Threading: Sharing same resource (List)

Question

Hi,
I'm facing one strange problem.
What I have done is as follows:
1. I have created two threads; Thread1 and Thread2. Both these share the same object (List)
2. Thread1: It add 50 elements to list.
3. Thread2: It reads 50 elements from the list.
Expected output is:
In Thread-2 it might throw ArrayIndexOutofBounds exception because it may try to read the value which Thread1 has not inserted yet.
Actual output is:
Yes; It throws the ArrayIndexOutofBounds exception but not due to above expected reason.
When second time Thread-2 is executed; list size is not updated although Thread1 and
Thread2 point to same references.
First I thought that it is due to Thread-1 and Thread-2 have separate stack; but this is
also not true since list variable is on the heap. And both Thread point to this.
This is very strange.
Please find the below code:
package xyz;
import java.util.ArrayList;
import java.util.List;
public class ThreadTry {
public static void main(String[] args) {
List list=new ArrayList<Integer>(50);
Thread myThread1=new Thread(new Thread1(list));
Thread myThread2=new Thread(new Thread2(list));
myThread1.start();
myThread2.start();
//writes to list
class Thread1 implements Runnable{
List list;
public Thread1(List list) {
this.list=list;
public void run() {
for(int i=0;i<50;i++){
System.out.println("{");
System.out.println(" Start Adding *********************************");
list.add(i);
System.out.println(" Thread-1 adding: "+i+" List size is: "+list.size());
System.out.println(" Adding is done ********************************");
System.out.println("}");
System.out.println("");
//read lists
class Thread2 implements Runnable{
List list;
public Thread2(List list) {
this.list=list;
public void run() {
for(int i=0;i<50;i++){
System.out.println("------Read : "+i+"-th element.");
System.out.println("------Thread-2 reading: "+" List size is: "+list.size()+" ; element: "+list.get(i));
}Regards,
Leena

Answer 1

It's perfectly possible that thread2 is being run before thread1, you can't depend on the order of execution between threads.
To make it safe you need to do something like:
// in thread 1
synchronized(list) {
      list.add(i);
      list.notifyAll();
// in thread 2
int v = 0;
synchronized(list) {
   while(i >= list.size()) {  // wait until list has enough entries
        list.wait();
        v = list.get(i);
   };