Develop Reference

Add JPanel to contentPane within actionListener

I'm trying to add a JPanel to my JFrame within an actionListener method, but it appears only after the second click on the button. This is a portion of my code where panCours is a JPanel and ConstituerData the targeted JFrame :
addCours.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent arg0) {
panCours.setBounds(215, 2, 480, 400);
panCours.setBorder(BorderFactory.createTitledBorder("Saisir les données concernant le cours"));
ConstituerData.this.getContentPane().add(panCours);
}
});
I don't understand why it doesn't appear as soon as I click on the button. Any explanation and help about how to fix this ?

You'll need to add a call to repaint(); (as well as probably revalidate();) to get the JPanel to show immediately. A basic example demonstrating your problem (and the solution) below;
public class Test {
public static void main(String[] args) {
final JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(null);
JButton button = new JButton("Test");
button.setBounds(20, 30, 100, 40);
button.addActionListener(new ActionListener(){
#Override
public void actionPerformed(ActionEvent e) {
JPanel panel = new JPanel();
panel.setBackground(Color.red);
panel.setBounds(215, 2, 480, 480);
frame.add(panel);
frame.revalidate(); // Repaint here!! Removing these calls
frame.repaint(); // demonstrates the problem you are having.
}
});
frame.add(button);
frame.setSize(695, 482);
frame.setVisible(true);
}
}
The above said, (as suggested by others) it's only right that I recommend against the use of a null layout in future. The swing layouts are a little awkward to begin with, but they will help you a great deal in the long run.

the answer can be found in the following snippet:
you need to revalidate() the contentPane, not repaint the frame. you can add any panel you want to the contentpane like this. if you declare contentPane as a private field you dont need your getContentPane() call. contentPane is global so it can be reffered to directly from anywhere within the class.
be careful about NullPointerExeptions which can be thrown if you refer to it before initialising.
public class testframe extends JFrame {
private JPanel contentPane;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
testframe frame = new testframe();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the frame.
*/
public testframe() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
contentPane = new JPanel();
setContentPane(contentPane);
JButton btnNewButton = new JButton("New button");
btnNewButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
JPanel a = new JPanel();
contentPane.add(a);
contentPane.revalidate();
}
});
contentPane.add(btnNewButton);
}
}

Thread not wanting to work

If so someone could explain how to get this to paint and why it isn't painting that would be awesome!!!
public class Main extends JPanel implements Runnable {
public void run() {
System.out.println("g");
repaint();
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawRect(50, 50, 200, 200);
}
public static void main(String[] args) {
JFrame frame = new JFrame("Physics!");
frame.setSize(500, 500);
frame.setBackground(Color.BLUE);
frame.setVisible(true);
Main physics = new Main();
Thread t = new Thread(physics);
t.start();
}
}

You never add physics to the JFrame
Main physics = new Main();
JFrame frame = new JFrame("Physics!");
frame.add(physics);
Side Note
When painting on JPanel override getPreferredSize() so the panel has a preferred size, then you can just pack() the frame, as you should be doing, instead of setting it's size
#Override
public Dimension getPreferredSize() {
return new Dimension(500, 500);
}
...
frame.pack();
// frame.setSize(500, 500);
Also, paintComponent should be protected not public
Also see Initial Threads for running Swing apps on the Event Dispatch Thread

You have to add the JPanel to the JFrame:
JFrame frame = new JFrame("Physics!");
Main physics = new Main();
Thread t = new Thread(physics);
t.start();
frame.setContentPane(physics); // Add it like this
frame.setSize(500, 500);
frame.setBackground(Color.BLUE);
frame.setVisible(true);

Try putting t.join() in the main method after t.start() .. Your main method should wait for the thread to finish.

JFrame and Rectangle

Can anyone tell me why the rectangle is not showing up on the frame?
I only see a button on the frame. Please help.
I tried to using the paint method for drawing the rectangle.
Should I use paintComponent() or just paint()?
public class GUI2 {
public static void main(String[] args) {
JFrame frame = new JFrame("Game");
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(500, 500);
frame.setLocation(500, 200);
JPanel panel = new JPanel();
frame.add(panel);
JButton button = new JButton("YO");
panel.add(button);
button.addActionListener(new Action());
frame.paint(null);
}
public void paint(Graphics g) {
g.drawRect(250, 250, 200, 100);
}
static class Action implements ActionListener {
public void actionPerformed(ActionEvent e) {
}
}
}

