Dan's: Thread Q 39

[Alfred Kemety]

Dan, your Thread Questions are very good: I don't understand though the answer of Q 39: I understand the it prints ABC, but why would the run method wait for ever and print nothing? You didn't explain that, and I am not sure it's a correct answer...
So, Dan - or anyone that can - explain it to me...

class A extends Thread {
  String[] sa;
  public A(String[] sa) {
    this.sa = sa;
  }
  public void run() {
    synchronized (sa) {
      while (!sa[0].equals("Done")) {
        try {sa.wait();} catch (InterruptedException ie) {}
      }
    }
    System.out.print(sa[1] + sa[2] + sa[3]);
  }
}
class B {
  private static String[] sa = new String[]{"Not Done","X","Y","Z"};
  public static void main (String[] args) {
    Thread t1 = new A(sa);
    t1.start();
    synchronized (sa) {
      sa[0] = "Done";
      sa[1] = "A";
      sa[2] = "B";
      sa[3] = "C";
      sa.notify();
    }
  }
}

[Dan Chisholm]

Alfred,
Thank you for asking that question. I now realize that the "waiting forever" answer option wasn't particularly instructive in that situation.
When working with threads there is always that possibility that the thread scheduler won't allow a particular thread to run. However, I agree that the possibility is not high. To avoid that issue I changed the question a little and eliminated the "wait forever" answer option.

class A extends Thread {
  String[] sa;
  public A(String[] sa) {
    this.sa = sa;
  }
  public void run() {
    synchronized (sa) {
      while (!sa[0].equals("Done")) {
        try {sa.wait();} catch (InterruptedException ie) {}
      }
    }
    System.out.print(sa[1] + sa[2] + sa[3]);
  }
}
class B {
  private static String[] sa = new String[]{"Not Done","X","Y","Z"};
  public static void main (String[] args) {
    Thread t1 = new A(sa);
    t1.start();
    synchronized (sa) {
      sa[0] = "Done";
      sa[1] = "A";
      sa[2] = "B";
      sa[3] = "C";
      sa.notify();
    }
  }
}

Which of the following are possible results of attempting to compile and run the program? 
a.  Prints: XYZ. 
b.  Prints: AYZ. 
c.  Prints: ABZ. 
d.  Prints: ABC. 
e.  Compiler error. 
f.  Run time error. 
g.  None of the above.