Java多线程，启动四个线程，两个执行加一，另外两个执行减一

package com.yu.learn.main;

public class MyThread {

public static void main(String[] args) {
OneThread one = new OneThread("add");
Thread t1 = new Thread(one);
Thread t2 = new Thread(one);

OneThread two = new OneThread("minus");
Thread t3 = new Thread(two);
t3.setPriority(9);
Thread t4 = new Thread(two);
t1.start();
t2.start();
t3.start();
t4.start();
}
}

class Singleton {
private static Singleton instance = null;
private static int number = 0;

private Singleton() {
}

public static Singleton getInstance() {
if (instance == null)
instance = new Singleton();
return instance;
}

public synchronized int Add(){
try{
System.out.println("执行+1操作前为 "+number);
return ++number;
}finally{
System.out.println("执行+1操作后为 "+number);
System.out.println("-------------------");
}
}

public synchronized int minus() {
try{
System.out.println("执行-1操作前为 "+number);
return --number;
}finally{
System.out.println("执行-1操作后为 "+number);
System.out.println("-------------------");
}
}
}

class OneThread implements Runnable {
private Singleton singleton;
private String status;

OneThread(String status) {
singleton = Singleton.getInstance();
this.status = status;
}

@Override
public void run() {
if ("add".equals(this.status)) {
System.out.println(singleton.Add());
} else {
System.out.println(singleton.minus());
}
}

}