求教，关于java中的线程同步问题。

public interface StackInterface
{
    public void push(int n);
    public int[] pop();
}

public class SafeStack implements StackInterface
{
    private int top = 0;
   
    private int[] values = new int[10];
   
    public void push(int n)
    {
        synchronized (this)
        {
            values[top] = n;
            System.out.println("压入数字" + n + "步骤1完成");
            top++;
            System.out.println("压入数字完成");
         }
    }
   
    public int[] pop()
    {     synchronized (this)
        {
            System.out.print("弹出");
            top--;
            int[] test = {values[top], top };
             return test;   
        }
    }
}


public class PopThread implements Runnable
{
    private StackInterface s;
   
    public PopThread(StackInterface s)
    {
        this.s = s;
    }   
    public void run()
    {
        System.out.print(s.pop()[1] + ":" +" "+ s.pop()[0]);   
        
    }
}


public class PushThread implements Runnable
{
    private StackInterface s;
   
    public PushThread(StackInterface s)
    {
        this.s = s;
    }   
   
    public void run()
    {
        /*for(int i = 0; i < 10; i ++)
        {
            System.out.print(".");   
        }*/   
        int k = 15;
        s.push(k);
    }
}


public class TestSafeStack
{
    public static void main(String[] args)
    {
        SafeStack a = new SafeStack();
        a.push(3);
        a.push(4);
        PushThread r1 = new PushThread(a);
        PopThread r2 = new PopThread(a);
        Thread t1 = new Thread(r1);
        Thread t2 = new Thread(r2);
        t1.start();
        t2.start();   
    }   
}