Thread sync weird: asking for pointers

public class BUYER {
public static void Main (string[] args) {
Testthread a=new Testthread ();
Testthread b=new Testthread ();
A.setname ("A");
B.setname ("B");
A.start ();
B.start ();
}
}
Class Testthread extends Thread
{
static int rest=10;//number of votes left
int buy_1=7;//buy Ticket number

public void Run () {
int a=rest;
if (a>buy_1) {
A=a-buy_1;

Rest=a; ① Why the statement is placed here can never appear at the same time the success of the purchase of the case

System.out.println (Thread.CurrentThread (). GetName () + "\ n Success.");
 // ②rest=a;
}else
System.out.println (Thread.CurrentThread (). GetName () + "\ n fail.");

}
}

The handling of ticketing at ① is always true. The situation of ticketing at ② may occur at the same time as success?

Know what the solution of the mankind to seek guidance.

Thread sync weird: asking for pointers