You shouldn't have to explicitly call paint
get rid of the paint method
Make an Inner JPanel class
Yes override paintComponent in the JPanel class
Call super.paintComponent in the paintComponent method.
Add the class JPanel to the JFrame
Don't do everything inside the main, as you'll find out, that static will cause a problem for you. Do everything inside a constructor
Run the program from the EDT SwingUtilitites.invokeLater().
Make the button a global variable so it can be accessed from the ActionListener
setVisible should be the last thing you do, after adding all the component.
When adding multiple components the JFrame you will want to use the BorderLayout positions, or set the layout to the JFrame to something else besides BorderLayout
Override getPrefferedSize in your JPanel when painting, so the JPanel has a respected preferred size.
Don't set the size of the JFrame just call pack();
Here is a refactor of your code
Also see Creating GUI with Swing | Graphics2D | Performing Custom Paintin for further details.
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
public class GUI2 {
JButton button = new JButton("YO");
public GUI2() {
button.addActionListener(new Action());
JFrame frame = new JFrame("Game");
frame.add(new DrawPanel(), BorderLayout.CENTER);
frame.add(button, BorderLayout.SOUTH);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
private class DrawPanel extends JPanel {
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawRect(250, 250, 200, 100);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(400, 400);
}
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new GUI2();
}
});
}
static class Action implements ActionListener {
public void actionPerformed(ActionEvent e) {
}
}
}

I can tell you why you'll get a NullPointerException
You've not overridden any paint method of any displayable component
You've not passed a valid Graphics context to you paint method, but I would discourage this any way
You should make use of the #Override annotation which would have prevented the class from compiling. Use it when you think you're overriding a method of a parent class, it will tell you when you're wrong
Start by taking a look at Performing Custom Painting and then take a look at Painting in AWT and Swing for more details about how painting is actually done in Swing

Here is working example of your code:
package test;
import javax.*;
import javax.swing.*;
import java.awt.event.*;
import java.awt.image.ImageObserver;
import java.awt.*;
import java.text.AttributedCharacterIterator;
public class GUI2 extends JPanel{
JButton button;
JFrame frame;
public GUI2() {
button = new JButton("YO");
//panel = new JPanel();
frame = new JFrame();
//panel = new JPanel();
this.add(button);
frame.add(this);
//button.addActionListener(new Action());
// this.paint(null);
frame.setSize(500, 500);
frame.setLocation(500, 200);
frame.setVisible(true);
}
public void paint(Graphics g) {
g.setColor(Color.red);
g.drawRect(250, 250, 200, 100);
}
public static void main(String[] args) {
GUI2 test = new GUI2();
}
}
I have removed some statements but you can add them later

JFrame not displaying contents

I know this question has been asked a lot, but ive read through about 10 different articles, all reccomending to different things such as "frame = this" nad frame.add(d)" Im not sure why, but none of these have been working. I typed something and the program worked fine, except the Jbuttons wouldnt show up until i clicked on the JFrame a few times. After some tweaking of that code, im back to the start. Now i just get a error:
Exception in thread "main" java.lang.NullPointerException
at Guis.Dynamic_JFrame.<init>(Dynamic_JFrame.java:37)
at Guis.Dynamic_JFrame.main(Dynamic_JFrame.java:46)
Heres my code:
public class Dynamic_JFrame extends JFrame{
static JFrame frame;
Graphics g;
Handler handler = new Handler();
JButton red = new JButton();
JButton green = new JButton();
JButton orange = new JButton();
public Dynamic_JFrame(){
red.setText("RED");
green.setText("GREEN");
orange.setText("orange");
add(green);
add(red);
add(orange);
red.addActionListener(handler);
green.addActionListener(handler);
orange.addActionListener(handler);
frame.setVisible(true);
}
public static void main(String[] args){
Dynamic_JFrame d = new Dynamic_JFrame();
frame = new JFrame("Changing colors");
frame.setPreferredSize(new Dimension(500,500));
frame.setMaximumSize(new Dimension(500,500));
frame.setMinimumSize(new Dimension(500,500));
frame.setLayout(new FlowLayout());
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public class Handler implements ActionListener{
public void actionPerformed(ActionEvent e) {
if(e.getSource()==red){
getContentPane().setBackground(Color.RED);
}
if(e.getSource()==green){
getContentPane().setBackground(Color.GREEN);
}
if(e.getSource()==orange){
getContentPane().setBackground(Color.ORANGE);
}
}
}
}
New code, Minor Changes. Program works as intended except for the buttons not updating until i click where they should be:
JFrame frame;
public Dynamic_JFrame(){
frame = new JFrame();
frame = this;
red.setText("RED");
green.setText("GREEN");
frame.add(green);
frame.add(red);
frame.setVisible(true);
}
public static void main(String[] args){
Dynamic_JFrame d = new Dynamic_JFrame();
d.frame.setPreferredSize(new Dimension(500,500));
d.frame.setMaximumSize(new Dimension(500,500));
d.frame.setMinimumSize(new Dimension(500,500));
d.frame.setLocationRelativeTo(null);
d.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
d.frame.setLayout(new FlowLayout());
}

