Circular Queue Array

Question

Hi everyone i am trying to implement a circular array, but something is not exactly right. I am not sure if is in the add method or display. When you run the debugger, the numbers are there but i cant get them in order. Please could you have a look and give me a feedback. Thank you.

public static void main (String[] args)
{
    Scanner input = new Scanner (System.in);
    Queue q1 = new Queue();
    int choice = 0;

    do
    {
        System.out.println ("Menu:");
        System.out.println ("1: Add");
        System.out.println ("2: Remove");
        System.out.println ("3: Display");
        System.out.println ("4: Exit");

        System.out.print ("\nChoice: ");
        choice = input.nextInt ();

        switch ( choice )
        {
            case 1:
                System.out.print ("\nEnter a number: ");
                int num = input.nextInt ();
                q1.add (num);
                break;
            case 2:
                q1.remove ();
                break;
            case 3:
                q1.display ();
                break;
            case 4:
                System.out.println ("Good Bye");
                break;
            default:
                System.out.println ("Wrong choice!");
                break;
        }

    } while ( choice != 4 );


}

}

public class Queue {

private final int SIZE;
private int first;
private int last;
private int[] q;

public Queue ()
{
    SIZE = 5;
    q = new int[ SIZE ];
    first = 0;
    last = 0;
}

public boolean isFull ()
{
    return last == SIZE;
}

public boolean isEmpty ()
{
    return last == first;
}

public void add (int x)
{
    if (  ! isFull () )
    {
        q[ ( first + last ) % q.length ] = x;
        last ++;
    } else
    {
        System.out.println ("\nThe queue is full!");
    }
}

int remove ()
{
    int x = 0;
    if (  ! isEmpty () )
    {
        x = q[ first ];
        first = ( first + 1 ) % q.length;
        last --;
    } else
    {
        System.out.println ("The queue is empy");
    }
    return x;

}

public void display ()
{
    if (  ! isEmpty () )
    {
        for ( int i = first; i < SIZE; i ++ )
        {
            System.out.println (q[ i ]);
        }


    } else
    {
        System.out.println ("The queue is emptry");
    }
}

Answer 1

It worked pretty well. Can you be more specific about your error ?

Answer 2

add() should only update last . remove() should only update first . add() needs to prevent the queue from going full, or the code needs to use a count of elements in the queue to check if the queue is full (versus empty, since last == first if either empty or full), in which case add() needs to prevent queue overflow. add() could return a code to indicate if add() was successful (no overflow) or if it failed (overflow). add() could optionally return a third code value to indicate that the queue was previously empty, in case some special action is needed when a queue goes from empty to non-empty.