Draw and redraw on a canvas in swing

Question

I know there's no direct replacement for java.awt.Canvas in swing, and I know I'm supposed to use a JPanel and override paintComponent, for example like so:

public void paintComponent(Graphics g) {
    g.setColor(Color.black);
    g.drawOval(0, 0, 100, 100);
}

And this would draw a black circle on the JPanel when it is created. The problem I have is that I want a dynamic canvas: I want to be able to draw things in response to user input, and redraw continuously, not just once when the application starts. An example would be having a moving object on a canvas, that would need to be redrawn at a rate of say 60 frames per second. How could I achieve this without using AWT components?

EDIT: what I mean is, in an actual canvas, I'd be able to arbitrarily call, say, drawOval anywhere in my code, and that would draw an oval on the canvas; is this doable with JPanel?

For better help sooner, post a [MCVE] or [Short, Self Contained, Correct Example](http://www.sscce.org/). — Andrew Thompson, Dec 26 '17 at 13:23
@AndrewThompson that would apply if I were asking "I'm trying to do X but my code is doing Y and I don't know why", whereas my question is "how do I even begin to do X?" — Bluefire, Dec 27 '17 at 13:01

score 3 · Answer 1 · answered Dec 26 '17 at 13:24

3

Store the information to be drawn (e.g. a Shape or a group of them) and call repaint() from a Swing Timer. Each time the paintComponent(..) method is called, first call the super(..) method to erase the previous drawings, then iterate the list of shapes, move them if necessary, and draw each one.

answered Dec 26 '17 at 13:24

Andrew Thompson

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Thanks! Is there a reason that swing doesn't have Canvas from AWT? It seems like a silly thing to omit... – Bluefire Dec 27 '17 at 13:02

sam46 · Accepted Answer · 2017-12-26T19:07:05.557

2

Here's one way to do it:

public class Renderer extends JComponent implements ActionListener {
    private int x;        

    public Renderer() {
        Timer timer = new Timer(1000/60, this);
        timer.start();
        x = 0;
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paint(g);
        // drawing code
        g.setColor(Color.black);
        g.drawOval(x, 0, 100, 100);
    }

    private void update() {
        this.x++;    
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        update();
        repaint();      
    }

}

Now just add this to your component (JPanel or whatever):

comp.add(new Renderer());

edited Dec 26 '17 at 19:07

answered Dec 26 '17 at 11:42

sam46

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Where is `repaint` defined? Can you give me an example? – Bluefire Dec 26 '17 at 11:56
in JComponent which Renderer inherits from. Example of what? – sam46 Dec 26 '17 at 11:58
An example of, say, painting a moving object, based on external variables such as `x` and `y` co-ordinates – Bluefire Dec 26 '17 at 12:01
Not what the question is asking, but okay. – sam46 Dec 26 '17 at 12:12
`repaint` calls `paint` (among other things), right? – Bluefire Dec 26 '17 at 12:18
Also what I meant was, in an actual canvas, I'd be able to arbitrarily call, say, `drawOval` anywhere in my code, and that would draw an oval on the canvas; is this doable with JPanel? – Bluefire Dec 26 '17 at 12:21
it does call paint() over and over. You want your drawing to happen in paint() – sam46 Dec 26 '17 at 12:37
1

`public void paint(Graphics g)` Don't override the `paint(..)` method for any `JComponent`. It should be `paintComponent(..)` as seen above. – Andrew Thompson Dec 26 '17 at 13:20
Yep my bad, this should be paintComponent() – sam46 Dec 26 '17 at 19:08
@Sam do you mean to call `super.paintComponent(g)` rather than `super.paint(g)`? – Bluefire Dec 27 '17 at 13:25

Draw and redraw on a canvas in swing

2 Answers2