A number of things...
Firstly, Dynamic_JFrame extends from JFrame so I don't know why you've then gone and create another frame...
Secondly, when Dynamic_JFrame calls frame.setVisible in the constructor, frame is null as it has not being initialised.
From my perspective, the simplest solution would be to extend Dynamic_JFrame from something like JPanel instead and simply add it to an instance of JFrame
For example...
public class Dynamic_JFrame extends JPanel {
static JFrame frame;
// Not sure that this is a good idea...
Graphics g;
//...
public Dynamic_JFrame(){
// Don't use this...
//frame.setVisible(true);
}
public static void main(String[] args){
EventQueue.invokeLater(new Runnable() {
public void run() {
Dynamic_JFrame d = new Dynamic_JFrame();
frame = new JFrame("Changing colors");
frame.setLayout(new FlowLayout());
frame.add(d);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
});
}

Java run+repaint = grey window

I'm trying to play around with JFrame/JPanels repaint(); and so forth, but when I start a thread, and call repaint via run while true, it only spits out a line of System.out.println("as"); which I put in place to check if loop was running.
So the question is:
Why is my drawings disappearing when calling repaint in a loop?
(It seems only a JFrame with the canvas_width/height is showing up, no panels etc.)
public static void main(String[] args) {
JFrame frame = new JFrame();
frame.setSize(CANVAS_WIDTH, CANVAS_HEIGHT);
frame.setPreferredSize(new Dimension(CANVAS_WIDTH, CANVAS_HEIGHT));
frame.setVisible(true);
frame.setResizable(false);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel p = new JPanel(new GridLayout());
frame.getContentPane().add(p);
Something s = new Something();
p.add(s);
p.setBackground(Color.black);
frame.pack();
}
And the something class:
public class Something extends JPanel implements Runnable {
public Something(){
Thread t = new Thread();
t.start();
run();
}
#Override
public void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(Color.cyan);
g.fillRect(0, 0, getWidth()-150, getHeight()-100);
g.setColor(Color.BLACK);
g.fillOval(10, 10, 25, 25);
}
#Override
public void run() {
while(true){
repaint();
System.out.println("as");
try {
Thread.sleep(1);
} catch (InterruptedException e){}
}
}
}
Any help regarding the contentpane is appreciated, since, I'm not sure this is done correctly.

Instead of calling Thread.sleep(n) in your Thread, implement a Swing Timer for repeating tasks. That ensures that repaint() is called on the Event Dispatch Thread.
See Concurrency in Swing for more details.
Also, repainting every 1 millisecond is being very optimistic.
Working SSCCE E.G. (Note this version actually changes the co-ords of the resulting paint operations, just so we know something is happening!
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Something extends JPanel {
static final int CANVAS_WIDTH = 400;
static final int CANVAS_HEIGHT = 100;
private int xDelta = 0;
public Something(){
ActionListener animater = new ActionListener() {
#Override
public void actionPerformed(ActionEvent ae) {
repaint();
}
};
Timer t = new Timer(10,animater);
t.start();
}
#Override
public void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(Color.cyan);
g.fillRect(0, 0, getWidth()-(xDelta--), getHeight()-100);
g.setColor(Color.BLACK);
g.fillOval(xDelta, 10, 25, 25);
if (xDelta<0) {
xDelta = CANVAS_WIDTH;
}
}
#Override
public Dimension getPreferredSize() {
return new Dimension(CANVAS_WIDTH, CANVAS_HEIGHT);
}
public static void main(String[] args) {
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel p = new JPanel(new GridLayout());
frame.getContentPane().add(p);
Something s = new Something();
p.add(s);
p.setBackground(Color.black);
frame.pack();
frame.setResizable(false);
frame.setVisible(true);
}
}

Starting a Runnable in a new Thread is done with java.lang.Thread.Thread( Runnable ) constructor.
Calling a method of a GUI component outside the Swing event loop is not a good practice, use a Swing timer instead.
Your code becomes:
public static void main(String[] args) {
...
new Timer( 40, new ActionListener() {
public void actionPerformed(ActionEvent evt) {
repaint
}
}).start();
...
}