Question

(Implement GenericQueue using inheritance) In Section 24.5, Stacks and Queues, GenericQueue is implemented using composition. Define a new queue class that extends java.util.LinkedList..

Answer 1

GenericQueue.java

import java.util.LinkedList;

public class GenericQueue<E> extends LinkedList<E>
{
/**
   *
   */
   private static final long serialVersionUID = 1L;
   E head;
   E tail;

   public GenericQueue()
   {
       head = null;
       tail = null;
}
  
   public boolean isEmpty()
   {
       if(size() == 0)
           return true;
       return false;
   }

public void enqueue(E c)
{
   add(c);
   // if the list was initially empty point both head and tail to element
   if(head == null && tail == null)
   {
       head = c;
       tail = c;
   }
   else
   {
       //only change tail and point it to recently added node
       tail = c;
   }

}

public E dequeue()
{
   if(isEmpty())
       return null;
  
   E firstElement = getFirst(); // getLast is a linkedList method;
   poll(); // this removes the element at the head
   if(isEmpty()) // if there was a single element in the list, point both head and tail to null
   {
       head = null;
       tail = null;
   }
   else
       head = getFirst(); // point the head to the current first node. No change in tail
  
return firstElement; // return the element we just removed from the queue
}
  
//return the first element of queue without removing it
public E peek()
{
  
   return (E) element(); // this is a linkedlist method which retrieves but does not remove the head of the queue
}
  
public void print()
{
   System.out.print("PrintList: ");
   Object[] ab = toArray(); //toArray is a linkedlist method that returns all elemnts of list in array from head to tail sequence
   for(int i=0; i<size(); i++)
   {
       System.out.print(ab[i] + " ");
   }
   System.out.println();
}
}

GenericQueueDemo.java

public class GenericQueueDemo
{
   public static void main(String args[])
   {
  
       GenericQueue<Integer> g1 = new GenericQueue<Integer>();
       System.out.println("Is queue empty? "+ g1.isEmpty());
       g1.enqueue(5);
       g1.print();
       g1.enqueue(6);
       g1.enqueue(45);
       System.out.println("Peek: "+ g1.peek());
       g1.print();
       System.out.println("Is queue empty? "+ g1.isEmpty());
       g1.dequeue();
       System.out.println("Peek: "+ g1.peek());
       g1.print();
      
       GenericQueue<String> g2 = new GenericQueue<String>();
       System.out.println("Is queue empty? "+ g1.isEmpty());
       g2.enqueue("Hello");
       g2.enqueue("World!");
       g2.enqueue("This");
       g2.enqueue("is");
       g2.enqueue("a");
       g2.enqueue("new");
       g2.enqueue("day");
       g2.print();
       g2.enqueue("Welcome");
       g2.enqueue("back!!");
       System.out.println("Peek: "+ g2.peek());

       g2.print();
       System.out.println("Is queue empty? "+ g1.isEmpty());
       g2.dequeue();
       System.out.println("Peek: "+ g2.peek());

       g2.print();  
       g2.dequeue();
       g2.print();  
       g2.dequeue();
       g2.print();  
   }  

}

Sample I/O and output