Описание тега onpaint
An event that is triggered when a GUI component needs to be repainted
onPaint is the method responsible for performing the paint act triggered from a PaintEvent. This event is triggered when a GUI component needs to be repainted, usually as a result of a change to the underlying data behind the GUI component. onPaint is responsible for the physical display of the component, and in particular, applying the themes and colors appropriate for the application/output device.