Develop Reference

How do I make a sprite move in a custom JPanel?

How do I make a sprite move in a custom JPanel?
I have looked at the similar questions and although one question is similar, it isn't addressing my problem. I have a sprite in a JPanel and I am unable to get it to move. One of the requirements I have to meet for the program is that it must begin moving when a JButton is pressed (Mouse Click). I have the code set-up in a way I believe should work, but it will spit out a long list of errors when I press the button. I'm also required to have the panel be a custom panel class.
What I need to know is this:
Methods (ha) of programming sprite movement.
Continuing to move the sprite without a trail.
Making the sprite bounce off the edges of the panel. Done (Unable to test due to no moving ball)
Here's the code I have (MainClient).
package clientPackage;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Color;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import logicPack.Logic;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class ClientClass
{
Ball mSolo = new Ball();
private JFrame frame;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
ClientClass window = new ClientClass();
window.frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the application.
*/
public ClientClass()
{
initialize();
}
/**
* Initialize the contents of the frame.
*/
Logic Logical;
Graphics g;
private void initialize() {
frame = new JFrame();
frame.setBounds(100, 100, 590, 520);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().setLayout(null);
SpriteField panel = new SpriteField();
panel.addMouseListener(new MouseAdapter()
{
public void mouseClicked(MouseEvent e)
{
/* int tX = e.getX();
Logical.MoveBallX();
int tY = e.getY();
Logical.MoveBallY();
panel.repaint();*/
Logical.MoveBallX();
Logical.MoveBallY();
panel.repaint();
}
});
panel.setForeground(Color.WHITE);
panel.setBackground(Color.GRAY);
panel.setBounds(64, 92, 434, 355);
frame.getContentPane().add(panel);
JButton btnStart = new JButton("Start");
btnStart.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e)
{
Graphics2D g2 = (Graphics2D)g;
mSolo.DrawSprite(g2 , Logical.MoveBallX(), Logical.MoveBallY());
}
});
btnStart.setBounds(64, 13, 174, 60);
frame.getContentPane().add(btnStart);
}
}
And here are my other Classes (Logic)
package logicPack;
import clientPackage.Ball;
public class Logic
{
Ball mSolo;
public int MoveBallX()
{
int NewX = mSolo.xPos + 50;
return NewX;
}
public int MoveBallY()
{
int NewY = mSolo.yPos + 50;
return NewY;
}
//Motion, force, friction and collision GO HERE ONLY
}
SpriteField
package clientPackage;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JPanel;
public class SpriteField extends JPanel
{
Ball mSolo;
SpriteField()
{
mSolo = new Ball();
repaint();
}
public void paintComponent(Graphics g)
{
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
mSolo.DrawSprite(g2 , mSolo.xPos , mSolo.yPos);
}
}
Ball
package clientPackage;
import java.awt.Color;
import java.awt.Graphics2D;
public class Ball
{
Ball()
{
}
public int xPos = 25;
public int yPos = 25;
int diameter = 25;
public void DrawSprite(Graphics2D g2, int xPos, int yPos)
{
g2.setColor(Color.BLACK);
g2.fillOval(xPos - diameter / 2 , yPos - diameter / 2 , diameter , diameter);
}
}
If you do not understand my Java comments, you can just ignore them.
If you need more details to help me, let me know.
EDIT 1:
Andrew, the closest article I could find used arrow keys to move a sprite. The article was "Sprite not moving in JPanel". All the other articles I found either addressed JPanels without sprites, or animating a sprite. However, I need a JButton that is MouseClicked to simply start the movement, and the ball does not change shape or color. I believe I have the collision part working, but I'm unable to test it until the ball starts moving.
EDIT 2:
LuxxMiner, Thanks for the hints. I have refined my collision portion to be a little more accurate using the getHeight and getWidth methods.
EDIT 3:
MadProgrammer, Thanks...? The problem is not the painting of the ball, I cannot get the ball to move in the first place to repaint it. And the example uses arrow keys, not a mouse click or JButton.

