Thread - Wait Doubt

[Micheal John]

From web page

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();
}}}
 

In the above pgm, why the output is coming? pls explain

[Ahmed Khan Mohammed]

When I ran the program I dont see any output. It just happened (on my machine) that the notify method was called before the wait method was called.
I modified the code.

    
class A extends Thread {
    String[] sa;
 
    public A(String[] sa) {
        this.sa = sa;
    }
 
    public void run() {
        synchronized (sa) {
            while (sa[0].equals("Done")) {
                try {
                    System.out.println("going to wait");
                    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();
            System.out.println("after notify");
        }
    }
}
 

The output is
after notify
going to wait