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How do I draw something in a JPanel after I already called the paintComponent method

I'm new to making GUIs in Java. As I understand, there's a class called Graphics which is in charge of drawing shapes in a JPanel. When my application starts, I call the paintComponent method, which draws the board of the game I'm programming, and the paintComponent method takes a Graphics g as input. However, later on, I want to update the board, so how do I tell the same g that drew the board at the start of the game to draw something else when the user does something like clicking?
I believe this should have a very simple answer.

Every JComponent (Swing component) has a repaint() method, just call it to tell the DrawingManager to redraw your component.
All your drawing code should be in paintComponent method, that means that you don't draw anything anywhere else (you draw only in the flow of invocation of paintComponent, you can have drawing code structured in methods of course).
This method needs to have access to the state that indicates what and where should be drawn. It is because the OS could request repainting, and then only the painting methods from JComponent are called.
When you invoke repaint() on your JComponent, then in short time the paintComponent() method of the component on which you requested repainting will be called by the drawing thread, and you should draw only in this drawing thread.

Try repaint() or revalidate(), it should work.

I call the paintComponent method
No, you should never call the paintComponent() method directly. Swing will call the method for you when the component needs to be repainted.
I want to update the board
Then you need a "setter" method. Think of other Swing components. They have methods like "setForeground(), setBackground(), setText()" etc.
So if you want to change your component then you need to create appropriate setter method to change the properties of your class. Then in the method you save the property and simply invoke repaint() and Swing will repaint your component. So now your paintComponent() method needs to check the property you just set to do the appropriate painting.
public void setSomeProperty(Obect someProperty)
{
this.someProperty = someProperty;
repaint();
}
....
protected void paintComponent(Graphics g)
{
super.paintComponent();
// paint the board
if (someProperty != null)
// paint the property
}

Force full redraw of a Jpanel Java2D

My question is that i need to make a GUI that keeps updating becuse i get values that can change from a DB and i got some problems in the graphical area.
well im using Drawline and Drawstring from Graphics2D that print values that are found on the database, this strings and lines move and change value, so i need to call repaint(); with a timer to make them apper in the jpanel, the problem is that repaint(); is not removing the old painting in the background before painting, but when i resize all updates perfecly.
i know a way to clear but the background color goes away too so,
There is a way to update the jpanel removing old paintings and keep the deafult background color?
Not updated
After changing a coordenate and a label text to "AXIS Y" (repaint called automatically from a timer)
Thanks.

From the looks of your image, it looks like you're just forgetting to call super.paintComponent in the paintComponent method. What this does is repaint the background for you, so aren't left with the previous paint artifacts.
#Override
protected voud paintComponent(Graphics g) {
super.paintComponent(g);
}
Note: For future reference, though the images gave us a good picture, it always best to post a Minimal, Complete, and Verifiable example along with those images, so we don't have to make guesses (educated or not)

Double buffered image example in Jpanel

I would know if my implementation is correct for double buffered image.. because i note that tremble the borders of my image that i move in the screen ... It is normal??
public void paintComponent(Graphics g) {
Image bufferimage= createImage(180,180);
Graphics dbg= bufferimage.getGraphics();
//clean the screen
dbg.setColor(new Color(100,100,100));
dbg.fillRect(0,0,getWidth(),getHeight());
if (game_is_running) {
// draw various type of object with drawImage
for(int i=0; list[i]!=null; i++) {
list[i].draw(dbg);
}
target.draw(dbg);
I.draw(dbg);
}
//finally draw the image linked to graphics
g.drawImage(bufferimage,0,0,this);
}

Move the creation of the bufferimage out of the paintComponent() method. You don't need to create this every time that method is called. You are drawing the whole surface anyway.
When you are done with the Graphics retrieved from bufferImage (i.e. dbg variable in your case) you are supposed to call dispose() on it.
Finally you might be able to get away without the second image if you ensure that your component and the components that contain it, have the property doubleBufferred set to true.

All the paintComponent() method should do is draw the image.
The "if game is running" code does not belong in the paintComponent() method. The idea is to have a Timer or something that changes the state of your game and does the custom drawing on your image. Then when Swing invokes the paintComponent() method you simple paint the image in its current state.
Take a look at the DrawOnImage example from Custom Painting Approaches. The code adds rectangles to the image by using the mouse. Then whenever the component is repainted the image is painted.
The ideas is to create/modify the image once and then repaint the image. In a game it may turn out that every time you modify the image you also paint it, but the code should not be part of the paintComponent() method.

panel with image background and mouse draw

How can I use a image as background in a JPanel if the paint () method is already used for other purposes? (I'm tried to draw over a image in a panel).
Here is my code to draw as a pencil, but I don´t know how to add the image as background ?
#Override
public void paint(Graphics g) {
if (x >= 0 && y >= 0) {
g.setColor(Color.BLACK);
g.fillRect(x, y, 4, 4);
}
}
Thanks Diego

Suggestions:
Don't draw in the JPanel's paint(...) method but rather use it's paintComponent(...) method. There are several reasons for this, one being that if you use the paint(...) method, then you are also responsible for drawing the JPanel's borders and child components and at risk of messing up the rendering of these guys. Also you lose Swing's automatic double buffering.
First call the parent class's super method before calling any other code in the method. This will allow the JPanel to refresh its background and do any graphics housekeeping that may need to be done.
Next draw your background image using g.drawImage(...),
Then do your pencil drawing.

Hovercraft Full Of Eels gave good advice on one direction to take. Here is another.
Display the image in a (ImageIcon in a) JLabel.
When it comes time to paint:
Call createGraphics() on the BufferedImage to gain a Graphics2D object.
paint the lines or other visual elements to the graphics instance.
dispose of the graphics instance.
Call repaint() on the label.
E.G. as seen in this answer.

How to measure Java2D drawing performance in a JComponent?

I'm trying something along these lines:
private static class TexturePanel extends JPanel {
#Override
protected void paintComponent(Graphics graphics) {
// drawing code
// calc fps
repaint();
}
}
Is calling repaint() in paintComponent() the right approach to this?

How to measure Java2D drawing performance in a JComponent?
A crude measure is to give the repaint Timer an arbitrarily short delay & count FPS.
Is calling repaint() in paintComponent() the right approach to this?
No. No it isn't. paintComponent() is fine, but don't trigger repaint() from within the method. See the Performing Custom Painting Lesson of the Java Tutorial for some tips.
If you cannot get it sorted from that, I suggest you prepare and post an SSCCE of your best effort.