First, take a look at Painting in AWT and Swing and Performing Custom Painting to understand how painting works in Swing.
Let's have a look at the code...
You have a Ball class, which has it's own properties, but then your DrawSprite method passes in values which override these properties?
public class Ball {
Ball() {
}
public int xPos = 25;
public int yPos = 25;
int diameter = 25;
public void DrawSprite(Graphics2D g2, int xPos, int yPos) {
g2.setColor(Color.BLACK);
g2.fillOval(xPos - diameter / 2, yPos - diameter / 2, diameter, diameter);
}
}
What's the point of that? The Ball should paint it's own current state. You should get rid of the additional parameters
public class Ball {
Ball() {
}
public int xPos = 25;
public int yPos = 25;
int diameter = 25;
public void DrawSprite(Graphics2D g2) {
g2.setColor(Color.BLACK);
g2.fillOval(xPos - diameter / 2, yPos - diameter / 2, diameter, diameter);
}
}
ClientClass, Logic and SpriteField all have their own Ball references, none of which is shared so if Logic where to update the state of it's Ball, neither ClientClass or SpriteField would actually see those changes.
In reality, only SpriteField needs an instance of Ball, as it's basically the "ball container", it has the information need to determine if the ball moves out of bounds and wants to know when the ball should be repainted, better to isolate the functionality/responsibility for the Ball to SpriteField at this time.
You also need a means to actually move the ball. While you could use other events, I'd be nice if the ball just moved itself, to this end, you can use a Swing Timer, which won't block the Event Dispatching Thread, but which notifies the registered ActionListener within the context of the EDT, making it safe to update the UI from within.
public class SpriteField extends JPanel {
private Ball mSolo;
private Timer timer;
private int xDelta, yDelta;
public SpriteField() {
mSolo = new Ball();
do {
xDelta = (int) ((Math.random() * 8) - 4);
} while (xDelta == 0);
do {
yDelta = (int) ((Math.random() * 8) - 4);
} while (yDelta == 0);
}
public void start() {
if (timer == null || !timer.isRunning()) {
timer = new Timer(40, new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
mSolo.xPos += xDelta;
mSolo.yPos += yDelta;
if (mSolo.xPos - (mSolo.diameter / 2) < 0) {
mSolo.xPos = mSolo.diameter / 2;
xDelta *= -1;
} else if (mSolo.xPos + (mSolo.diameter / 2) > getWidth()) {
mSolo.xPos = getWidth() - (mSolo.diameter / 2);
xDelta *= -1;
}
if (mSolo.yPos - (mSolo.diameter / 2) < 0) {
mSolo.yPos = (mSolo.diameter / 2);
yDelta *= -1;
} else if (mSolo.yPos + (mSolo.diameter / 2) > getHeight()) {
mSolo.yPos = getHeight() - (mSolo.diameter / 2);
yDelta *= -1;
}
repaint();
}
});
timer.start();
}
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
mSolo.DrawSprite(g2);
}
}
Now, all you need to do, is when the "Start" button is clicked, call the start method
public class ClientClass {
private JFrame frame;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
ClientClass window = new ClientClass();
window.frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the application.
*/
public ClientClass() {
initialize();
}
/**
* Initialize the contents of the frame.
*/
// Logic Logical;
private void initialize() {
frame = new JFrame();
frame.setBounds(100, 100, 590, 520);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
SpriteField panel = new SpriteField();
panel.setForeground(Color.WHITE);
panel.setBackground(Color.GRAY);
frame.getContentPane().add(panel);
JButton btnStart = new JButton("Start");
btnStart.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
panel.start();
}
});
frame.getContentPane().add(btnStart, BorderLayout.SOUTH);
}
}

New to java can't figure out how to use paint method

I am trying to learn the paint method and get a ball to move across the frame. here is my code so far. w=.
I currently have two classes One is the main and one for the ball.
this is the main class
import java.awt.;
import javax.swing.;
public class PaintTest extends JPanel {
int x = 0;
int y = 0;
public void moveBall(){
x = x + 1;
y = y + 1;
}
public static void main(String[] args){
JFrame frame = new JFrame();
frame.setSize(500,500);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
Ball ball = new Ball(x,y);
while(true){
ball.moveBall();
repaint();
}
}
protected void paintComponent(Graphics g){
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
g.setColor(Color.magenta);
g.drawLine(0,100,500,100);
g.drawLine(0,101,500,101);
g.drawLine(0,102,500,102);
g.drawLine(0,103,500,103);
g2.fillOval(x,y,35,35);
}
}
and here is the ball class
public class Ball {
int x,y;
public Ball(int x, int y){
this.x = x;
this.y = y;
}
}
now when i compile I get an error saying cannot find symbol ball in class PaintTest even though I am calling it from the class Ball. I am aware of the repaint error as i do not know what to put in front of it.

