public Dimension getPreferredSize (JComponent c)

Returns the specified component's preferred size appropriate for the look and feel. If null is returned, the preferred size will be calculated by the component's layout manager instead (this is the preferred approach for any component with a specific layout manager installed). The default implementation of this method returns null.

Parameters:
c    the component whose preferred size is being queried; this argument is often ignored, but might be used if the UI object is stateless and shared by multiple components

See also:
javax.swing.JComponent.getPreferredSize, java.awt.LayoutManager.preferredLayoutSize