1Z0-809 Question #255: Real Exam Question with Answer & Explanation

Given:

class Counter extends Thread {
 int i = 10;
 public synchronized void display(Counter obj) {
 try {
 Thread.sleep(5);
 obj.increment(this);
 System.out.println(i);
 } catch (InterruptedException ex) { }
 }
 public synchronized void increment (Counter obj) {
 i++;
 }
}
public class Test {
 public static void main(String[] args) {
 final Counter obj1 = new Counter();
 final Counter obj2 = new Counter();
 new Thread(new Runnable() {
 public void run() { obj1.display(obj2); }
 }).start();
 new Thread(new Runnable() {
 public void run() { obj2.display(obj1); }
 }).start();
 }
}

From what threading problem does the program suffer?