}
  				}
  				Demo.rocket--;
  				System.out.println("[1111]卖第:" + Demo.rocket + "张票");
  			}
  		}
  	}
  }
  
  class MyThread2 extends Thread {
  	
     public Demo demo;
  	
  	public MyThread2(Demo demo){
  		this.demo=demo;
  	}
  	
  	public void run() {
  		synchronized (demo) {
  			while (Demo.rocket > 0) {
  
  				Demo.rocket--;
  				System.out.println("[2222]第 :" + Demo.rocket + "张票");
  			}
  		}
  	}
  }
  

------ Solution ------------------------------------- -------
you two are not the same lock , a lock does not work , got to lock the same object .
------ For reference only -------------------------------------- -
you lock rocket variables , other threads do not get in the right, or you can lock the Demo class lines . You still see Thread Synchronization bar
------ For reference only ------------------------------- --------
synchronized (this), on ok !
------ For reference only -------------------------------------- -

public class Demo {
public static int rocket = 20;
public static void main (String [] args) {
MyThread mt1 = new MyThread ();
Thread t1 = new Thread (mt1);
MyThread2 t2 = new MyThread2 ();
t1.start ();
t2.start ();
}
}
class MyThread implements Runnable {
public void run () {
synchronized (this) {
while (Demo.rocket> 0) {

if (Demo.rocket == 1) {
try {
Thread.sleep (1000);
} catch (Exception e) {
e.printStackTrace ();
}
}
Demo.rocket--;
System.out.println ("1111 ticket , remaining :" + Demo.rocket + " tickets " ) ;
}
}
}
}
class MyThread2 extends Thread {
public void run () {
synchronized (this) {
while (Demo.rocket> 0) {

Demo.rocket--;
System.out.println ("22222222 selling tickets remaining :" + Demo.rocket + " tickets ........ " ) ;
}
}
}
}


------ For reference only ---------------------------------- -----
Thank above answer