Problem with Queue Implementation

Question

Hi there !
I can't solve the implementation of an exercize about Queue.My teacher wrote down just the interface Queue and the first part of the class ArrayQueue implementation.They are the following :
public interface Queue<E>
   public int size();
   public boolean isEmpty();
   public E front() throws EmptyQueueException;
   public void enqueue (E element);
   public E dequeue() throws EmptyQueueException;
public class ArrayQueue<E> implements Queue<E>
   public ArrayQueue(int cap)
      capacity = cap;
      Q = (E[]) new Object[capacity];
   public ArrayQueue()
      this(CAPACITY);
}So the teacher asked to complete the code. I have tried and retried so many times for 3 days.I know it's so easy and I understood how it should work but when I try to write down the code I can't get the final solutions but only mistakes.
During my attempts my better code was this :
package queue;
public class ArrayQueue<E> implements Queue<E>
     @SuppressWarnings("unchecked")
     public ArrayQueue(int cap)
        capacity = cap;
        Q = (E[]) new Object[capacity];
     public ArrayQueue()
        this(CAPACITY);
     public String toString()
        int count = 0;
        for(E element : Q)
             return count +")"+(String)element;
        count++;
        return null;
     public void enqueue(E element)
          try
               if(element == null)
                    throw new IllegalArgumentException();
               if (size() == capacity)
                 throw new FullQueueException();
               if(Q[rear] == null)
                  Q[rear] = element;
                  rear++;
          catch (FullQueueException e)
               System.out.println("The Queue is Full !");
     public E dequeue() throws EmptyQueueException
          try
             E temp;
             if (isEmpty())
                  Q = (E[]) new Object[capacity];
                  front = 0;
                  rear = 0;
                 throw new EmptyQueueException();
            temp = Q[front];
             Q[front] = null;
             front++;
             return temp;
          catch(EmptyQueueException e)
               System.out.println("The Queue is Empty !");
          return null;
     public E front() throws EmptyQueueException
          try
             if(Q[front] == null)
                       front++;
             if (isEmpty())
                 throw new EmptyQueueException();
             return Q[front];
          catch(EmptyQueueException e)
               System.out.println("The Queue is Full !");
          return null;
     public int size()
          return (front + rear);
     public boolean isEmpty()
        return (front == rear);
    public Object[] getQueue()
        return Q;
     protected int capacity;
     public static final int CAPACITY = 1000;
     protected E Q[];
     protected int front = 0;
     protected int rear = 0;
}But the problems are that I can add the element through the method enqueue() and delete it through dequeue() then,but after deleting it the array size is the same and when I print the array elements I see some null,so if I add a new element I get the message of the class FullQueueException because of the size which it's the same.
Moreover if I delete all the elements and then I print the value returned by the method front() I get the NullPointerExceptionError because the returned value is null,but I think it should print it ! But he doesn't!
How should I fix these problems ?
I also wondered during all my attempts how to repeat this procedure after the array size limit is reached.What I really mean is if I delete the element Q[0] and the index front = 1 and rear = n - 1 for example,how can I add with the method enqueue() a new element in Q[0] again ?
Can you help me courteously ?
I hope my explanation was clear because of my English !
Thanks in Advance !

Answer 1

Thanks for all your suggestions men ^^ !
I changed some things in the code :
package queue;
public class ArrayQueue<E> implements Queue<E>
     @SuppressWarnings("unchecked")
     public ArrayQueue(int cap)
        capacity = cap;
        Q = (E[]) new Object[capacity];
     public ArrayQueue()
        this(CAPACITY);
     public String toString()
          String s = "[";
          int count = 0;
          for (int i = front; i < rear; i++) {
               count++;
               s += Q.toString() + (i < rear - 1 ? "," : "");
          s += "] (" + count + " elements)";
          return s;
     public void enqueue(E element)
          try
               if (size() == capacity - 1)
          throw new FullQueueException();
               Q[rear] = element;
               rear = (rear + 1) % capacity;
          catch (FullQueueException e)
               System.out.println("The Queue is Full !");
     public E dequeue() throws EmptyQueueException
          try
          E temp;
          if(isEmpty())
          throw new EmptyQueueException();
     temp = Q[front];
          Q[front] = null;
          front = (front + 1) % capacity;
          return temp;
          catch(EmptyQueueException e)
               System.out.println("The Queue is Empty !");
          return null;
     public E front() throws EmptyQueueException
          try
          if (isEmpty())
          throw new EmptyQueueException();
          return Q[front];
          catch(EmptyQueueException e)
               System.out.println("The Queue is Empty !");
          return null;
     public int size()
          return (capacity - front + rear) % capacity;
     public boolean isEmpty()
return (front == rear);
     protected int capacity;
     public static final int CAPACITY = 1000;
     protected E Q[];
     protected int front;
     protected int rear;
}Now after I delete an element and I use the method enqueue() to add a new one the algorithm does,but
after I delete all the elements I get an element null when I add the element in the array position n - 1.
I have fixed the method toString() as pgt told me to do and it's really great,but dunno why sometimes when I delete an element it says the array has 0 elements also if the array is almost full !
About my code changements,according to my university friend I should reduce front and rear mod the capacity and that's what marlin314 suggested me to do too and I had to change the expression if(size() == capacity) to if(size() == capacity - 1).But the algorithm doesn't work as it should do !