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https://docs.oracle.com/javase/tutorial/uiswing/painting/closer.html

 

The paintComponent method is where all of your custom painting takes place. 

 

 In most cases the object that is actually received by this method will be an instance of java.awt.Graphics2D (a Graphics subclass), which provides support for sophisticated 2D graphics rendering.

 

The invocation of super.paintComponent(g) passes the graphics context off to the component's UI delegate, which paints the panel's background.

 

 Whenever a MouseEvent received, it is forwarded to the moveSquare method, which updates the square's coordinates and repaints the component in an intelligent manner.

 

repaint method. This method is defined by java.awt.Component and is the mechanism that allows you to programatically repaint the surface of any given component. It has a no-arg version (which repaints the entire component)