Java previous button cycle

Question

I am trying to create previous and next buttons in addition to my first and last buttons. I have managed to get them all to work. However, I cannot seem to get the previous and next buttons to cycle through the array and instead I get errors when reaching the end. I'm not even sure where to start, but all help is very much appreciated!

    JButton firstButton = new JButton("First");
    buttonPanel.add(firstButton);
    firstButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            bookIndex = 0;
            prepareDisplay(inventoryBook[bookIndex], textArea);
        }
    });

    JButton previousButton = new JButton("Previous");
    buttonPanel.add(previousButton);
    previousButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            bookIndex = bookIndex - 1;
            prepareDisplay(inventoryBook[bookIndex], textArea);
        }
    });

    JButton nextButton = new JButton("Next");
    buttonPanel.add(nextButton);
    nextButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            bookIndex = bookIndex + 1;
            prepareDisplay(inventoryBook[bookIndex], textArea);
        }
    });

    JButton lastButton = new JButton("Last");
    buttonPanel.add(lastButton);
    lastButton.addActionListener(new ActionListener()
    {   
        public void actionPerformed(ActionEvent e)
        {
            bookIndex = (inventoryBook.length - 1);
            prepareDisplay(inventoryBook[bookIndex], textArea);
        }
    });

Answer 1

In the actionPerformed for the previousButton and nextButton you need to check what the current bookIndex is before you decrement or increment, respectively. In the case of previousButton, if the current bookIndex == 0, set the bookIndex to inventoryBook.length-1 instead of decrementing. For the nextButton, if the bookIndex == inventoryBook.length-1, set bookIndex to 0 instead of incrementing. So for nextButton:

        JButton nextButton = new JButton("Next");
        buttonPanel.add(nextButton);
        nextButton.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                if(bookIndex == inventoryBook.length - 1) {
                    bookIndex = 0;
                } else {
                    bookIndex = bookIndex + 1;
                }

                prepareDisplay(inventoryBook[bookIndex], textArea);
            }
        });

Answer 2

if you have an out index range in the end on the next button, check the index in

JButton nextButton = new JButton("Next");
    buttonPanel.add(nextButton);
    nextButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            // i guess that here is the problem
            bookIndex = bookIndex + 1;
            prepareDisplay(inventoryBook[bookIndex], textArea);
        }
    });

Answer 3

% Modulo is the key. It will make the desired cycle simulation.

This will make next work as a cycle, that means when you will use next at the last index it will take you to the start. Same goes for prev just change it for bookIndex-1

JButton nextButton = new JButton("Next");
buttonPanel.add(nextButton);
nextButton.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {
        bookIndex = (bookIndex+1)%inventoryBook.length;
        prepareDisplay(inventoryBook[bookIndex], textArea);
    }
});

Answer 4

You will have to add some bounds checking to make sure previous doesn't go below 0, and next doesn't make it go above the length.