JPanel Graphics clearing and repainting?

Question

I have a JPanel with a paintComponent() function. I'll call it once, then when the user clicks a different JButton, I'll set some flag and want to call this function again as it will do something slightly different after the flag is set.

So here's what I'm wondering: how do I clear the existing stuff from paintComponent? And to redraw, do I just call paintComponent again?

Currently I'm trying the following:

flag2 = true;
repaint(); //I expect (want) paintComponent to be called again

In paint component, I do stuff like:

if (flag2==true) {
    g.drawRect(...);
} else {
    g.drawLine(...);
}

But through testing it seems like there is something wrong with what I'm doing.

Thanks for any help.

You're not calling repaint() from within the paintComponent method, are you? Are you sure the repaint() call is associated with the panel you want refreshed? (e.g. myPanel.repaint())? — Amir Afghani, Aug 01 '11 at 18:41

score 15 · Accepted Answer · answered Aug 01 '11 at 19:01

When you change a property of the panel then you need to invoke:

panel.repaint();

to cause the component to be repainted.

Then the first statement in the paintComponent() method should be:

super.paintComponent(g);

This will paint the background so you can now do your custom painting.

If you need more help then post your SSCCE that demonstrates the problem.

mre · Answer 2 · 2016-05-11T00:55:16.533

14

To clear all previously drawn graphics, invoke g.clearRect(0, 0, getWidth(), getHeight()).

edited May 11 '16 at 00:55

answered Aug 01 '11 at 18:58

mre

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score 2 · Answer 3 · answered Aug 01 '11 at 18:46

2

First, why not use an enum instead of a boolean?

enum Enum { 
    RECTANGLE,
    LINE,
    CIRCLE
}

Enum choice = RECTANGLE; //default to RECTANGLE

switch(choice) { 
   // case RECTANGLE, LINE, CIRCLE
}

With regards to your issue, can you answer my comments in your question?

answered Aug 01 '11 at 18:46

Amir Afghani

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The logic I have is fine, I could probably use enum but whatever that's not my problem. My problem is that I need to clear all drawn graphics and then call paintComponent again somehow for this JPanel. – JDS Aug 01 '11 at 18:55
You shouldn't need to clear all the graphics to achieve what you want. – Amir Afghani Aug 01 '11 at 19:32
See camickr's answer, if you aren't doing what he's outlined, focus on that first. – Amir Afghani Aug 01 '11 at 19:33

score 2 · Answer 4 · answered Aug 01 '11 at 20:40

I would suggest calling revalidate(); instead of repaint(). revalidate() needs to be called when changing the size / layout or when you add/remove objects onto your jpanel and will update all of it's children. From what I can tell, you're still using the same paint object tho but changing it's layout.

JPanel Graphics clearing and repainting?

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