paintComponent

PaintComponent is a method in the Swing library used for custom painting of a component. It is invoked by the Swing painting system whenever a portion of the component needs to be redrawn, such as after a resize, uncovering, or a call to repaint.

In Swing, developers override protected void paintComponent(Graphics g) in a subclass of JComponent (for example, a JPanel) to render the component’s appearance. It is customary to call super.paintComponent(g) at the start to clear the background when the component is opaque. After that, drawing code using the Graphics context determines the component’s visuals, such as shapes, text, and images.

Within paintComponent, the Graphics parameter provides the drawing context. It is common to cast it to Graphics2D for advanced operations, set rendering hints (such as anti-aliasing), and perform the actual rendering. Never dispose of the Graphics object passed to paintComponent; the painting system manages it. Keeping painting code efficient and free of heavy computation helps avoid UI delays.

PaintComponent is part of the painting process triggered by repaint() and the Swing RepaintManager. Swing uses double buffering by default to reduce flicker, so painting should be designed with this in mind. Do not perform long-running tasks inside paintComponent, and avoid allocating unnecessary objects on every repaint.

In practice, developers override paintComponent for custom components and rely on the default behavior of painting backgrounds, borders, and children through the standard Swing painting pipeline. Overriding paint() directly is discouraged in Swing in favor of paintComponent.