public interface Queue 
{ 
    // postcondition: returns true if queue is empty, false otherwise 
  boolean isEmpty(); 

    // precondition:  queue is [e1, e2, ..., en] with n >= 0 
    // postcondition: queue is [e1, e2, ..., en, x] 
  void enqueue(Object x); 

    // precondition:  queue is [e1, e2, ..., en] with n >= 1 
    // postcondition: queue is [e2, ..., en]; returns e1 
    //                 throws an unchecked exception if the queue is empty 
  Object dequeue(); 

    // precondition:  queue is [e1, e2, ..., en] with n >= 1 
    // postcondition: returns e1 
    //                 throws an unchecked exception if the queue is empty 
  Object peekFront(); 
}