Draw in a JPanel
In its paintComponent method not in its paint method -- this gives you double buffering.
Call the super's paintComponent method in your override. This allows the JPanel to do housekeeping drawing including erasing the oval image at its old position.
Don't use a while (true) loop as this can cause serious Swing threading issues. Use a Swing Timer instead.
In the Swing Timer, increment your animation variables and then call repaint(). This will tell Swing to repaint the component which will re-draw the oval in the new location.
Don't guess at this stuff as that leads to frustration since Swing graphics coding is a different beast. Instead check the tutorials. You can find links to the Swing tutorials and to other Swing resources here: Swing Info. Also check out Performing Custom Painting with Swing.
Graphics2D goodies: RenderingHints can be used to smooth out your image jaggies.
More Graphics2D goodies: Stroke can be used to draw thicker lines when needed.
For example:
import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.Stroke;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
#SuppressWarnings("serial")
public class PaintTest extends JPanel {
private static final int PREF_W = 600;
private static final int PREF_H = PREF_W;
private static final int TIMER_DELAY = 20;
private static final Stroke STROKE = new BasicStroke(5f);
private int x;
private int y;
public PaintTest() {
new Timer(TIMER_DELAY, new TimerListener()).start();
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
// to smooth graphics
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g2.setColor(Color.magenta);
Stroke initialStroke = g2.getStroke();
g2.setStroke(STROKE);
g.drawLine(0, 100, 500, 100);
g2.setStroke(initialStroke);
g2.fillOval(x, y, 35, 35);
}
#Override
public Dimension getPreferredSize() {
if (isPreferredSizeSet()) {
return super.getPreferredSize();
}
return new Dimension(PREF_W, PREF_H);
}
private class TimerListener implements ActionListener {
#Override
public void actionPerformed(ActionEvent e) {
x++;
y++;
repaint();
}
}
private static void createAndShowGui() {
PaintTest mainPanel = new PaintTest();
JFrame frame = new JFrame("PaintTest");
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.getContentPane().add(mainPanel);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}

You have to put the paintComponent method in a JPanel. You can do it by using something like this.
JPanel panel = new JPanel(){
#Overide
public void paintComponent(Graphics g){
super.paint();
// Draw Stuff Here
}
};
The reason you are not getting the ball to move across the frame is that you are not calling the repaint method. You should do so on a thread.
Thread th = new Thread(new Runnable(){
#Overide
public void run(){
while(frame.isVisible()){
ball.moveBall();
panel.repaint();
try{Thread.sleep(5);}catch(Exception e){e.printStackTrace();}
}
}
});
Also, why are you making ball a instance of the PaintTest class? To get only one frame and ball you would want to add a class named Ball and use that to make an instance:
public class Ball{
int x, y;
public Ball(int x, int y){
this.x = x;
this.y = y;
}
}
That is why you were getting 2 frames.
Then you would want to get rid of the x and y variables in the main class. To make an instance using this class you would do:
Ball ball = new Ball(x, y);
Then to paint the ball in the paintComponent method you would do:
g.fillOval(ball.x, ball.y, 35, 35);

You didn't call the repaint(); method.
You don't need the y + 1 part.
Instead of using the while(true) loop, you should use a for loop.
You didn't call the super.paint() method.
You didn't use any Thread.sleep(), which made the ball move across instantaneously.
Here is the code:
import java.awt.*;
import javax.swing.*;
public class PaintTest extends JFrame {
int x = 8;
int y = 30;
public void moveBall(){
x = x + 1;
//y = y + 1;
try{
Thread.sleep(500);
} catch(InterruptedException e){
}
repaint();
}
public static void main(String[] args){
PaintTest frame1 = new PaintTest();
PaintTest ball = new PaintTest();
for(int i = 0; i<100; i++){
//while(true){
ball.moveBall();
}
}
public PaintTest() {
super("Paint Test");
setSize(500,500);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setVisible(true);
}
public void paint(Graphics g){
Graphics2D g2 = (Graphics2D) g;
super.paint(g);
super.paint(g2);
g.setColor(Color.magenta);
g.drawLine(0,100,500,100);
g.drawLine(0,101,500,101);
g.drawLine(0,102,500,102);
g.drawLine(0,103,500,103);
g.fillOval(x,y,35,35);
}
}
This code will make the ball move across the screen VERY slowly. If you want to speed it up, change the number of miliseconds in the Thread.sleep(miliseconds) part to a smaller number of miliseconds.

