java JFrame graphics

Question

I have the following simple code in a JFrame constructor

 super(name);
 setBounds(0,0,1100,750);
 setLayout(null);
 setVisible(true);
 g = this.getGraphics();
 int[] x =new int[]{65, 122, 77, 20, };
 int[] y =new int[]{226, 258, 341, 310};
 g.setColor(Color.RED); 
 g.drawPolygon (x, y, x.length);
 System.out.println(g);

I get the output on console as:

sun.java2d.SunGraphics2D[font=java.awt.Font[family=Dialog,name=Dialog,style=plain,size=12],color=java.awt.Color[r=255,g=0,b=0]]

But no red polygon drawn on JFrame but just the blank JFrame.

Why ??

• FThompson: This is not the correct way to draw on a JFrame. You should subclass JPanel and override paintComponent(Graphics), and do the painting on the graphics object passed there.

Answer 1

• Dont overridepaint(..) in JFrame

• Rather add custom JPanel with overridden paintComponent(Graphics g) to JFrame

• Dont use Null/AbsoluteLayout use an appropriate LayoutManager

• Dont call setBounds(..) on JFrame instance (not that its not allowed but cant see it being relevant in this application)

• Dont forget to use EDT for creating and changing GUI components:

  javax.swing.SwingUtilities.invokeLater(new Runnable() {
   @Override
   public void run() {
   Test test = new Test();
   }
  });
  

you would then do something like this:

public class Test {
 /**
 * Default constructor for Test.class
 */
 public Test() {
 initComponents();
 }
 public static void main(String[] args) {
 /**
 * Create GUI and components on Event-Dispatch-Thread
 */
 javax.swing.SwingUtilities.invokeLater(new Runnable() {
 @Override
 public void run() {
 Test test = new Test();
 }
 });
 }
 /**
 * Initialize GUI and components (including ActionListeners etc)
 */
 private void initComponents() {
 JFrame jFrame = new JFrame();
 jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 jFrame.add(new MyPanel());
 //pack frame (size JFrame to match preferred sizes of added components and set visible
 jFrame.pack();
 jFrame.setVisible(true);
 }
}
class MyPanel extends JPanel {
 @Override
 protected void paintComponent(Graphics g) {
 super.paintComponent(g);
 int[] x = new int[]{65, 122, 77, 20};
 int[] y = new int[]{226, 258, 341, 310};
 g.setColor(Color.RED);
 g.drawPolygon(x, y, x.length);
 }
 //so our panel is the corerct size when pack() is called on Jframe
 @Override
 public Dimension getPreferredSize() {
 return new Dimension(400, 400);
 }
}

which produces:

enter image description here

• MadProgrammer: +1, nice example. Only point of curiosity is the comment about setBounds? What's the difference between that at using setLocation & setSize?
• David Kroukamp: @MadProgrammer +1 thank you... As for setBounds(..) I have no concrete evidence, but I see it as an amalgamation of setSize() and setLocation(). I dont have a problem with setLocation() (besides the fact that the position could be off screen), but setSize() has known issues such as components being hidden due to incorrect size given etc and the same can happen when setting the height and width using setBounds(x,y,width,height). Though I am open for criticism :)
• MadProgrammer: Personally I have no particular issue, pack and setLocationRelativeXxx are generally more useful day to day, IMHO, although its sometimes necessary/desirable to specify the location size directly. It would "discourage" the use of absolute positioning in this manner without good reason, but that's just me. Just wanted to get your perspective ;)
• David Kroukamp: @MadProgrammer, +1 very true, and not a problem, I edited the post to show the change and OP and others can see the comments
• aditya parikh: Hey I am adding the following image in the Jpanel msu-footprints.org/2011/Aditya/map.jpg and then adding polygon int[] x = new int[]{65, 122, 77, 20}; int[] y = new int[]{226, 258, 341, 310}; g.setColor(Color.RED); g.drawPolygon(x, y, x.length); The polygon should coincide with building GER but is a little bit offset. My proff says the co-ordinates are okay. Is there anything wrong with how i put the image position.

Answer 2

You should better override paint(Graphics g) or paintComponent(Graphics g) than the approach you are trying. Add the lines below and remove the lines after setVisible in your code.

public void paint(Graphics g) {
 int[] x =new int[]{65, 122, 77, 20};
 int[] y =new int[]{226, 258, 341, 310};
 g.setColor(Color.RED); 
 g.drawPolygon (x, y, x.length);
}

• mKorbel: and not to use getGraphics(), this method is for Printing or save Graphics to the File
• aditya parikh: But paint method runs at start automatically. I want to programatically decide when to draw.
• Dan D.: Then put a flag and draw the stuff under paint only when the flag decides that.
• MadProgrammer: You really shouldn't override a top level containers paint method. Apart from anything else, it's not double buffered, better to override something that extends from JComponent
• Dan D.: That's true. However, I showed where the wrong part is. Things can definitely be improved from this point.