Can't add JPanel to BorderLayout.CENTER

Question

I am just Starting out with JAVA. I have say a JPanel x, a JPanel y and a BorderLayout JPanel z. When I try to change the contents of the center of z from default x t y, it works but it doesn't go back to x. I AM calling revalidate() after each. Help please. The class below is where the problem is.

Main Class Below

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.LayoutManager;

import javax.swing.BorderFactory;
import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingConstants;
import javax.swing.border.Border;
import javax.swing.border.EmptyBorder;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

@SuppressWarnings({ "serial", "unused" })
public class Manager extends JFrame {

    private JPanel contentPane;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Manager frame = new Manager();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public Manager() {
        setTitle("Popper");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        double width = screenSize.getWidth();
        double height = screenSize.getHeight();
        height = height/5.1;
        setSize((int)width, (int)height);
        setExtendedState(getExtendedState() | JFrame.MAXIMIZED_BOTH);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(0,0,0,0));
        setContentPane(contentPane);
        contentPane.setBackground(new Color(14,99,165));
        contentPane.setLayout(new BorderLayout(0, 0));
        ImageIcon image = new ImageIcon("D:/popper26.png");
        setIconImage(image.getImage());
        JPanel pane = new JPanel();
        calcu cal = new calcu();
        curr nup = new curr();
        stopc newst = new stopc();
        pane.setLayout(new FlowLayout(FlowLayout.CENTER));
        JPanel mainpanel = new JPanel();
        BorderLayout x =new BorderLayout(0,0);
        mainpanel.setLayout(x);
        mainpanel.setBackground(Color.WHITE);
        JLabel madeby = new JLabel("Project By Anant Bhasin");
        madeby.setHorizontalAlignment(SwingConstants.RIGHT);
        mainpanel.add(madeby, BorderLayout.SOUTH);
        JPanel logo = new JPanel();
        logo.setLayout(new FlowLayout(FlowLayout.CENTER));
        JLabel jk = new JLabel(new ImageIcon("D:/popper2.png"));
        logo.add(jk, BorderLayout.NORTH);
        logo.setBackground(Color.decode("#1abc9c"));
        mainpanel.add(logo, BorderLayout.NORTH);
        mainpanel.add(cal, BorderLayout.CENTER);
        contentPane.add(mainpanel, BorderLayout.CENTER);
        JPanel newj = new JPanel();
        BoxLayout bxl = new BoxLayout(newj, BoxLayout.PAGE_AXIS);
        newj.setLayout(bxl);
        newj.setBackground(new Color(58,115,144));
        contentPane.add(newj, BorderLayout.WEST);
        Border emptyBorder = BorderFactory.createEmptyBorder();
        JButton calc = new JButton(new ImageIcon("D:/calc.png"));
        newj.add(calc);
        calc.setBorder(emptyBorder);
        calc.setFocusPainted(false);
        calc.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                mainpanel.add(BorderLayout.CENTER, cal);
                mainpanel.revalidate();
            }
        });
        JButton currb = new JButton(new ImageIcon("D:/curr.png"));
        currb.setBorder(emptyBorder);
        newj.add(currb);
        currb.setFocusPainted(false);
        currb.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                mainpanel.add(BorderLayout.CENTER, nup);
                mainpanel.revalidate();
            }
        });
        JButton stop = new JButton(new ImageIcon("D:/stop.png"));
        stop.setBorder(emptyBorder);
        newj.add(stop);
        stop.setFocusPainted(false);
        stop.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                mainpanel.add(BorderLayout.CENTER, newst);
                mainpanel.revalidate();
            }
        });
        JButton timer = new JButton(new ImageIcon("D:/timer.png"));
        timer.setBorder(emptyBorder);
        newj.add(timer);
        timer.setFocusPainted(false);
        JButton memo = new JButton(new ImageIcon("D:/memo.png"));
        memo.setBorder(emptyBorder);
        newj.add(memo);
        memo.setFocusPainted(false);
    }
}

Answer 1

A BorderLayout is not designed to display multiple components with the same constraint because of the way ZOrder painting works in Swing.

If you need the ability to swap panels, then you should be using a CardLayout.

A CardLayout lets you specify the name of the panel that you want to display. Read the section from the Swing tutorial on How to Use CardLayout for more information and working examples.

You set up the layout with code like:

JPanel main = new JPanel( new CardLayout() );
main.add(panelx, "X");
main.add(panely, "Y");

Then to swap a panel you use code like:

CardLayout cl = (CardLayout)(main.getLayout());
cl.show(main, "X");