timer.schedule() in not working at all (in Java)

I made a simple program in Java which draws a rectangle on a canvas. And then the rectangle starts moving along X-axis, from left to right.
But the timer.schedule() function is not working. Following is the code:-
package firstanimation;
import java.awt.*;
import java.util.Timer;
public class FirstAnimation {
public static void main(String[] args) {
Frame frame = new Frame("SomeRandomName");
frame.setBounds(50, 50, 700, 500);
frame.setBackground(Color.red);
MyCanvas canvas = new MyCanvas();
frame.add(canvas);
frame.setVisible(true);
Graphics graph = frame.getGraphics();
Timer timer = new Timer();
Task task = new Task(canvas, graph);
timer.schedule(task, 1000,1000);
}
}
package firstanimation;
import java.awt.*;
public class MyCanvas extends Canvas{
public int x,y,width,height;
public MyCanvas()
{
x = 0;
y = 0;
width = 50;
height = 50;
}
#Override
public void paint(Graphics g) {
g.setColor(Color.LIGHT_GRAY);
g.fillRect(x, y, width, height);
}
#Override
public void update(Graphics g) {
x+=10;
g.fillRect(x, y, width, height);
}
}
package firstanimation;
import java.util.TimerTask;
import java.awt.Graphics;
public class Task extends TimerTask{
private MyCanvas canvas;
private Graphics graphics;
public Task(MyCanvas can, Graphics g)
{
super();
canvas = can;
graphics = g;
canvas.paint(g);
}
#Override
public void run() {
canvas.update(graphics);
//throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
}
But the strange thing is... Every time i'm maximizing and restoring the frame, the box is moving.
Why is this happening?

"But the strange thing is... Every time i'm maximizing and restoring the frame, the box is moving. Why is this happening?"
Because repaint() is called when you resize which update the graphics, which you should be doing, instead of trying to call paint.
But...
Still many things wrong.
Seeing as this is your first animation (package firstanimation;), let me get you started in the right direction.
Don't use Canvas. Use JPanel or JComponent instead. When you do, don't override paint but paintComponent instead. Also make sure you call super.paintComponent so you aren't let with any paint artifact during the animation.
public class MyCanvas extends JPanel {
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
//painting code
}
}
Never use getGraphics on a component to do any painting.
You shouldn't ever have to explicitly call paint. The paint[Component] method will be implicitly called for you. A simple call to repaint() will repaint the component.
I just realized you're using all AWT components. Don't use them, they're out-dated. Instead use Swing component. The majority of them are just prefixed with a J, like Frame -> JFrame. They are in the javax.swing.* package.
For animation use a javax.swing.Timer. You can see more at How to Use Timers. The basic construct is
Timer ( int delayInMillis, ActionListener listener )
where delayInMillis is the time to delay between ticks(in this case animations) and the ActionListener listens for "ticks". Each tick, the actionPerformed of the ActionListener is called. There, you can put the code to update any variables you use for animation.
I suggest you read the tutorials Performing Custom Painting to see the proper way to paint.
Here's a simple example with all the points above mentioned
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;
public class AnimateBall extends JPanel {
private static final int D_W = 500;
private static final int D_H = 300;
private Ball ball;
public AnimateBall() {
Random rand = new Random();
int randX = rand.nextInt(D_W);
int randY = rand.nextInt(D_H);
ball = new Ball(randX, randY);
Timer timer = new Timer(15, new ActionListener() {
public void actionPerformed(ActionEvent e) {
ball.animate();
repaint();
}
});
timer.start();
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
ball.drawBall(g);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(D_W, D_H);
}
public class Ball {
int x = 0;
int y = 0; // Current ball position
int dx = 4; // Increment on ball's x-coordinate
int dy = 4; // Increment on ball's y-coordinate
int radius = 15; // Ball radius
public Ball(int x, int y) {
this.x = x;
this.y = y;
}
Color color = new Color((int) (Math.random() * 256),
(int) (Math.random() * 256), (int) (Math.random() * 256));
public void drawBall(Graphics g) {
g.setColor(color);
g.fillOval(x - radius, y - radius, radius * 2, radius * 2);
}
public void animate() {
if (x < 0 || x > getWidth()) {
dx = -dx;
}
if (y < 0 || y > getHeight()) {
dy = -dy;
}
// Adjust ball position
x += dx;
y += dy;
}
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
JFrame frame = new JFrame();
frame.add(new AnimateBall());
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
}

How would I insert a string into a rectangle?

