How do I change the JPanel inside of a JFrame?

Question

I'm working on this basic Pong-style game for fun but I'm having some difficulty with the GUI aspect of it. Essentially I have a JFrame called window which houses everything, and a JPanel called content. content is supposed to refer to the ContentPanel I have. Here's my source code so far (sorry it's so long, a lot of it pertains exclusively to the game and not this question but I'm not sure which parts of the source code I need for my question to make sense):

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

public class Pong extends JApplet {


   public static void main(String[] args) {
      JFrame window = new JFrame("Pong: The Game of Champions");
      JPanel content = new JPanel();
      window.setContentPane(content);
      window.setSize(1000,600);
      window.setLocation(100,100);
      window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      window.setVisible(true);
      window.setResizable(false);

      JMenuBar settings = new JMenuBar();
      ButtonHandler listener = new ButtonHandler();
      window.setJMenuBar(settings);
      JMenu toolsMenu = new JMenu("Settings");
      settings.add(toolsMenu);
      JMenuItem preferences = new JMenuItem("Preferences");
      JMenuItem about = new JMenuItem("About");
      toolsMenu.add(preferences);
      toolsMenu.add(about);      
      about.addActionListener(listener);
      preferences.addActionListener(listener);

   }

   public void init() {
      setContentPane( new ContentPanel() );
   }

   public static class ButtonHandler implements ActionListener {
       public void actionPerformed(ActionEvent evt) {

           String command = evt.getActionCommand();

           if (command.equals("About")) {
               JOptionPane.showMessageDialog(null, "Made by Kyle Evans for Mr. Richardson's AP Computer Science class. Aww yeah.");
           }

           else if (command.equals("Preferences")) {

           }
       }
   }

   public static class ContentPanel extends JPanel implements KeyListener, FocusListener, MouseListener, ActionListener {

      private Timer timer = new Timer(10, this);
      private static final int PADDLE_WIDTH = 10;  // Length of side of square.
      private static final int PADDLE_HEIGHT = 100;
      private static final int BALL_WIDTH = 10;
      private static final int UP = 1, DOWN = 2;

      private Color squareColor;  // The color of the square.

      private int paddleOneDirection = -1;
      private int paddleTwoDirection = -1;

      private int paddleOneTop, paddleOneLeft, paddleTwoTop, paddleTwoLeft, paddleSpeed, 
                            ballTop, ballLeft, ballSpeedHorizontal, ballSpeedVertical;



      public ContentPanel() {

         paddleOneTop = 230;  // Initial position of top-left corner of square.
         paddleOneLeft = 900;
         paddleTwoTop = 230;
         paddleTwoLeft = 90;
         paddleSpeed = 4;
         ballTop = 270;
         ballLeft = 490;
         ballSpeedHorizontal = (int)(Math.random()*2);
            if (ballSpeedHorizontal == 0) {
                ballSpeedHorizontal = -1;
            }
         ballSpeedVertical = (int)(Math.random()*6) - 3;
         squareColor = Color.WHITE;  // Initial color of square.

         setBackground(Color.BLACK);

         addKeyListener(this);     // Set up event listening.
         addFocusListener(this);
         addMouseListener(this);

      } // end init();

      public void paintComponent(Graphics g) {

         super.paintComponent(g);  // Fills the panel with its
                                   // background color, which is white.

         g.setColor(squareColor);
         g.fillRect(paddleOneLeft, paddleOneTop, PADDLE_WIDTH, PADDLE_HEIGHT);
         g.fillRect(paddleTwoLeft, paddleTwoTop, PADDLE_WIDTH, PADDLE_HEIGHT);
         g.fillRect(ballLeft, ballTop, BALL_WIDTH, BALL_WIDTH);
         String messageH = "false";
         String messageV1 = "false";
         String messageV2 = "false";
         if (isHittingPaddleHorizontal()) {
             messageH = "true";
         }
         if (!isHittingPaddleHorizontal()) {
             messageH = "false";
         }
         if (isHittingPaddleOneVertical()) {
             messageV1 = "true";
         }
         if (!isHittingPaddleOneVertical()) {
             messageV1 = "false";
         }
         if (isHittingPaddleTwoVertical()) {
             messageV2 = "true";
         }
         if (!isHittingPaddleTwoVertical()) {
             messageV2 = "false";
         }
         g.drawString("Is hitting horizontal: " + messageH,7,20);
         g.drawString("Is hitting vertical right: " + messageV1,7,40);
         g.drawString("Is hitting vertical left: " + messageV2,7,60);

      }  // end paintComponent()

