Make a package PQ, and within it, place class PriorityQueue. The method must satisfy this API specification exactly as it is written:

/** class PriorityQueue  implements a priority queue, in which
objects are stored with their priority numbers.  */
public class PriorityQueue

/** method  insert  places a new object into the queue
  * @param num - the object's priority number; it must be a positive
  *   number.  (The lower the number, the higher the priority ---
  *   1 is highest, then 2, etc.)
  * @param ob - the object stored in the queue
  * @throw RuntimeException  if  num  is not a positive number or if
  *   ob  has value null.  */
public void insert(int num, Object ob)

/** method  retrieve  removes from the queue the highest priority object.
  *  If there are multiple objects that both have the highest priority, then
  *  the object that has resided in the queue for the longest time is
  *  retrieved.
  * @return  the (address of) the object retrieved
  * @throw  RuntimeException if the priority queue is empty */
public Object retrieve()

/** function  isEmpty  checks whether the queue is empty.
  * @return true, if the queue is empty; otherwise, return false. */
public boolean isEmpty()

/** method  toString  returns a string representation of the contents of the
  *   queue.  (This is useful for debugging purposes.)
  * @return  a string that lists the objects and their priority numbers,
  *   enclosed by square brackets.   For example, if the queue holds
  *   ob A with priority 5  and ob B with priority 3, the string would say
  *   "[5,A  3,B]".
  *   (But the order in which the items are listed in the string is unimportant.
  *   so  "[3,B  5,A]"  is OK also.  If the queue is empty,  "[]"  is returned.)
  *   Note:  for each object,  ob,  in the queue,   ob.toString()  is
  *   used to make the string that represents  ob  in the answer string.  */
public String toString()

Test your implementation with one or more test programs, like this one:

import PQ.*;
/** Test gives a script for testing a priority queue */
public class Test
{ public static void main(String[] a)
   { PriorityQueue q = new PriorityQueue();
     System.out.println(q.toString());  // prints  []
     if ( q.isEmpty() )
        { System.out.println("OK so far"); }

     q.insert(8, "abc");
     q.insert(3, "def");
     q.insert(4, "ghi");
     if ( q.isEmpty() )
          { System.out.println("wrong answer"); }
     else { System.out.println(q.toString()); } // prints  [8,abc  3,def  4,ghi]  

     String s = (String)(q.retrieve());   // "def" is returned
     System.out.println(s);  // prints  def

     q.insert(4, "jkl");
     System.out.println(q.toString());  // prints  [8,abc  4,ghi  4,jkl]

     System.out.println(q.retrieve().toString());  // prints  ghi
     System.out.println(q.toString());  // prints  [8,abc  4,jkl]

     // An experiment: make a second queue and insert a string into it:
     PriorityQueue r = new PriorityQueue().insert(1, "strange");
     q.insert(2, r);  // it's strange but legal
     System.out.println(q.toString()); // prints  [8,abc  4,jkl,  2,[1,strange]]
     // now, retrieve  r   and print it:
     System.out.println(q.retrieve().toString()); // what does it print?

     q.retrieve();  // "jkl" is returned
     System.out.println(q.toString());  // prints  [8,abc]
   }
}

Implementation and submission requirements:

Make a folder named Assign7. Within it, place package PQ and any other packages it uses. Document all classes and package them accordingly, and run javadoc on your completed program. Submit Assign7.jar at http://www.cis.ksu.edu/~schmidt/300f04/Assign/submit.html.