Develop Reference

Prevent JPanel from redrawing background on repaint() calls

Some animations (such as some gif files) only store in each frame the pixels that change from the previous frame. To draw these animations correctly, you have to paint the first frame, then paint the succeeding frames on top of them without erasing the previous frames. JPanels, however, always redraw the background when you call repaint(), erasing the previous frames. Is there a way to prevent a JPanel from redrawing the background? I've been told that calling super.paintComponent(g) in the JPanels paintComponent(Graphics g) method is what redraws the background, but I tried commenting that out and it lead to some strange behaviour, so I'm assuming it does more than just repaint the background.

I'd recommend to build your code upon the already existent code provided by the API instead of messing with it. Just store the image as BufferedImage. This allows you to display it using an ImageIcon, so it's an additional simplification. This allows you to update single pixels without any hassles with the API. If you absolutely insist on excluding the JPanel from the repaint-routine, this question might help.
In general:
Follow the conventional use of the API. If you want to permanently store data of an image, us a BufferedImage. JComponents are supposed to be entirely overridden every time the frame is updated.

Depending on your exact requirement there are two common approaches:
Draw on a BufferedImage and then display the BufferedImage
Keep an ArrayList of Objects you want to paint and paint all the Objects in the list every time paintComponent() is invoked.
Check out Custom Painting Approaches for more information and working examples of each approach.

Does Java repaint the components when resize or mouse over?

I am not asking how to use Java swing nor I am asking for suggestion on using layout managers. I am just curious how Java behaves.
All along it has been a myth and many people speculate that Java automatically repaints the components when you resize the frame OR mouse over the components in the frame.
So my question is: Is it true that Java does the repainting automatically when we carry out one of the above actions?
There has been several post with similar title such as: Java repainting a component at mouse-over.
But no one can give a definite answer whether Java does the repainting automatically upon certain user actions (such as resizing & mouse over).

All along it has been a myth and many people speculate
There is no myth or speculation.
automatically repaints the components when you resize
This makes sense because the layout manager is invoked and the size or location may change which means some components may need to be repainted.
automatically repaints the components when you mouse over the components in the frame.
It depends on the component. If a MouseListener has been added to the component to do special processing (ie. rollover a button) then the component may be repainted, otherwise nothing happens. But there is no default painting unless it has been specifically added as part of the UI for the component.
These question is easily verified. Just override the paintCompent() method of your components to display a message when the component is painted and see what happens.

Animation with JComponents on Top

i wanted to ask, if somebody might have a solution about a problem i face. I am working at an application, which draws an animation - for instance a map with objects moving onto. My problem is, that on top of the drawing, a Jtable, Jlist as well as other Components are also placed.
In my particular example all of those components have been added to the Panel, which holds the map. In result each component gets redrawn as often as good my fps is. Therefore making one of the tables invisible reduces the already high cpu usage of sometimes around 50% to less than 30%.
My question is, how can i avoid calling somewhat static visual contents paintComponent() method, without having the "background" - the map - whited out the menu.
Since the animation redraws permanently the menu is not shown at all, if its separated from the corresponding JPanel.
First thoughts move into following directions:
Clipping - actually not as good as i would like to, since id like to enable moving around the menus.
JLayeredPane - already tried but seemed to turn out, that the paintComponent method of menus still gets called frequently.
JWindow/Internal Frame - had that thought a couple of minutes ago. Having a complete independent container shall be able to handle my regard, or?
I am looking forward, if somebody has an elegant idea, how to fix that and reduce the cpu usage significantly.
Thanks!!
Best regards.

I would create a custom Shape for clip. Use Area class and subtract from the Area all the children components' bounds.

For painting over JComponent(s) placed into JPanel you have look at
JLayer (Java7) based on JXLayer(Java6)
GlassPane, notice all JComponents must be lightweight, otherwise is GlassPane behind heavyweight (J)Components
is possible painting to the JViewport,
EDIT
you have to use Swing Timer for moving with Icon (in the JLabel) placed into JXLayer, GlassPane or JViewport, don't use Runnable#Thread and never, not by using plain Thread.sleep(int)

repaint() method causes a second, non-functional JMenuBar to appear?

I am creating a program in which the user can draw lines and erase them. I am able to create my window and create a functioning JMenuBar. When the drawing capability is not enabled, everything is fine. But when the drawing capability is enabled, a second JMenuBar is painted below the original. This menu bar is not functional, and sometimes when it is drawn, ALL of the graphics on the screen are repainted lower as well. Re-sizing the window in any way remedies these problem.
I would post pictures but, you know, I'm a new user and can't.
I have been able to narrow the problem down to a point in my paint() method. Need to know: When the mouse is dragged, the repaint() method is called. It uses my paint() method which calls the paintObject(Graphics g) method in my Line class. The JMenuBar error is appearing at the END of the paint method in the top class.
public void paint(Graphics g)//paint() in the top class
{
for(int i = 0; i < objs.size(); i++)//Loop that finds all Tool(Line) objects that
((Tools)objs.get(i)).paintObject(g);//have been drawn, and redraws them.
//This part of the code completes successfully with no error.
}//ERROR OCCURS HERE!!!!!!
I have no idea why this is happening. As a side note, there is also a JTextField on the screen that doesn't appear until it is highlighted. Thanks for at least reading this far!

Edit: I'll go on a limb, I'll bet you in fact are drawing directly on the top level window (JFrame or JApplet or JDialog...) and have added the mouse listener to this window as well -- hence the drawing of components translated down a bit.
The solution is to look at the tutorials first before attempting something fairly complex like this. They'll tell you how to draw correctly including the suggestions I've given in my comments, such as drawing in a JComponent or JPanel's paintComponent method and calling the super.paintComponent(g) method as the first method call of your paintComponent override.

Java SWT - Repainting a canvas after a resize

I have a shell that contains a canvas, which in turn draws some text in its PaintListener. The problem is, when I dynamically reduce size of the shell with its setBounds method (this is going to happen every once in a while), the text doesn't get redrawn. In fact, the canvas' PaintListener doesn't get called again.
Is there a way to force repaint? I've tried calling canvas.redraw() but this does not work.

Hmm, it appears that I did not set a layout on the shell. After setting a FillLayout, it works.