GroceryLinked & GLink classes

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GroceryLinked Class
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package stacksQueuesLinkedLists;

import java.io.*; //for serialization
import javax.swing.*;
import java.util.*; //for ListIterator

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;


public class GroceryLinked extends JFrame implements ActionListener{


    int[] filled = new int[5];

    static JFrame mainFrame = new JFrame();
    JPanel main = new JPanel(new GridLayout(3,1));
    JPanel titlePanel = new JPanel();
    JPanel customerQueuePanel = new JPanel(new GridLayout(1,9));
    JPanel buttonPanel = new JPanel(new GridLayout(1,3));

    JLabel title = new JLabel("Customer Queue Manager");
    JLabel[] customers = new JLabel[9];

    JButton processC = new JButton("Process Next Customer");
    JButton info = new JButton("Info");
    JButton addC = new JButton("Add Customer");


    JPanel infoPanel = new JPanel(new BorderLayout());
    JTextArea infoArea = new JTextArea("In an effort to remind employees and enforce policies around distancing customers in cashier queues, spaces will be added after each customer has been added. As queues only have 9 spaces, there can only be a maximum of 5 customers in each queue, after which you will not be able to add any more without processing them. Stay healthy!");
    JButton back = new JButton("back");
    JTextArea margin1 = new JTextArea("    ");
    JTextArea margin2 = new JTextArea("    ");
    JTextArea margin3 = new JTextArea("    ");


    GLink linked = new GLink();

    public GroceryLinked()
    {


        for(int x = 0; x < 5; ++x) {
            filled[x] = 0;
        }

        for(int x = 0; x < 9; ++x) {
            customers[x] = new JLabel();
            customerQueuePanel.add(customers[x]);
        }


        add(main);
        main.add(titlePanel);
        main.add(customerQueuePanel);
        main.add(buttonPanel);

        titlePanel.add(title);

        buttonPanel.add(addC);
        buttonPanel.add(info);
        buttonPanel.add(processC);

        infoPanel.add(infoArea, BorderLayout.CENTER);
        infoPanel.add(back, BorderLayout.SOUTH);
        infoPanel.add(margin1, BorderLayout.NORTH);
        infoPanel.add(margin2, BorderLayout.EAST);
        infoPanel.add(margin3, BorderLayout.WEST);

        processC.addActionListener(this);
        info.addActionListener(this);
        addC.addActionListener(this);
        back.addActionListener(this);

        infoArea.setLineWrap(true);
        infoArea.setEditable(false);
        margin1.setEditable(false);
        margin2.setEditable(false);
        margin3.setEditable(false);

    }


    public static void main(String[] args)
    {

        mainFrame = new GroceryLinked();
        mainFrame.setBounds(500,500,998,300);
        mainFrame.setTitle("Customer Queue Manager");
        mainFrame.setVisible(true);


    }
    @Override
    public void actionPerformed(ActionEvent temp) {
        Object e = temp.getSource();

        String cName = "";
        String cCost = "";




        if(e.equals(processC)) {
            linked.remove(0);
            linked.remove(0);
            for(int x = 0; x < 9; ++x) {
                customers[x].setText("" + linked.get(x));
            }
        }


        if(e.equals(info)) {
            getContentPane().invalidate();
            getContentPane().removeAll();
            add(infoPanel);

            getContentPane().repaint();
            getContentPane().revalidate();
        }


        if(e.equals(addC)) {
            if(filled[0] == 0) {
                cName = JOptionPane.showInputDialog(null, "Please input the customers name.");
                cCost = (JOptionPane.showInputDialog(null, "Please input cost of the customers order."));
                if(cName != null || cCost != null) {
                    linked.set(0, "" + cName + ", $" + cCost);
                    filled[0] = 1;
                }
            }
            else {
                if(filled[1] == 0) {
                    cName = JOptionPane.showInputDialog(null, "Please input the customers name.");
                    cCost = (JOptionPane.showInputDialog(null, "Please input cost of the customers order."));
                    if(cName != null || cCost != null) {
                        linked.set(2, "" + cName + ", $" + cCost);
                        filled[1] = 1;
                    }
                }
                else {

                    if(filled[2] == 0) {
                        cName = JOptionPane.showInputDialog(null, "Please input the customers name.");
                        cCost = (JOptionPane.showInputDialog(null, "Please input cost of the customers order."));
                        if(cName != null || cCost != null) {
                            linked.set(4, "" + cName + ", $" + cCost);
                            filled[2] = 1;
                        }
                    }
                    else {

