Question about graphics in Java?

Question

I am trying to learn how to do graphics in java and I've written a simple program which does not work as I wish. I have one class called MyCanvas which extends JPanel and is the "surface" which I am trying to draw on. I have another class which creates a frame and an instance of MyCanvas and adds that object to the frame. I see the red circle which is drawn when paint is called which happens when the object is added to the frame. I do not see the blue circle when i call the drawOval method from MyCanvas though. The graphics object is defined in MyCanvas though. I have tried using both the update method and the repaint method. Neither work. Any help?

import java.awt.*;
import javax.swing.*;

public class Test {

	JFrame frame;
	MyCanvas canvas;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        new Test();
    }

    public Test(){
    	frame = new JFrame("Test");
    	canvas = new MyCanvas();

		//Sets conditions for frame
    	frame.add(canvas);
    	frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    	frame.setSize(500,400);
    	frame.setLocationRelativeTo(null);
    	frame.setVisible(true);

		//Should draw blue circle on screen
		canvas.drawOval(10,10,100,100);
    }
}

------------------------------------------------------------------------------------------------------
import java.awt.*;
import javax.swing.*;

public class MyCanvas extends JPanel {

	public MyCanvas(){
		super();
	}

	public void drawOval(int x, int y, int width, int height){
		Graphics g = getGraphics();
		g.setColor(Color.blue);
		g.fillOval(x,y,width,height);
		update(g);
	}

    public void paint(Graphics g) {
        g.setColor(Color.red);
        g.fillOval(100, 100, 200, 200);
    }
}

TekuConcept · Accepted Answer

I'm not big in java any more but I have done alot of graphics coding lately: http://fc03.deviantart.net/fs70/i/2011/306/d/d/visual_concept___minecraft_by_tekuconcept-d4ek1ui.png

Here would be my c# version: 
(don't be discouraged by the fact that this is in c# because it is practically the same syntax as java just named differently)
public class MyPanel : System.Windows.Forms.Panel
{
     // generated code
     private System.ComponentModel.Container components = null;
     public Panel()
     {
          InitializeComponent();
     }
     private void InitializeComponent()
     {
          components = new System.ComponentModel.Container();
          // set defaults
          this.Size = new Size(600, 400);
          this.BackColor = Color.White;
          this.Location = Point.Empty; // returns a point set to (0,0)
     }
     protected override void Dispose(bool disposing)
     {
          if(disposing)
          {
               if(components != null)
                    components.Dispose();
          }
          base.Dispose(disposing);
     }

// The actual painting to the panel
     protected override void OnPaint(PaintEventArgs e)
     {
          // the control's custom paint handeling
          e.Graphics.DrawEllipse(new Pen(Color.Black), new Rectangle(100,100,200,200));
          // the control's base paint handeling
          base.OnPaint(e);
     }
}

From what it sounds like, you aren't calling the base's Paint() method; but then again, after reviewing through your code again, I realized you didn't define the size nor location of your canvas. I bet that canvas' size is 0x0. Try including some of these properties into your canvas before adding your canvas to your frame.

Anuk · Answer

im also not a pro on graphics in java, but if i remember correct, any drawing u wanna do should be done inside the paint() method... i just checked with the java api... and what the update() method does is that it also calls paint() just like repaint() does.. so im guessing that ur blue circle gets erased by this second drawing of the red circle... (just my guess though.. i could be wrong)...

Question about graphics in Java?

2 個解答