public class Block {
  static int top = 3;              // stack[0:5] with four defined values
  static char stack[] = new char[] {'a', 'b', 'c', 'd', '$', '$'};

  public static void main (String[] a) {
    System.out.println ("Main thread starts executing.");
    System.out.println ("Initial value of top = " + top + ".");
    System.out.println ("Initial value of stack top = " + stack[top] + ".");
    System.out.println ("Main thread will now fork two threads.");

    AcquireBlock t1 = new AcquireBlock ();
    ReleaseBlock t2 = new ReleaseBlock ();
    t2.start ();                   // added to ready queue 
    t1.start ();                   // added to ready queue
    try {                          // main thread waits for thread termination
      t1.join ();
      t2.join ();
    }
    catch (InterruptedException e) { };
    System.out.println ("System terminates normally.");
    System.out.println ("Final value of top = " + top + ".");
    System.out.println ("Final value of stack top = " + stack[top] + ".");
    System.out.println ("Final value of stack top-1 = " + stack[top-1] + ".");
  }
}

class AcquireBlock extends Thread {
  private char block;
  private char copy;
  private int  oldtop;
  private int  newtop;

  public void run () {
    System.out.println ("AcquireBlock thread starts executing.");
    yield ();

    System.out.println ("AcquireBlock thread requests Ms block.");
    oldtop = Block.top; copy = Block.stack[oldtop];
    System.out.println ("AcquireBlock thread obtains  Ms block " + copy +
                       " from position " + oldtop + ".");
    block = Block.stack[oldtop];
    newtop = Block.top - 1;
    System.out.println ("AcquireBlock decrements pointer to " + newtop + ".");
    --Block.top;
    System.out.println ("Acq: Current value of top = " + Block.top + ".");
    System.out.println ("Acq: Current value of stack top = " +
                       Block.stack[Block.top] + ".");
    System.out.println ("AcquireBlock thread terminates.");
  }
}

class ReleaseBlock extends Thread {
  private char block = 'e';        // block to be returned
  private int  oldtop;
  private int  newtop;

  public void run () {
    System.out.println ("ReleaseBlock thread starts executing.");
    yield ();

    newtop = Block.top + 1;
    System.out.println ("ReleaseBlock increments pointer to " + newtop + ".");
    Block.top++;
    yield ();
    System.out.println ("ReleaseBlock thread returns  Ms block " + block +
                       " to position " + Block.top + ".");
    Block.stack[Block.top] = block;
    System.out.println ("Rel: Current value of top = " + Block.top + ".");
    System.out.println ("Rel: Current value of stack top = " +
                       Block.stack[Block.top] + ".");
    System.out.println ("ReleaseBlock thread terminates.");
  }
}