Develop Reference

paintComponent is not working at all

I'm using paintComponent to make a GUI for a class assignment and it's just not affecting the appearance of the GUI at all. To start, I'm just setting the background to white. The following code works:
import javax.swing.*;
import java.awt.*;
public class PA05a extends JPanel {
public static void main(String[] args) {
JFrame window = new JFrame("MouseDrawDemo");
JPanel content = new JPanel();
content.setBackground(Color.WHITE);
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(400,300);
window.setVisible(true);
}
}
but this does not:
import javax.swing.*;
import java.awt.*;
public class PA05a extends JPanel {
public static void main(String[] args) {
JFrame window = new JFrame("MouseDrawDemo");
JPanel content = new JPanel();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(400,300);
window.setVisible(true);
}
#Override
public void paintComponent(Graphics g) {
//add backdrop
super.paintComponent(g);
g.setColor(Color.WHITE);
g.fillRect(0,0,getWidth(),getHeight());
}
}
I can't just not use paintComponent because I'll later be adding things that will change from frame to frame. Can someone pinpoint where I'm missing something?

JPanel content = new PA05a();
You did not create an object of PA05a. ;)

You just forgot to create your object. Change your code to:
import javax.swing.*;
import java.awt.*;
public class PA05a extends JPanel {
public static void main(String[] args) {
JFrame window = new JFrame("MouseDrawDemo");
JPanel content = new PA05a();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(400,300);
window.setVisible(true);
}
#Override
public void paintComponent(Graphics g) {
//add backdrop
super.paintComponent(g);
g.setColor(Color.WHITE);
g.fillRect(0,0,getWidth(),getHeight());
}
}

Image not showing on the window

I have reduced my codes to such a simple function: to show a picture on a window. But why does the picture not show up however I tried? I create a JFrame, and then created a JPanel which is expected to show the picture. Then add the panel to the frame. By the way, I imported the picture and double clicked it to get the url.
import java.awt.*;
import javax.swing.*;
import com.sun.prism.Graphics;
public class GUI {
JFrame frame=new JFrame("My game");
JPanel gamePanel=new JPanel();
public static void main(String[] args){
GUI gui=new GUI();
gui.go();
}
public void go(){
frame.setSize(300, 400);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Background backPic=new Background();
backPic.setVisible(true);
frame.getContentPane().add(backPic);
JPanel contentPane=(JPanel) frame.getContentPane();
contentPane.setOpaque(false);
frame.setVisible(true);
}
class Background extends JPanel{
public void paintComponent(Graphics g){
ImageIcon backgroundIcon=new ImageIcon("file:///E:/eclipse/EL/backgroundPicture.jpg");
Image backgroundPic=backgroundIcon.getImage();
Graphics2D g2D=(Graphics2D) g;
g2D.drawImage(backgroundPic,0,0,this);
}
}
}

It's because you've imported com.sun.prism.Graphics. It should be java.awt.Graphics.
I would also get rid of the "file:///" bit from your path. And you also probably don't want to be loading the image on each paint event. Here's a better version of the Background class;-
class Background extends JPanel {
Image backgroundPic;
public Background() {
ImageIcon backgroundIcon=new ImageIcon("E:/eclipse/EL/backgroundPicture.jpg");
backgroundPic=backgroundIcon.getImage();
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2D=(Graphics2D) g;
g2D.drawImage(backgroundPic,10,10,this);
}
}

Java: paintComponent() Oval is not appearing in Netbeans

I'm trying to learn how to draw an oval in java but the paintComponent I made is not being called by anything, and attempting to call it only causes more issues.
The program runs successfully but the image I want displayed isn't showing up.
import java.awt.*;
import javax.swing.*;
public class TEST2{
public void paintComponent(Graphics g){
g.drawOval(70, 70, 100, 100);
}
public static void main(String[] args) {
TEST2 gui = new TEST2();
gui.setUpFrame();
}
public void setUpFrame(){
JFrame frame = new JFrame();
frame.setTitle("Images should be in this program");
frame.setSize(600,300);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}

Start by taking a look at Painting in AWT and Swing and Performing Custom Painting
In order to be able to perform custom painting in Swing, you must...
Inherit from a swing based component (like JComponent or JPanel)
You must then override it's paintComponent method and perform you custom painting within this method.
Add this component to something that is displayable (like a JFrame)
You should make sure to call super.paintComponent before doing any custom painting
To ensure that you're not making any (common) mistakes, you should use the #Override annotation
As an example...
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class Test2 extends JPanel {
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawOval(70, 70, 100, 100);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}
public static void main(String[] args) {
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();
frame.setTitle("Images should be in this program");
frame.add(new Test2());
frame.pack();
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
});
}
}

