paintComponent()

Our paintComponent() overrides the method of the same name in the super class of JPanel. Call the super class method super.paintComponent( gr ) before doing custom painting. If you forget to do this the background color will be the default color.

class DemoPanel extends JPanel
{
  . . . 
  
   public void paintComponent ( Graphics gr )
   { 
      int width  = getWidth();
      int height = getHeight();

      super.paintComponent( gr );
      gr.setColor( Color.BLUE );
      gr.drawLine( 0, 0, width-1, height-1 );
   }

}

To draw a diagonal line, the method needs the width and height of the panel. This does not include the width and height of the borders and menu bar of the frame.

getWidth()  — the current width in pixels of this component
getHeight()  — the current height in pixels of this component

The method then sets the pen color to BLUE and draws a diagonal line. The setColor() method of a Graphics object changes the color that is used in drawing. This color will be used for all drawing until is it changed again.