A problem that I will give to my AP computer science class to solve

Problem.java

import java.text.NumberFormat;
import java.util.Locale;

public class Problem{
	static int test = 0;
	static boolean complete = false;

	public static void main(String[] args){
		new Thread(){
			@Override
			public void run(){
				for(int i = 0; i < 1000000000; i++){ //1 billion
					
					test = test + 1; // add 1 to our variable
					
				}

				done();
			}
		}.start();

		new Thread(){
			@Override
			public void run(){
				for(int i = 0; i < 1000000000; i++){ //1 billion
					
					test = test + 1; // add 1 to our variable
					
				}

				done();
			}
		}.start();
	}
	 
	static synchronized void done(){
		if(complete){
			// If the other thread is finished, print the result
			// adding 1 billion to 1 billion makes 2 billion, right?

			System.out.println( formatNumber(test) );
		}
		complete = true; // Mark a thread as complete
	}

	static String formatNumber(int i){
		return NumberFormat.getNumberInstance(Locale.US).format(i);
	}
}