Develop Reference

Changing the sizes of a coloured square after pressing a button

I have written this code however I am having trouble with one aspect I wish to code into it. I want to make the green square change size when I press one of the three buttons I have so when I press the button 'small' the square changes size to small e.g. 100 and when I press the button 'medium' it changes size to medium e.g. 400. This is my code so far:
package Lab2;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Main {
public static void main(String[] args) {
FilledFrame frame = new FilledFrame();
frame.setVisible( true );
}
}
class FilledFrame extends JFrame {
int size = 400;
public FilledFrame()
{
JButton butSmall = new JButton("Small");
JButton butMedium = new JButton("Medium");
JButton butLarge = new JButton("Large");
JButton butMessage = new JButton("Say Hi");
SquarePanel panel = new SquarePanel(this);
JPanel butPanel = new JPanel();
butPanel.add(butSmall);
butPanel.add(butMedium);
butPanel.add(butLarge);
butPanel.add(butMessage);
add(butPanel, BorderLayout.NORTH);
add(panel, BorderLayout.CENTER);
setSize( size+100, size+100 ); } }
class SquarePanel extends JPanel {
FilledFrame theApp;
SquarePanel(FilledFrame app)
{
theApp = app;
}
public void paintComponent ( Graphics g)
{
super.paintComponent(g);
g.setColor(Color.green);
g.fillRect(20, 20, theApp.size, theApp.size);
}
}
class buttonHandler implements ActionListener {
FilledFrame theApp;
int size;
public buttonHandler(FilledFrame app, int size) {
theApp = app;
this.size = size;
}
#Override
public void actionPerformed (ActionEvent e){
theApp.setSize(this.size, this.size);
}
}

As I don't see any event listeners for your buttons, I assume this is all the code you have. Your buttons will not do anything unless you tell them to do it. You need to add event listeners, and through that change the size and update the panel.
Example:
button.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
theApp.size = 200;
frame.getContentPane().repaint();
//OR frame.repaint();
}
});
EDIT:
The problem with using the button handler class is you would need to find which buttton was pressed, instead its easier to use the way I showed above. I edited the code above, try copy pasting to one of the buttons.

My JFrame doesn't show anything

I am trying to build a simple basic to a game for my class in Game Engine Architecture. But my JFrame just wont show anything.
My code is currently structured like this:
Implementation.java (This is some arbitrary implementation of the Engine-package I'm creating)
public class Implementation {
public static void main(String[] args){
World w = new World("Hej", "M:\\workspace\\SP6\\pics\\tulips.jpg",1024,768);
}
}
World.java
public class World extends JFrame{
private static final long serialVersionUID = 1L;
private SpritePanel spritePanel;
private JPanel bottom;
private int width;
private int height;
public World(String windowCaption, String bgPath, int width, int height){
super(windowCaption);
spritePanel = new SpritePanel(bgPath);
add(spritePanel, BorderLayout.CENTER);
System.out.println(spritePanel);
bottom = new JPanel();
bottom.add(new JLabel("Hej"));
add(bottom, BorderLayout.SOUTH);
Dimension size = new Dimension(width,height);
setSize(size);
setLocationRelativeTo(null);
setDefaultCloseOperation(EXIT_ON_CLOSE);
pack();
setVisible(true);
validate();
repaint();
}
SpritePanel.java
public class SpritePanel extends JPanel {
private static final long serialVersionUID = 1L;
private ImageIcon background;
private ArrayList<Sprite> sprites = new ArrayList<Sprite>();
public SpritePanel(String bgPath){
background = new ImageIcon(bgPath);
setLayout(null);
Dimension size = new Dimension(background.getIconWidth(), background.getIconHeight());
setPreferredSize(size);
setMaximumSize(size);
setMinimumSize(size);
}
#Override
protected void paintComponent(Graphics g){
super.paintComponent(g);
g.drawImage(background.getImage(), 0, 0, this);
System.out.println("painted panel");
}
}
So the basic action flow at the moment (from what I can see):
I create a new World object in the Implementation
The World-constructor is called with the given parameters
The window caption is set (this works)
I create a new SpritePanel object with the given bgPath
The SpritePanel-constructor is called and sets the ImageIcon to the image that exists in the given path
The SpritePanel is added to the frame in BorderLayout.CENTER
I add a new JPanel to the frame in BorderLayout.CENTER
I set size and stuff
I pack and set the frame to visible
I validate and repaint
The thing is the paintComponent methods in the JPanel and SpritePanel doesn't seem to get called. As you can see I added a System.out.println in the paintComponent for SpritePanel and that line is never executed.
Another thing I noticed is that the pack seems to know that the components are there. Because if I comment the three lines
spritePanel = new SpritePanel(bgPath);
add(spritePanel, BorderLayout.CENTER);
System.out.println(spritePanel);
The windows size when I run the program is reduced to the size of the "bottom"-JPanel. I can't see the JLabel that I added to the panel but the window size is the size of it. So the pack() method seems to be finding the dimensions of my components.
If I comment the three lines that adds the bottom panel as well the window size is reduced to no height and the width it gets from the standard icons.
I've been trying different things to get it to work but to no avail. I have programmed with Swing before and made working programs but I just can't seem to find the problem here.
Help very much appreciated.