                        if(filled[3] == 0) {
                            cName = JOptionPane.showInputDialog(null, "Please input the customers name.");
                            cCost = (JOptionPane.showInputDialog(null, "Please input cost of the customers order."));
                            if(cName != null || cCost != null) {
                                linked.set(6, "" + cName + ", $" + cCost);
                                filled[3] = 1;
                            }
                        }
                        else {
                            if(filled[4] == 0) {
                                cName = JOptionPane.showInputDialog(null, "Please input the customers name.");
                                cCost = (JOptionPane.showInputDialog(null, "Please input cost of the customers order."));
                                if(cName != null || cCost != null) {
                                    linked.set(8, "" + cName + ", $" + cCost);
                                    filled[4] = 1;
                                }
                            }
                            else {
                                JOptionPane.showMessageDialog(null, "The customer queue is full! process customers to make space.");
                            }
                        }
                    }
                }
            }
            for(int x = 0; x < 9; ++x) {
                customers[x].setText("" + linked.get(x));
            }
        }

        if(e.equals(back)) {
            getContentPane().invalidate();
            getContentPane().removeAll();
            add(main);

            getContentPane().repaint();
            getContentPane().revalidate();
        }



    }

}



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GLink Class
_______________________________________________________________________________________________________________________________________

package stacksQueuesLinkedLists;

import java.io.*; //for serialization
import javax.swing.*;
import java.util.*; //for ListIterator

public class GLink implements IndexedList, Serializable {

    private Node head; // pointer to first node
    private static int listSize; // list size
    private Node nodeAtIndex; // The node at index position or null if past the end of the list
    private Node nodeBefore; // The node before index position or null if before the beginning of the list
    private Node pointer;

    public GLink() {
        head = new Node("no customer", null);
        head = new Node("SPACE", head);
        head = new Node("no customer", head);
        head = new Node("SPACE", head);
        head = new Node("no customer", head);
        head = new Node("SPACE", head);
        head = new Node("no customer", head);
        head = new Node("SPACE", head);
        head = new Node("no customer", head);
        listSize = 9;
    }


    public void add(int index, Object obj)
    {
        if(index < 0 || index >= listSize)
        {
            throw new RuntimeException("Index = " + index + " is out of the list bounds");
        }
        if(index == 0)
        {
            head = new Node(obj, head);
        }
        else
        {
            locateNode(index);
            nodeBefore.next = new Node(obj, nodeAtIndex);
        }
        listSize++;
    }

    public boolean isEmpty()
    {
        return listSize == 0;
    }

    public boolean isFull()
    {
        return false;
    }

    public Object get(int index)
    {
        if(index < 0 || index >= listSize)
        {
            throw new RuntimeException("Index = " + index + " is out of the list bounds");
        }

        locateNode(index);
        return nodeAtIndex.value;
    }

    public Object remove(int index)
    {
        if(index < 0 || index >= listSize)
        {
            throw new RuntimeException("Index = " + index + " is out of the list bounds");
        }

        Object removedObj = null;

        if(index == 0)
        {
            removedObj = head.value;
            head = head.next;
        }

        else
        {
            locateNode(index);
            nodeBefore.next = nodeAtIndex.next;
            removedObj = nodeAtIndex.value;
        }

        listSize--;
        return removedObj;
    }

    public Object set(int index, Object obj)
    {
        Object replacedObj = null;

        try
        {
            if(index == 0)
            {
                replacedObj = head.value;
                head.value = obj;
            }
            else
            {
                locateNode(index);
                replacedObj = nodeAtIndex.value;
                nodeAtIndex.value = obj;
            }
        }

        catch(RuntimeException e)  //8
        {
            JOptionPane.showMessageDialog(null, "Index = " + index + " is out of the list bounds",
            "Out of Bounds Error", JOptionPane.ERROR_MESSAGE);
            System.out.println("The list size is only" + size());
        }

        return replacedObj;
    }

    public int size()
    {
        return listSize;
    }

    public String toString()
    {
        String message = "";

        for(Node pointer = head; pointer != null; pointer = pointer.next)
        {
            message += pointer.value + " ";
        }
        return message;
    }

    private void locateNode(int index)
    {
        nodeBefore = null;
        nodeAtIndex = head;

        for(int i = 1; i < listSize && i <= index; i++)
        {
            nodeBefore = nodeAtIndex;
            nodeAtIndex = nodeAtIndex.next;
        }

        if(index == listSize)
        {
            nodeBefore = nodeAtIndex;
            nodeAtIndex = null;
        }
    }

    //private inner class for Node
    private class Node
    {
        private Object value;
        private Node next;
        private Node()
        {
            value = null;
            next = null;
        }
        private Node(Object value, Node next)
        {
            this.value = value;
            this.next = next;
        }
    }


}