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about overwritting jpanel paintcomponent and relative position

i am making a custom jpanel acting like a canvas for a game.
i was thinking of making the JPanel have a fixed size and use a LayoutManager to make it align to center.
but now i have the following problem:
if i draw something in my custom JPanel like,
#Override
public synchronized void paintComponent(Graphics g) {
g.fillRect(0, 0, 10, 10);
}
will the coordinates i give to the Graphics object be relative to my JPanel top left corner or relative to the inside of the window top left corner? (aka the rectangle within the decoration)

Here's a somewhat simple example I created for a different question.
Change the location of the rectangles and see where they wind up.
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Rectangle;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class ShapesExample extends JFrame {
private static final long serialVersionUID = 1L;
private List<Rectangle> rectangles;
public ShapesExample() {
rectangles = new ArrayList<>();
rectangles.add(new Rectangle(10, 10, 20, 20));
rectangles.add(new Rectangle(41, 41, 20, 20));
setDefaultCloseOperation(EXIT_ON_CLOSE);
setTitle("Shapes Example");
add(new DrawingPanel());
pack();
setLocationByPlatform(true);
setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new ShapesExample();
}
});
}
class DrawingPanel extends JPanel {
private static final long serialVersionUID = 1L;
public DrawingPanel() {
this.setPreferredSize(new Dimension(300, 300));
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.RED);
for (Rectangle rectangle : rectangles) {
g.drawRect(rectangle.x, rectangle.y,
rectangle.width, rectangle.height);
}
}
}
}

Repaint() does not work after adding more components to JPanel in Swing, Java

I am starting with Java and want to make a simple pong game to get into the ways of displaying stuff in java. I have created 2 classes that both extend JPanel and call the repaint() function every 16.6 milliseconds. I added both to a JPanel which I added to the frame, but only the component I added first displays.
I've tried to revalidate() after repaint() in both classes and made sure that the Timer actually works and that it's actionPerformed() method is actually called.
This is the main method:
public class Main {
public static void main(String[] args){
JFrame frame = new JFrame("Pong");
//...
JPanel mainPanel = new JPanel();
mainPanel.add(new Player());
mainPanel.add(new Ball(frameWidth/2, frameHeight/2 -40));
frame.add(mainPanel);
}
}
This is the important code for the classes (It's basically the same for both of them):
public class Player extends JPanel {
public Player(){
setPreferredSize(new Dimension(Main.frameWidth, Main.frameHeight));
setBackground(Color.BLACK);
new Timer(1000/60, new ActionListener(){
public void actionPerformed(ActionEvent e){
update();
repaint();
}
}).start();
}
//...
public void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(Color.WHITE);
g.fillRect(x, y, width, height);
}
}
(Of coure I left things like #Override or unneseccary functions out to shorten the code)
This code only paints the Player, altough I want it to display all the components of mainPanel.
This is an example that you can (hopefully) run. I had to split it up into 3 files, since I suck at anonymus classes:
Main.java
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Main {
public static void main(String[] args){
JFrame frame = new JFrame("Pong");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(800, 800);
JPanel mainPanel = new JPanel();
mainPanel.add(new Player());
mainPanel.add(new Ball());
frame.add(mainPanel);
frame.setVisible(true);
}
}
///////
Player.java
//////
import javax.swing.JPanel;
import javax.swing.Timer;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Color;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class Player extends JPanel{
private int x = 20, y = 300, width = 20, height = 100;
public Player(){
setPreferredSize(new Dimension(800, 800));
setBackground(Color.BLACK);
new Timer(1000/60, new ActionListener(){
#Override
public void actionPerformed(ActionEvent e){
repaint();
}
}).start();
}
#Override
public void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(Color.WHITE);
g.fillRect(x, y, width, height);
}
}
//////
Ball.java
//////
import javax.swing.JPanel;
import javax.swing.Timer;
import java.awt.Graphics;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class Ball extends JPanel{
private int x = 20, y = 300, width = 20, height = 100;
public Ball(){
setPreferredSize(new Dimension(800, 800));
setBackground(Color.BLACK);
new Timer(1000/60, new ActionListener(){
#Override
public void actionPerformed(ActionEvent e){
repaint();
}
}).start();
}
#Override
public void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(Color.WHITE);
g.fillRect(x, y, width, height);
}
}