This is my code:
package com.Bench3.simplyMining;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Keying extends JPanel {
private static final long serialVersionUID = 1L;
public Rectangle miningRock;
public Rectangle Rocks;
public int mRockW = 150;
public int mRockH = 100;
public long rocks = 0;
public String rockAmount = rocks + " Rocks";
public Keying(Display f, Images i){
miningRock = new Rectangle(0, 658, mRockW, mRockH);
Rocks = new Rectangle(1024, 0, 0, 0);
f.addKeyListener(new KeyAdapter() {
public void keyPressed(KeyEvent e){
}
public void keyReleased(KeyEvent e){
/* if(e.getKeyCode() == KeyEvent.VK_A){
left = false;
} -- Used for setting key combinations and stuff */
}
});
}
public void paintComponent(Graphics g){
if(Main.f.i.imagesLoaded){
super.paintComponent(g);
g.drawImage(Main.f.i.miningRock, miningRock.x, miningRock.y, miningRock.width, miningRock.height, null);
g.drawString(rockAmount, Rocks.x, Rocks.y);
this.setBackground(Color.WHITE); // Sets background color
// g.setColor(Color.WHITE); -- Used for setting colors
repaint();
}
}
}
I've been trying to insert the string "rockAmount" into the rectangle "Rocks" using g.drawString(), but when I try it doesn't output the string inside of the rectangle. What am I doing wrong?
EDIT: Solved, thanks to Paul for providing the answer.

The y position of text represents the base line minus the font ascent. This means that it appears that text is rendered above the y position.
When rendering text, you need to take into consideration the FontMetrics of the text and font.
For example, the following renders the text at the rectangles x/y position on the left and then calculates the x/y position so it can centre the text within the give rectangle.
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class TestTextRect {
public static void main(String[] args) {
new TestTextRect();
}
public TestTextRect() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new TestPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TestPane extends JPanel {
public TestPane() {
}
#Override
public Dimension getPreferredSize() {
return new Dimension(400, 200);
}
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
int mRockW = 150;
int mRockH = 100;
int x = ((getWidth() / 2) - mRockW) / 2;
int y = (getHeight() - mRockH) / 2;
badRect(g2d, x, y, mRockW, mRockH);
x = (getWidth() / 2) + (((getWidth() / 2) - mRockW) / 2);
goodRect(g2d, x, y, mRockW, mRockH);
g2d.dispose();
}
protected void badRect(Graphics2D g2d, int x, int y, int mRockW, int mRockH) {
g2d.drawRect(x, y, mRockW, mRockH);
String text = "rockAmount";
g2d.drawString(text, x, y);
}
protected void goodRect(Graphics2D g2d, int x, int y, int mRockW, int mRockH) {
g2d.drawRect(x, y, mRockW, mRockH);
FontMetrics fm = g2d.getFontMetrics();
String text = "rockAmount";
x = x + ((mRockW - fm.stringWidth(text)) / 2);
y = (y + ((mRockH - fm.getHeight()) / 2)) + fm.getAscent();
g2d.drawString(text, x, y);
}
}
}
Take a look at Working with text APIs for more details
You should avoid calling repaint or any method that might call repaint from within your paintXxx methods. Because of the way painting is scheduled in Swing, this will set up an infinite loop that will eventually consume you CPU cycles and make your application unresponsive...
You should, also, always call super.paintComponent, regardless of what ever else you want to do...
I would also recommend Key bindings over KeyListener as it provides better control over the focus level

You need to add to x and y position 1/2 of the width and height of the rectangle.
g.drawString(rockAmount,Rocks.x+rectangleWidth/2,Rocks.y+rectangleHeight/2);

How to create Graphics object for drawing Polygon?

