多线程和以及同步，Thread,Runable, synchronized

public class Test {

??? public static void main(String[] args) {

?

??? ??? S run = new S();
??? ??? new Thread(run).start();??????????? //多线程
??? ??? new Thread(run).start();
??? }

?

}

?

class S implements Runnable{
???
??? private int x = 1;
??? private int y = 1;
??? public void run(){
??? ??? for(;;){
??? ??? ??? synchronized(this){???????????????? //如果同步，x始终等于y，2877590:2877590
??? ??? ??? ??? System.out.println(x++ + ":" + y++);? // 如果不同步 出现1934945:1934940，为啥会出现这种情况？
??? ??? ??? }
??? ??? }
??? }
}