In method paintComponent() of class Player, you paint the exact same Rectangle each time. In order to achieve animation, each time you paint the rectangle you need to change either its size or location or both. What do you expect the Player to do? Should it move up and down along the left edge of the window? Have you seen the lesson entitled How to Use Swing Timers which is part of Oracle's Java Tutorial?
EDIT
I see now that Player hides Ball, because of the default layout manager of JPanel. The below code is essentially the code you posted but I set GridLayout as the layout manager for mainPanel. Also, a java source code file may contain more than one class but exactly one class must be public. Hence in the below code only class Main is public.
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
public class Main {
public static void main(String[] args) {
JFrame frame = new JFrame("Pong");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel mainPanel = new JPanel(new GridLayout(0, 2));
mainPanel.add(new Player());
mainPanel.add(new Ball());
frame.add(mainPanel);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
}
class Player extends JPanel {
public Player() {
setBorder(BorderFactory.createLineBorder(Color.RED, 2, false));
setPreferredSize(new Dimension(800, 800));
setBackground(Color.BLACK);
new Timer(1000 / 60, new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
repaint();
}
}).start();
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.WHITE);
g.fillRect(20, 100, 20, 100);
}
}
class Ball extends JPanel {
public Ball() {
setBorder(BorderFactory.createLineBorder(Color.CYAN, 2, false));
setPreferredSize(new Dimension(800, 800));
setBackground(Color.BLACK);
new Timer(1000 / 60, new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
repaint();
}
}).start();
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.WHITE);
g.fillRect(300, 300, 10, 10);
}
}
And here is a screen capture of the GUI...

I just realized that if I extend my window manually, the second JPanel shows up under the one responsible for the Player! This means that I'll need to set the Panels position somehow, right?
#Abra

custom JPanel not updating

the code:
import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class PaintWindow {
private JFrame frame;
private aJPanel panel;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
PaintWindow window = new PaintWindow();
window.frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the application.
*/
public PaintWindow() {
initialize();
}
/**
* Initialize the contents of the frame.
*/
private void initialize() {
frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
panel = new aJPanel();
frame.getContentPane().add(panel, BorderLayout.CENTER);
JButton btnNewButton = new JButton("New button");
btnNewButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
panel.stam();
}
});
frame.getContentPane().add(btnNewButton, BorderLayout.NORTH);
frame.setBounds(100, 100, 450, 300);
frame.setVisible(true);
}
public class aJPanel extends JPanel {
private static final long serialVersionUID = 8874943072526915834L;
private Graphics g;
public aJPanel() {
super();
System.out.println("Constructor");
}
public void paint(Graphics g) {
super.paint(g);
System.out.println("paint");
g.fillRect(10, 10, 10, 10);
this.g = g;
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
System.out.println("paintComponent");
g.fillRect(20, 20, 20, 20);
this.g = g;
}
public void stam() {
System.out.println("stam");
this.g.fillRect(40, 40, 40, 40);
this.repaint();
}
}
}
what I want to do is paint custom shapes (lines, rectangles etc) after the user clicks a button or triggers a mouse event. but calling aJPanel.stam() does not show the rectangle its supposed to. I am fairly new to JPanel. any suggestions?

First of all:
class names should start with a capital letter
class names should be descriptive.
"aJPanel" does not follow either of the above.
but calling aJPanel.stam() does not show the rectangle its supposed to
See Custom Painting Apporoaches for the two common ways to do dynamic painting:
Add objects to a List and iterate the List to paint all the objects
Paint directly to a BufferedImage and then paint the BufferedImage
The examples add the Rectangle by dragging the mouse, but you can easily add Rectangles by just invoking the addRectangle(...) method when you click a button.

paint method draws outside jFrame if first coord 0,0

First coord in this case should be 0,0 and not 8,30. What am i doing wrong(i am using NetBeans)
import java.awt.Color;
import java.awt.Graphics;
public class TEST extends javax.swing.JFrame {
#Override
public void paint(Graphics g){
super.paint(g);
g.setColor(Color.blue);
g.drawRect(8, 30, 200, 200);
repaint();
}}

Add a JPanel to the frame and paint in that. The frame's coordinates include the decorations (title bar, borders, etc.). It would look something like this:
public class Test extends JFrame {
public static void main(String[] args) {
new Test();
}
private Test() {
add(new MyPanel());
setDefaultCloseOperation(EXIT_ON_CLOSE);
setSize(600, 600);
setVisible(true);
}
private class MyPanel extends JPanel {
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.blue);
g.drawRect(8, 30, 200, 200);
}
}
}
Also, don't call repaint(); in paint();. That will cause an infinite loop and will freeze the entire program.