Something may be wrong with your with your String path. What you should do though, instead o reading a file, you should be reading a URL, loading from the class path. At time of deployment, you'll find out that the file path won't work, so it's better to package your image as an embedded resource, putting the image in the class path. I had no problems when doing this. Here's what I did.
Change the path to a path relative to my class path
World w = new World("Hej", "/resources/stackoverflow5.png", 1024, 768);
Changed loading from a file to loading from a URL from my class path
background = new ImageIcon(SpritePanel.class.getResource(bgPath));
Put the image in resources package in my class path
Everything works fine
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Graphics;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import static javax.swing.JFrame.EXIT_ON_CLOSE;
import javax.swing.JLabel;
import javax.swing.JPanel;
public class Implementation {
public static void main(String[] args) {
World w = new World("Hej", "/resources/stackoverflow5.png", 1024, 768);
}
}
class World extends JFrame {
private static final long serialVersionUID = 1L;
private SpritePanel spritePanel;
private JPanel bottom;
private int width;
private int height;
public World(String windowCaption, String bgPath, int width, int height) {
super(windowCaption);
spritePanel = new SpritePanel(bgPath);
add(spritePanel, BorderLayout.CENTER);
System.out.println(spritePanel);
bottom = new JPanel();
bottom.add(new JLabel("Hej"));
add(bottom, BorderLayout.SOUTH);
Dimension size = new Dimension(width, height);
setSize(size);
setLocationRelativeTo(null);
setDefaultCloseOperation(EXIT_ON_CLOSE);
pack();
setVisible(true);
validate();
repaint();
}
class SpritePanel extends JPanel {
private static final long serialVersionUID = 1L;
private ImageIcon background;
//private ArrayList<Sprite> sprites = new ArrayList<Sprite>();
public SpritePanel(String bgPath) {
background = new ImageIcon(SpritePanel.class.getResource(bgPath));
setLayout(null);
Dimension size = new Dimension(background.getIconWidth(), background.getIconHeight());
setPreferredSize(size);
setMaximumSize(size);
setMinimumSize(size);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawImage(background.getImage(), 0, 0, this);
System.out.println("painted panel");
}
}
}
Side Notes
No need to setSize(). You already pack(). It better to just pack anyway.
Put the pack() before the setLocationRelativeTo(). If you pack() after, you'll notice your frame won't be in the desired location.
Run your Swing apps from the Event Dispatch Thread (EDT) like this
public static void main(String[] args){
SwingUtilities.invokeLater(new Runnable(){
#Override
public void run() {
new World("Hej", "/resources/stackoverflow5.png", 1024, 768);
}
});
}
If you want to make the frame fill the screen, don't set the size. Different machines have different screen sizes. Instead use setExtendedState(JFrame.MAXIMIZED_BOTH);
Also, your setSize() won't work anyway because you're calling pack() after.

My problem was that I hade overridden the getWidth() and getHeight() methods in my World-class.
But look to peeskillets response for how to make things better!

How can I switch between jpanels?

I'm still very new to java programming, so please help me to correct any mistakes I might have overlooked or give tips on how to improve this program.
Okay, so a lot of problems have been solved, and now I have a CardLayout, but I still have questions about how I should make my pipes show inside it.
When I tried to add in my refresh rate timer and my speed timer, I have problems about how I need to declare and initialize boolean variables.
Also, when I compile and run this game, I get files such as Game$1.class. Is there a way for me to clean this up, and could someone explain why this happens? Do these have an affect on the finished product? (When the game is compiled and packaged into a JAR.)
I want to set playerIsReady to true when the play button is clicked. And from there, when the if statement is true, then switch to a panel that displays the pipes, and start moving the pipe across the screen. Preferably 3 instances of that pipe, each starting at different times, but whatever you can help with is fine.
Some of this code needs work, so I have commented some parts out and left notes.
My other questions about this game can be found here.
This is my current code
Game.java
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.border.EmptyBorder;
import javax.swing.SwingUtilities;
public class Game {
public static void main(String[] args) {
Runnable r = new Runnable() {
#Override
public void run() {
// the GUI as seen by the user (without frame)
final CardLayout cl = new CardLayout();
final JPanel gui = new JPanel(cl);
// remove if no border is needed
gui.setBorder(new EmptyBorder(10,10,10,10));
JPanel menu = new JPanel(new GridBagLayout());
JButton playGame = new JButton("Play!");
ActionListener playGameListener = new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
cl.show(gui, "game");
}
};
playGame.addActionListener(playGameListener);
Insets margin = new Insets(20, 50, 20, 50);
playGame.setMargin(margin);
menu.add(playGame);
gui.add(menu);
cl.addLayoutComponent(menu, "menu");
final JPanel pipes = new Pipes();
gui.add(pipes);
cl.addLayoutComponent(pipes, "game");
JFrame f = new JFrame("Pipes Game");
f.add(gui);
// Ensures JVM closes after frame(s) closed and
// all non-daemon threads are finished
f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
// See https://stackoverflow.com/a/7143398/418556 for demo.
f.setLocationByPlatform(true);
// ensures the frame is the minimum size it needs to be
// in order display the components within it
f.pack();
// should be done last, to avoid flickering, moving,
// resizing artifacts.
f.setVisible(true);
/*if (playerIsReady) {
Timer speed = new Timer(10, new ActionListener() { //pipe speed
#Override
public void actionPerformed(ActionEvent e) {
pipes.move();
}
});
speed.start();
Timer refresh = new Timer(30, new ActionListener() { //refresh rate
#Override
public void actionPerformed(ActionEvent e) {
pipes.repaint();
}
});
refresh.start();
}*/
}
};
// Swing GUIs should be created and updated on the EDT
// http://docs.oracle.com/javase/tutorial/uiswing/concurrency
SwingUtilities.invokeLater(r);
}
}
Pipes.java
// What import(s) do I need for ArrayList?
public class Pipes {
List<Pipe> pipes = new ArrayList<Pipe>();
public Pipes() {
pipes.add(new Pipe(50, 100));
pipes.add(new Pipe(150, 100));
pipes.add(new Pipe(250, 100));
}
protected void paintComponent(Graphics g) {
super.paintComponent(g);
for ( Pipe pipe : pipes ){
pipe.drawPipe(g);
}
}
}
PipeObject.java
import java.awt.Graphics;
public class PipeObject {
//Declare and initialiaze variables
int x1 = 754; //xVal start
int x2 = 75; //pipe width
//total width is 83
int y1 = -1; //yVal start
int y2 = setHeightVal(); //pipe height
int gap = 130; //gap height
public void drawPipe(Graphics g) {
g.clearRect(0,0,750,500); //Clear screen
g.drawRect(x1,y1,x2,y2); //Draw part 1
g.drawRect(x1-3,y2-1,x2+6,25); //Draw part 2
g.drawRect(x1-3,y2+25+gap,x2+6,25); //Draw part 3
g.drawRect(x1,y2+25+gap+25,x2,500-y2-49-gap); //Draw part 4
}
public void move() {
x1--;
}
public int getMyX() { //To determine where the pipe is horizontally
return x1-3;
}
public int getMyY() { //To determine where the pipe is vertically
return y2+25;
}
public int setHeightVal() { //Get a random number and select a preset height
int num = (int)(9*Math.random() + 1);
int val = 0;
if (num == 9)
{
val = 295;
}
else if (num == 8)
{
val = 246;
}
else if (num == 7)
{
val = 216;
}
else if (num == 6)
{
val = 185;
}
else if (num == 5)
{
val = 156;
}
else if (num == 4)
{
val = 125;
}
else if (num == 3)
{
val = 96;
}
else if (num == 2)
{
val = 66;
}
else
{
val = 25;
}
return val;
}
}

The best way to approach this is using a CardLayout.
Notes
A button with an ActionListener is far better than a MouseListener over a rectangle.
The button will show focus when the mouse is pointed at it, or the component is tabbed to via the keyboard.
The button is keyboard accessible.
The button has facility to support multiple icons built in (e.g. for 'initial look', focused, pressed etc.)
White space in the GUI is provided around the menu panel and game by adding an EmptyBorder
The button is made larger by setting a margin.
Adjust margins, borders and preferred size according to need. These sizes were set by me so as not to make the screenshots too large.
See more tips in the code comments.
Code
Here is the MCTaRE (Minimal Complete Tested and Readable Example) that produced the above screenshots.
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.border.EmptyBorder;
public class PipesGame {
public static void main(String[] args) {
Runnable r = new Runnable() {
#Override
public void run() {
// the GUI as seen by the user (without frame)
final CardLayout cl = new CardLayout();
final JPanel gui = new JPanel(cl);
// remove if no border is needed
gui.setBorder(new EmptyBorder(10,10,10,10));
JPanel menu = new JPanel(new GridBagLayout());
JButton playGame = new JButton("Play!");
ActionListener playGameListener = new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
cl.show(gui, "game");
}
};
playGame.addActionListener(playGameListener);
Insets margin = new Insets(20, 50, 20, 50);
playGame.setMargin(margin);
menu.add(playGame);
gui.add(menu);
cl.addLayoutComponent(menu, "menu");
JPanel pipes = new Pipes();
gui.add(pipes);
cl.addLayoutComponent(pipes, "game");
JFrame f = new JFrame("Pipes Game");
f.add(gui);
// Ensures JVM closes after frame(s) closed and
// all non-daemon threads are finished
f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
// See https://stackoverflow.com/a/7143398/418556 for demo.
f.setLocationByPlatform(true);
// ensures the frame is the minimum size it needs to be
// in order display the components within it
f.pack();
// should be done last, to avoid flickering, moving,
// resizing artifacts.
f.setVisible(true);
}
};
// Swing GUIs should be created and updated on the EDT
// http://docs.oracle.com/javase/tutorial/uiswing/concurrency
SwingUtilities.invokeLater(r);
}
}
class Pipes extends JPanel {
Pipes() {
setBackground(Color.BLACK);
setForeground(Color.WHITE);
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawString("Pipes game appears here..", 170, 80);
}
#Override
public Dimension getPreferredSize() {
// adjust to need
return new Dimension(500,150);
}
}

"Is there a way for me to add my GameMenu jpanel to my jframe, and then replace it with the Pipes jpanel?"
As other have suggested, for this you want a CardLayout. It is very simple to you. Personally, I always wrap my CardLayout in a JPanel rather than the JFrame, just force of habit.
What you want to do is have a mainPanel that will have the CardLayout
CardLayout card = new CardLayout();
JPanel mainPanel = new JPanel();
Then you want to add your panels to the mainPanel. What the CardLyaout does is layer the panels, making just one visible at a time. The first one you add, will the in the foreground. Also when you add the panel, you'll also want to issue it a key it can be called from. The key, can be any String you like.
mainPanel.add(gameMenu, "menu");
mainPnael.add(pipes, "pipe");
Now gameMenu is the only panel shown. To show pipes, all you do is use this method
public void show(Container parent, String name) - Flips to the parent that was added to this layout with the specified name, using addLayoutComponent. If no such component exists, then nothing happens.
So you'd use, card.show(mainPanel, "pipes");
Whatever even you want to trigger the showing of pipes, just add that line in that event handler. You could add a button or something to the GameMenu that will allow movement to the Pipes panel.

