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 ??

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: