drawLine（）方法不绘制任何内容

Question

So I have this class inside a class which is an implementation of JPanel.

    private static class Line extends JComponent {

    private static final long serialVersionUID = 1L;

    @Override
    public void paintComponent(Graphics g) {
        System.out.println("Pozvan paintComponent()");
        g.setColor(Color.YELLOW);
        g.drawLine(20, 20, 100, 20);
        super.paintComponent(g);
    } 
    }

This is a snippet of code which creates a single instance of Line:

        Line line = new Line();
        line.setOpaque(true);
        add(line);

I really don't know what I am doing wrong here. When I draw a rectangle, everything is nicely drawn.

Answer 1

when I set the height to remotely big number it works.

The default size of a Swing component is (0, 0). Since the size is 0, there is nothing to paint.

g.drawLine(20, 20, 100, 20);

Using the above information this means your component needs a size of (120, 40). That is, width = 20 + 100 and height = 20 + 20, in order for the component to be painted.

I added line.setBounds(20, 20, 80, 50); 

Only part of your line will be painted, since you set the width to 80, not 120.

Read the section from the Swing tutorial on Custom Painting for more information and examples.