The paintComponent() method is a method that you override and it should be accessed inside a class that extends JPanel. You can create a new class that extends JPanel and override the paintComponent() method to draw your oval. You will also have to add the new JPanel to your JFrame for it to display. I modified your code below it should display the oval now. As Madprogrammer noted you should probably construct your GUI within the context of the edt to avoid concurrency issues but I will omit that for simplicity.
import java.awt.*;
import javax.swing.*;
public class Test {
public static void main(String[] args) {
Test gui = new Test();
gui.setUpFrame();
}
public void setUpFrame() {
JFrame frame = new JFrame();
frame.setTitle("Images should be in this program");
frame.setSize(600, 300);
JPanel oval = new oval();
frame.setContentPane(oval);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public class oval extends JPanel{
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawOval(70, 70, 100, 100);
}
}
}

An ImageIcon not even used is affecting my JFrame/JPanel. Why?

I hope this isn’t a stupid first question; I can’t seem to find an answer anyway.
I have this JFrame constructor where a JPanel is added to the JFrame. The JPanel paints a Rectangle in the JFrame, and that’s fine. However, if I add an ImageIcon object as in the code below (for later use), the rectangle isn’t painted. It does appear if I resize the window though.
One solution is to put the setVisible(true) as the last line, or to instantiate the ImageIcon above the constructor, but I really want to understand this. It doesn’t make sense to me that an object not even used can cause this behaviour. Thanks.
public class AJFrame extends JFrame {
ImageIcon ii;
public AJFrame() {
setSize(400,400);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLocationRelativeTo(null);
setVisible(true);
ImageIcon ii = new ImageIcon("Untitled.png");
JPanel jp = new JPanel() {
#Override
protected void paintComponent(Graphics g) {
g.fillRect(0, 0, 50, 50);
}
};
add(jp);
}
public static void main(String[] args) {
AJFrame jf = new AJFrame();
}
}

All actions within a frame should be done in the EDT (Event Dispatching Thread) of Swing. Therefore the right way to start your frame is
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new AJFrame().setVisible(true);
}
});
So maybe it all comes down to the wrong start of your frame.
The main routine of a Java program is not started within the EDT. All Swing actions that are not within the EDT could produce strange refresh/visibility issues.
Here is the complete sourcecode:
public class AJFrame extends JFrame {
ImageIcon ii;
public AJFrame() {
setSize(400, 400);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLocationRelativeTo(null);
//setVisible(true);
//ImageIcon ii = new ImageIcon("Untitled.png");
JPanel jp = new JPanel() {
#Override
protected void paintComponent(Graphics g) {
g.fillRect(0, 0, 50, 50);
}
};
add(jp);
}
public static void main(String[] args) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new AJFrame().setVisible(true);
}
});
}
}

Issue with this simple code

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class MyDrawPanel extends JPanel{
public void paintComponents(Graphics g){
g.setColor(Color.orange);
g.fillRect(20,50,100,100);
}
public static void main(String[] args){
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(500, 500);
frame.getContentPane().add(paintComponents(g));
frame.setVisible(true);
}
}
I think I should add something arguments in frame.getContentPane().add(paintComponents(g));.
I looked up Graphics class but I'm still struggling with it. What should be the parameter of it?

try this
public class MyDrawPanel extends JPanel{
MyDrawPanel()
{
setOpaque(true);
}
protected void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(Color.orange);
g.fillRect(20,50,100,100);
}
public static void main(String[] args){
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(new MyDrawPanel());
frame.setBounds(10,10,500, 500);
frame.setVisible(true);
}
}

I'm no awt expert, but what I think you want to do is add a Canvas object to your content pane from the JFrame, then paint a Graphics object on it.

Okay, this is what I came up with:
public class MyDrawPanel extends JPanel
{
private static void createAndShowGUI()
{
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(500, 500);
MyDrawPanel panel = new MyDrawPanel();
panel.setOpaque(true);
frame.getContentPane().add(panel);
frame.setVisible(true);
}
#Override
public void paintComponent(Graphics g)
{
super.paintComponent(g);
g.setColor(Color.orange);
g.fillRect(20,50,100,100);
}
public static void main(String[] args)
{
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run()
{
createAndShowGUI();
}
});
}
}
Notes:
There are several problems with your line frame.getContentPane().add(paintComponents(g));. What you said is "add to the content pane the result of calling paintComponents on g. Where did g come from? You can not used a variable until declared. The result of calling paintComponents is void which means the result cannot be used as an argument to a method. Presumably you had compiler errors.
I changed paintComponents to paintComponent. The former is used to control painting of subcomponents and in general should not be overridden.
Swing objects should not be created on the main thread. The details are complicated for a beginner (and described here in detail). Mostly you can just memorize the SwingUtilities.invokeLater pattern used above.