Multithreading error in Java

I implemented a simple thread program in which one thread pushes element into a stack and the other pops element from the stack. I'm getting an unexpected output in which thread1 pushes an element twice.

import java.io.*;
import java.lang.*;
import java.util.*;

public class Threading_Sample implements Runnable
{
    private Thread T1,T2;
    String ThreadName="";

    Stack<Integer> Stack1=new Stack<Integer>();


    public void Push_Element_to_Stack(int element)
    {
        Stack1.push(element);
    }

    public void Pop_Element_from_Stack()
    {
        Stack1.pop();
    }

    public void run()
    {
      try
       {
        //while(Thread.currentThread().isAlive())
        for(int i=0;i<10;i++)
        {
            if(T1.getName().equals("THREAD1"))
                {
                    System.out.println("Current Thread: "+T1.getName());
                    System.out.println("DOING TASK 1...");

                    Push_Element_to_Stack(i);
                    Thread.sleep(100);
                    System.out.println("Stack1 of Thread1: "+Stack1);
                }
                if(T2.getName().equals("THREAD2"))
                {
                    System.out.println("Current Thread: "+T2.getName());
                    System.out.println("DOING TASK 2...");

                System.out.println("Stack of Thread2 before: "+Stack1);
                    Pop_Element_from_Stack();
                    Thread.sleep(100);
                    System.out.println("Stack of Thread2 after: "+Stack1);
                }
          }

       }
       catch(Exception e)
       {
           System.out.println("Thread interrupted...");
           e.printStackTrace();
       }
    }

    public void start()
    {
        if(T1==null)
        {
            T1=new Thread(this,"THREAD1");
            T1.start();
            try
            {
                T1.join();
            }
            catch(Exception e)
            {
                e.printStackTrace();
            }
        }
        if(T2==null)
        {
            T2=new Thread(this,"THREAD2");
            T2.start();
            try
            {
                T2.join();
            }
            catch(Exception e)
            {
                e.printStackTrace();
            }
        }
    }

    public static void main (String[] args)
    {
        Threading_Sample TASK1=new Threading_Sample();

        TASK1.start();
    }
}

OUTPUT

--------------------Configuration: <Default>--------------------
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0]
Thread interrupted...
java.lang.NullPointerException
    at Threading_Sample.run(Threading_Sample.java:39)
    at java.lang.Thread.run(Thread.java:722)
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 0]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 0]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 1]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 1]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 2]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 2]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 3]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 3]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 4]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 4]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 5]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 5]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 6]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 6]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 7]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 7]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 8]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 8]
Stack of Thread2 after: [0]
Current Thread: THREAD1
DOING TASK 1...
Stack1 of Thread1: [0, 9]
Current Thread: THREAD2
DOING TASK 2...
Stack of Thread2 before: [0, 9]
Stack of Thread2 after: [0]

Process completed.

Why is the element 0 pushed twice into the stack ?

Regards, Rajesh.