This works with a mouse click on the menu. You can change it later, to a click on some button or whatever you want.
I added a MouseListener to the Game class. When the user presses the mouse on the menu JPanel, it adds the Pipes JPanel to JFrame and calls the pack method.
Game.java:
import java.awt.Point;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.Rectangle2D;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.Timer;
public class Game {
GameMenu menu = new GameMenu();
Pipes game;
boolean start = false;
JFrame f;
Rectangle2D menuRect = new Rectangle2D.Double(20, 20, 60, 40);
public Game() {
f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.add(menu);
f.setTitle("Pipe Game");
f.setResizable(false);
f.pack();
f.setLocationRelativeTo(null);
f.setVisible(true);
menu.addMouseListener(new MouseAdapter() {
#Override
public void mousePressed(MouseEvent e) {
Point click = new Point(e.getX(), e.getY());
System.out.println("Clicked on the Panel");
if(menuRect.contains(click))
{
System.out.println("Clicked inside the Rectangle.");
start = true;
menu.setVisible(false);
game = new Pipes();
f.add(game);
f.pack();
Timer timer = new Timer(10, new ActionListener() { //pipe speed
#Override
public void actionPerformed(ActionEvent e) {
game.move();
}
});
timer.start();
Timer refresh = new Timer(30, new ActionListener() { //refresh rate
#Override
public void actionPerformed(ActionEvent e) {
game.repaint();
}
});
refresh.start();
}
}
#Override
public void mouseReleased(MouseEvent e) {
}
});
}
public static void main(String args[]) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new Game();
}
});
}
}

Why won't my content pane show up on my JFrame?

This is my graphics class, but when I create and add a JPanel as a content pane to it nothing shows up. I have done many tests to see if my content pane is visible but it still will not show.
import java.awt.Color;
import java.awt.Font;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class GraphicsMain extends JFrame{
public static final long serialVersionUID = 7610350056926018727L;
static GraphicsMain frame = new GraphicsMain();
static final int WIDTH = 1024, HEIGHT = 768;
static Listener listener = new Listener();
public static void init() {
createGUI();
}
public static void createGUI() {
frame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
frame.setFocusable(true);
frame.setLayout(null);
frame.setResizable(false);
frame.setSize(WIDTH, HEIGHT);
frame.setTitle("Game of Life");
frame.setContentPane(frame.createMainPanel());
frame.setVisible(true);
}
public JPanel createMainPanel() {
JPanel totalGUI = new JPanel();
totalGUI.setSize(HEIGHT, WIDTH);
totalGUI.setBackground(Color.red);
totalGUI.setLayout(null);
JPanel buttonPanel = createButtonPanel();
totalGUI.add(buttonPanel);
totalGUI.setVisible(true);
System.out.println("Is returning!");
return totalGUI;
}
public JPanel createButtonPanel() {
JPanel buttonPanel = new JPanel();
buttonPanel.setSize(WIDTH, HEIGHT);
buttonPanel.setLocation(0, 0);
Font buttonFont = new Font("Button Font", Font.PLAIN, 12);
JButton goButton = createButton("Go", WIDTH/16, HEIGHT/12, 10, 10, listener, buttonFont, Color.black);
buttonPanel.add(goButton);
JButton clearButton = createButton("Clear", WIDTH/16, HEIGHT/12, 10 + HEIGHT/12, 10, listener, buttonFont, Color.black);
buttonPanel.add(clearButton);
JButton exitButton = createButton("Exit", WIDTH/16, HEIGHT/12, 10 + 2*HEIGHT/12, 10, listener, buttonFont, Color.black);
buttonPanel.add(exitButton);
return buttonPanel;
}
public JButton createButton(String text, int width, int height, int x, int y, ActionListener listener, Font font, Color color) {
JButton button = new JButton(text);
button.setSize(width, height);
button.setLocation(x, y);
button.addActionListener(listener);
button.setFont(font);
button.setForeground(Color.red);
return button;
}
}
this class is called by
import javax.swing.SwingUtilities;
public class GameOfLifeMain {
static boolean running = true;
static int UPS = 60;
static GameOfLifeMain main = new GameOfLifeMain();
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
main.init();
long startTime = System.nanoTime();
double ns = 1000000000.0 / UPS;
double delta = 0;
long secondTimer = System.nanoTime();
while(running) {
long now = System.nanoTime();
delta += (now - startTime) / ns;
startTime = now;
while(delta >= 1) {
main.update();
delta--;
}
main.render();
if(System.nanoTime() - secondTimer > 1000000000) {
secondTimer += 1000000000;
}
}
}
});
}
public void init() {
GraphicsMain.init();
}
public void update() {
}
public void render() {
//GraphicsMain.render();
}
}

