Java Swing: Revalidate() or Repaint()? When Should You Use Each Method?

Java Swing: revalidate() vs. repaint()

When modifying the contents of a JPanel in Swing, you may encounter the dilemma of whether to call revalidate() or repaint() or both. This article examines the differences between these methods and their appropriate usage.

removeAll() and revalidate()

removeAll() removes all components from the JPanel, while revalidate() updates the layout of the panel. Typically, you would call revalidate() after modifying the panel's contents to ensure the new components are properly laid out.

repaint() vs. revalidate()

repaint() updates the graphics of the specified component. Calling it forces the component to redraw itself, making it essential when the component's appearance changes (e.g., due to removing or adding components).

When to use each method

repaint():

• To update the visual appearance of a component (e.g., after changing its color or size)
• If the component is outside a JPanel (panels automatically repaint their children)
• If you need to force an immediate repaint (e.g., after updating a label or thumbnail)

revalidate():

• To relayout a component after modifying its contents (e.g., after removing or adding components)
• When the preferred size of the layout may have changed (e.g., after adding or removing a large component)
• If you need to recalculate the position and size of child components

Best practice

While revalidate() will often force a repaint of child components, it may not repaint the panel itself. Therefore, it is generally recommended to call both repaint() and revalidate() to ensure both the contents and layout are updated as expected.

Alternative approach

Rather than reusing the existing panel, consider creating a new panel with the desired contents and swapping it with the original panel in the parent container. This approach avoids the potential paint issues and ensures a clean and consistent state.

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