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

class user{
   public String name;
}

class Thread1 implements Runnable
{
   Thread mythread ;
   ArrayList<user> list;
   Thread1(ArrayList<user> l)
   {
     list = l;
      mythread = new Thread(this, "Thread1");
      System.out.println("Thread1 created");
      mythread.start();
   }
   public void run()
   {
      try
      {
        for (int i=0 ;list.size()<1000;i++)
        {

          list.add(new user());
          System.out.println("Adding items " + i);
          Thread.sleep(1);

        }
     }
     catch(InterruptedException e)
     {
        System.out.println("Thread1 interrupted");
     }
     System.out.println("Thread1 run is over" );
   }
}

class Thread2 implements Runnable
{
   Thread mythread ;
   ArrayList<user> list;
   Thread2(ArrayList<user> l)
   {
      mythread = new Thread(this, "Thread2");
      System.out.println("Thread2 created");
      list = l;
      mythread.start();
   }
   public void run()
   {
      try
      {
        for (int i=0 ;i<1000;i++)
        {
          System.out.println("Removing objects " + i);

          list.remove(list.size() -1);
          Thread.sleep(1);
        }
     }
     catch(InterruptedException e)
     {
        System.out.println("Thread2 interrupted");
     }
     System.out.println("Thread2 run is over" );
   }
}
public class DemoThread
{
    public static void main(String args[])
    {
       ArrayList<user> list = new ArrayList<user>();
       Thread1 t1 = new Thread1(list);
        Thread2 t2 = new Thread2(list);
       try
       {
          while(t1.mythread.isAlive() || t2.mythread.isAlive())
          {
            System.out.println(" Size : " + list.size());
            Thread.sleep(1);
          }

       }
       catch(InterruptedException e)
       {
          System.out.println("Main thread interrupted");
       }
       System.out.println("Main thread run is over" );
    }
}

After 6

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

class user{
   public String name;
}

class Thread1 implements Runnable
{
   Thread mythread ;
   ArrayList<user> list;
   Thread1(ArrayList<user> l)
   {
     list = l;
      mythread = new Thread(this, "Thread1");
      System.out.println("Thread1 created");
      mythread.start();
   }
   public void run()
   {
      try
      {
        for (int i=0 ;list.size()<1000;i++)
        {
          while(list.size() == 1000){
             Thread.sleep(1);
          }
          list.add(new user());
          System.out.println("Adding items " + i);


        }
     }
     catch(InterruptedException e)
     {
        System.out.println("Thread1 interrupted");
     }
     System.out.println("Thread1 run is over" );
   }
}

class Thread2 implements Runnable
{
   Thread mythread ;
   ArrayList<user> list;
   Thread2(ArrayList<user> l)
   {
      mythread = new Thread(this, "Thread2");
      System.out.println("Thread2 created");
      list = l;
      mythread.start();
   }
   public void run()
   {
      try
      {
        for (int i=0 ;i<1000;i++)
        {
          System.out.println("Removing objects " + i);
          while (list.size() == 0){
              Thread.sleep(1);
          }
          list.remove(list.size() -1);
        }
     }
     catch(InterruptedException e)
     {
        System.out.println("Thread2 interrupted");
     }
     System.out.println("Thread2 run is over" );
   }
}
public class DemoThread
{
    public static void main(String args[])
    {
       ArrayList<user> list = new ArrayList<user>();
       Thread1 t1 = new Thread1(list);
        Thread2 t2 = new Thread2(list);
       try
       {
          while(t1.mythread.isAlive() || t2.mythread.isAlive())
          {
            System.out.println(" Size : " + list.size());
            Thread.sleep(1);
          }

       }
       catch(InterruptedException e)
       {
          System.out.println("Main thread interrupted");
       }
       System.out.println("Main thread run is over" );
    }
}