Your problems are (at least) two fold:
As #Radiodef astutely notes, you're calling a long-running loop on the Swing event thread. Since this thread is completely responsible for the rendering of your GUI and the interacting with the user, your GUI becomes completely frozen.
You are using null layouts which make you completely responsible for the positioning and size of all components added to the null-layout using component.
I suggest:
As #Radiodef suggests, do your long-running loop in a background thread such as a SwingWorker. This will allow easier interaction between your GUI and the background process. His link is a good one: Concurrency in Swing.
Use nested JPanels, each one using its own layout to achieve the layout desired, one that will run well on any platform and on any video card setting.
Better to override your drawing JPanels getPreferredSize(...) method to have it sized correctly.
Setting a JPanel to opaque via setOpaque(true) will not help since JPanels are already opaque by default.
Edit
I stand corrected on the last point per Jan Bodnar:
"However, the default value for this property on most standard JComponent subclasses (such as JButton and JTree) is look-and-feel dependent." For example, the JPanel of the GTK look and feel is not opaque by default.
Thanks, Jan

Call setOpaque(true) on your Panel, then the background will be painted. I also noticed that you mixed up WIDTH and HEIGHT when defining the size of your panel (this might be an error).

Java - repainting JPanel gives an error

I'm a beginner in Java, and I'm trying to create an application that draws a rectangle where ever the cursor is located. I've already done everything, but I can't get the mouseMoved(MouseEvent) method to repaint the JPanel. Without the repaint, the rectangle is only drawn once and that's it. With the repaint, it compiles fine, but when I run it, every time the mouse is moved, I get this big "Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException" error.
So, can anyone please help me out on this?
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.event.*;
public class Game extends JPanel implements MouseMotionListener
{
public static void main(String[] args) {
new Game().game();
}
JPanel panel;
JButton button2;
JButton button;
public void game() {
JPanel panel = new Game();
button = new JButton("Ok");
panel.setLayout(new FlowLayout());
panel.add(button);
button2 = new JButton("Cancel");
JFrame frame = new JFrame("Game");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(500,500);
frame.setResizable(false);
frame.add(panel);
frame.setVisible(true);
panel.addMouseMotionListener(this);
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
PointerInfo a = MouseInfo.getPointerInfo();
Point b = a.getLocation();
int x = (int) b.getX();
int y = (int) b.getY();
g.fillRect(x,y,100,100);
}
public void mouseMoved(MouseEvent evt) {
panel.repaint; //This is the line of code that I need help with. Thanks!
}
public void mouseDragged(MouseEvent evt) {}
}

Hopefully the comments in the code example, be able to tell what you doing wrong in your code :-), otherwise there is always a reason to put forth your doubts...
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Game extends JPanel {
/*
* Now x and y are instance variables,
* whose values you can change at each
* MouseMove Event, and call repaint()
* to see the effects
*/
private int x;
private int y;
private MouseAdapter mouseActions =
new MouseAdapter() {
#Override
public void mouseMoved(MouseEvent me) {
/*
* Now as the Mouse moves, we simply
* updating the instance variables,
* i.e. x and y to the new values
* of the Mouse Location and calling
* repaint() to draw the rectangle.
* Since this class (Game) extends JPanel,
* hence all the functions of the JPanel
* belongs to this class, hence like
* as we call any other method of this
* class, without using the object,
* we can call repaint, likewise.
*/
x = me.getX();
y = me.getY();
repaint();
}
};
/*
* This JPanel panel is unnecessary in
* this case, since the class itself
* extends JPanel, hence you can use
* this (keyword) to access the instance
*/
//JPanel panel;
// Not needed for this case.
//JButton button2;
//JButton button;
public void game() {
JFrame frame = new JFrame("Game");
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.setResizable(false);
addMouseMotionListener(mouseActions);
/*
* Here this means the instance
* of the current class
*/
frame.add(this);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
/*
* While overriding methods of the
* super class, try to keep the
* ACCESS SPECIFIER, as close to
* the original thingy as possible
* In this case, it's protected
* and not public
*/
#Override
protected void paintComponent(Graphics g) {
/*
* Do not perform calculation in this method
* atleast.
*/
super.paintComponent(g);
g.fillRect(x, y, 100, 100);
}
public static void main(String[] args) {
Runnable runnable = new Runnable() {
#Override
public void run() {
new Game().game();
}
};
EventQueue.invokeLater(runnable);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(500, 500);
}
}

Change this :
public void game() {
JPanel panel = new Game();
to this :
public void game() {
panel = new Game();
You are just creating a local variable in the first case. To fix this you need to instantiate the class variable.