      // ------------------- Game controlling methods -------------------------

      public void moveBall() {

          ballLeft -= ballSpeedHorizontal;
          ballTop -= ballSpeedVertical;

          if (isHittingPaddleHorizontal()) {
              if (isHittingPaddleOneVertical()) {
                      ballSpeedHorizontal = -ballSpeedHorizontal;
                     // ballSpeedHorizontal++;
              } 
              else if (isHittingPaddleTwoVertical()) {
                     // ballSpeedHorizontal++;
                      ballSpeedHorizontal = -ballSpeedHorizontal;
              }
          }

          if ((ballTop <= 0) || (ballTop >= 565))     // Sets the vertical boundaries so the ball bounces off the top and bottom.
              ballSpeedVertical = -ballSpeedVertical;

          if ((ballLeft > 1000) || ballLeft < 0) {
              gameOver();
          }
      }

      public boolean isHittingPaddleOneVertical() {
          for (int i = 0; i < PADDLE_HEIGHT + (2 * BALL_WIDTH); i++) {
              if (ballTop == (paddleOneTop - BALL_WIDTH + i)) {
                  return true;
              }
          }
          return false;
      }

      public boolean isHittingPaddleTwoVertical() {
          for (int i = 0; i < PADDLE_HEIGHT + (2 * BALL_WIDTH); i++) {
              if (ballTop == (paddleTwoTop - BALL_WIDTH + i)) {
                  return true;
              }
          }
          return false;
      }

      public boolean isHittingPaddleHorizontal() {
          for (int i = 0; i < 50; i++) {
              if ((ballLeft == paddleOneLeft + i) || (ballLeft == paddleTwoLeft + PADDLE_WIDTH - i)) { 
                  return true;
              }
          }
          return false;
      }

      public void robotControl() {
          paddleTwoTop = ballTop - 45;
      }

      public void gameOver() {
          paddleOneTop = 230;
          paddleTwoTop = 230;
          ballTop = 270;
          ballLeft = 490;
          ballSpeedHorizontal = (int)(Math.random()*2);
            if (ballSpeedHorizontal == 0) {
                ballSpeedHorizontal = -1;
            }
          ballSpeedVertical = (int)(Math.random()*6) - 3;
      }

      // ------------------- Event handling methods ----------------------

      public void focusGained(FocusEvent evt) {
         repaint();  // redraw with cyan border
      }

      public void focusLost(FocusEvent evt) {
         repaint();  // redraw without cyan border
      }

      public void keyTyped(KeyEvent evt) {

         char ch = evt.getKeyChar();  // The character typed.

      }  // end keyTyped()

      public void keyPressed(KeyEvent evt) { 

         int key = evt.getKeyCode();  // keyboard code for the pressed key

         timer.start();

         if (key == KeyEvent.VK_UP) {  // up arrow key
             paddleOneDirection = UP;
         }
         else if (key == KeyEvent.VK_DOWN) {  // down arrow key
             paddleOneDirection = DOWN;
         }
         else {
             paddleOneDirection = -1; // This prevents other keys from interfering with the paddle movement.
         }
         if (key == KeyEvent.VK_W) {
             paddleTwoDirection = UP;
         }
         else if (key == KeyEvent.VK_S) {
             paddleTwoDirection = DOWN;
         }
         else {
             paddleTwoDirection = -1;
         }

      if ((paddleOneTop > 1000 - PADDLE_HEIGHT))
          paddleOneTop = 1000 - PADDLE_HEIGHT;
      if (paddleTwoTop > 1000 - PADDLE_HEIGHT)
          paddleTwoTop = 1000 - PADDLE_HEIGHT;


      }  // end keyPressed()

      public void keyReleased(KeyEvent evt) {
          paddleOneDirection = -1;
          paddleTwoDirection = -1;
      }

      public void mousePressed(MouseEvent evt) {
         requestFocus();
      }   

      public void mouseEntered(MouseEvent evt) { }  // Required by the
      public void mouseExited(MouseEvent evt) { }   //    MouseListener
      public void mouseReleased(MouseEvent evt) { } //       interface.
      public void mouseClicked(MouseEvent evt) { }
      public void actionPerformed(ActionEvent evt) {


          switch (paddleOneDirection) {
          case UP:
              paddleOneTop -= paddleSpeed;
              break;
          case DOWN:
              paddleOneTop += paddleSpeed;
              break;
          }
          switch (paddleTwoDirection) {
          case UP:
              paddleTwoTop -= paddleSpeed;
              break;
          case DOWN:
              paddleTwoTop += paddleSpeed;
              break;
          }
          robotControl();
          moveBall();
          repaint();
      }
   } // end nested class ContentPanel


} // end class

(The code is a bit messy, I apologize, I'm pretty new to Java and programming in general.)

Here are my questions:

1. How do I initialize the JFrame to display the JPanel content by default?

2. How do I begin creating a separate JPanel that I can switch between? My ultimate goal is to have a separate JPanel appear when the user clicks on "Preferences" and then be able to switch back and forth between the preferences JPanel and the content JPanel .

I would greatly appreciate any help.

Answer 1

Initialize as many panels with different content on JFrame creation and place them in CardLayout . Switch cards to show required panel when necessary.