The problem is your paint(..) method is not taking into account the JFrame Insets by calling getInsets which as docs state:
If a border has been set on this component, returns the border's
insets.
this code works fine:
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
public class Test {
public Test() {
createAndShowGui();
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new Test();
}
});
}
private void createAndShowGui() {
JFrame frame = new JFrame() {
#Override
public void paint(Graphics g) {
super.paint(g);
g.setColor(Color.blue);
g.drawRect(0 + getInsets().left, 0 + getInsets().top, 200, 200);
}
};
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);
}
}
however this is not best practice.
Rather add JPanel to JFrame and override paintComponent(Graphics g) of JPanel dont forget call to super.paintComponent(g) as first call in the overridden method and than draw there (dont forget to override getPreferredSize() and return correct Dimensions so the JPanel will fit its drawing/graphic content) this problem will no longer persist as JPanel is added at correct co-ordinates on contentPane like so:
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class Test {
public Test() {
createAndShowGui();
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new Test();
}
});
}
private void createAndShowGui() {
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel panel = new JPanel() {
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
g2d.addRenderingHints(new RenderingHints(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY));
g2d.setColor(Color.blue);
g2d.drawRect(0, 0, 200, 200);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(300, 300);
}
};
frame.add(panel);
frame.pack();
frame.setVisible(true);
}
}
The above includes Graphics2D and RenderingHints i.e anti-aliasing. Just for some better looking drawings :)

How to update java canvas dynamically?

Problem: I am trying to update the canvas with new painting objects based on user action. The canvas dosent get updated.
What i have done: The user interacts with the DnD action,The transferrable object reaches the canvas, Calls an update graphics method created by me. And the method simply uses the aldready created graphics 2d object and draws images using it.I have checkd the DnD action,the object is properly recived at canvas class and i was able to print them out using System.out.println.
A sample code,that has a similar function to that of mine,
Paint class:
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JPanel;
public class PaintPanel extends JPanel{
private Graphics2D drawImage;
public PaintPanel()
{
}
#Override
public void paint(Graphics g) {
drawImage = (Graphics2D) g;
drawImage.setColor(Color.WHITE);
drawImage.fillRect(0, 0, getWidth(), getHeight());
}
public void updateGraphics(int length,int width)
{
drawImage.setColor(Color.black);
drawImage.drawRect(100, 150, length, width);
repaint();
}
}
mainframe class:
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
public class MainPaint extends JFrame{
public MainPaint()
{
setTitle("test paint");
setSize(400,400);
setLayout(new BorderLayout());
final PaintPanel paintPan = new PaintPanel();
JButton testButon = new JButton("Display shape");
add(paintPan,BorderLayout.CENTER);
add(testButon,BorderLayout.PAGE_END);
testButon.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent arg0) {
paintPan.updateGraphics(50,50);
repaint();
}
});
setVisible(true);
}
public static void main(String[] args)
{
new MainPaint();
}
}

Graphics2D drawImage; //source of the problem!
Don't attempt to cache a Graphics (or Graphics2D) instance! Instead:
Add the new objects to a list
Call repaint().
In paintComponent(Graphics) draw the list of objects.
An alternative to that is to use a BufferedImage as the drawing object. See this answer for an example.
Update - SSCCE based on latest code.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class MainPaint extends JFrame {
public MainPaint() {
setTitle("test paint");
setSize(400, 400);
setLayout(new BorderLayout());
final PaintPanel paintPan = new PaintPanel();
JButton testButon = new JButton("Display shape");
add(paintPan, BorderLayout.CENTER);
add(testButon, BorderLayout.PAGE_END);
testButon.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent arg0) {
paintPan.updateGraphics(50, 50);
repaint();
}
});
setVisible(true);
}
public static void main(String[] args) {
new MainPaint();
}
}
class PaintPanel extends JPanel {
private int x, y;
private Color color = null;
public PaintPanel() {
setBackground(Color.ORANGE);
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D drawImage = (Graphics2D) g;
if (color != null) {
drawImage.setColor(color);
drawImage.drawRect(100, 150, x, y);
}
}
public void updateGraphics(int length, int width) {
color = Color.RED;
x = length;
y = width;
repaint();
}
}
Note
There are still a number of things about that code that need changing. I decided to stop at the earliest variant that worked to display the rectangle on button click.

I think you need to call the validate() method.