I need to draw a Polygon - by connecting consecutive points and then connecting the last point to the first.
With this goal I tried to use drawPolygon(xPoints, yPoints, nPoints). To my mind it's much more convenience approach to achieve this aim
But the Graphics class is abstract class and I we can't create instance object and call drawPolygon() method?
Code:
public void draw() {
Graphics g = null;
int xPoints [] = new int[pointsList.size()];
int yPoints [] = new int[pointsList.size()];
int nPoints = pointsList.size();
for (int i = 0; i < pointsList.size(); i++) {
xPoints [i] = (int) pointsList.get(i).getX();
yPoints [i] = (int) pointsList.get(i).getY();
}
g.drawPolygon(xPoints, yPoints, nPoints);
}
Can we circumvent calling this method at any way?
Maybe exist some other ways to achieve this aim?

The reason the developers made Graphics abstract was that a graphics object needs to come from somewhere. For instance a JPanel or JFrame object have a graphics object associated with them since they render viewable areas to the screen. A graphics object is usually assigned with the getGraphics() method. Here is a quick example:
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
public class Polygon extends JFrame {
public static void main(String args[]){
Test a = new Test();
a.drawAPolygon();
}
public Polygon(){
setSize(300,300);
setVisible(true);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
void drawAPolygon(int[] xPoints, int[] yPoints, int numPoints){
Graphics g = getGraphics();
g.drawPolygon(xPoints, yPoints, numPoints);
}
//#override
public void paint(Graphics g){
super.paint(g);
//could also do painting in here.
}
}

I have had the same problem, this was how I circumvented it:
//assuming you are displaying your polygon in a JFrame with a JPanel
public class SomeDrawingFrame extends JPanel{
SomeDrawingFrame(){
}
#Override //JFrame has this method that must be overwritten in order to
display a rendered drawing.
public void paintComponent(Graphics g){
super.paintComponent(g);
Polygon square = new Polygon();
// these points will draw a square
square.addPoint((0, 0)); //use this.getWidth() method if you want to
create based on screen size
square.addPoint((0, 100));
square.addPoint((100, 100));
square.addPoint((100, 0));
int y1Points[] = {0, 0, 100, 100};
g.draw(polygon);
}
}
now just create an instance of this and add it to a JFrame of minimum height and width of 100 each. You can use JFrame's getWidth() method which will return the size of the JFrame and use this to set your points instead (which is better) because then the image will render relative to the size of the frame itself.

Painting is controlled by the RepaintManager. Painting in Swing is done via a series of methods which are called on your behalf when the RepaintManager decides your component needs to be update (you can, of course, request repaints, but the RepaintManager will decided when, what and how much).
In order to perform custom painting in Swing, you need to override one of the methods that are called as part of the paint cycle.
It is recommended that you override paintComponent
You can check out
Performing Custom Painting
Painting in AWT and Swing
For more details.
In your example, your Graphics is null...Graphics g = null; which isn't going to help...
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class SimplePloy {
public static void main(String[] args) {
new SimplePloy();
}
public SimplePloy() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new BorderLayout());
frame.add(new PloyPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class PloyPane extends JPanel {
private int[] xPoints;
private int[] yPoints;
public PloyPane() {
}
#Override
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}
#Override
public void invalidate() {
xPoints = null;
yPoints = null;
super.invalidate();
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
if (xPoints == null || yPoints == null) {
int width = getWidth() - 1;
int height = getHeight() - 1;
int halfWidth = width / 2;
int halfHeight = height / 2;
int innerWidth = width / 8;
int innerHeight = height / 8;
xPoints = new int[9];
yPoints = new int[9];
xPoints[0] = halfWidth;
yPoints[0] = 0;
xPoints[1] = halfWidth - innerWidth;
yPoints[1] = halfHeight - innerHeight;
xPoints[2] = 0;
yPoints[2] = halfHeight;
xPoints[3] = halfWidth - innerWidth;
yPoints[3] = halfHeight + innerHeight;
xPoints[4] = halfWidth;
yPoints[4] = height;
xPoints[5] = halfWidth + innerWidth;
yPoints[5] = halfHeight + innerHeight;
xPoints[6] = width;
yPoints[6] = halfHeight;
xPoints[7] = halfWidth + innerWidth;
yPoints[7] = halfHeight - innerHeight;
xPoints[8] = halfWidth;
yPoints[8] = 0;
}
g2d.drawPolygon(xPoints, yPoints, xPoints.length);
g2d.dispose();
}
}
}