opaque
Sets isOpaque - required for background to actually paint. Requires a JComponent target.
A look and feel writes this flag from contentAreaFilled on every change to that property, so a button holds no flag of its own while the two agree: removing the declaration restores that rule rather than the value read when the declaration attached. A menu item is left out of it - nothing ties its flag to the fill, so what it carries is its own.
Return
this modifier with the opaque flag declared on it.
Parameters
opaque
true promises the component paints every pixel of its bounds, letting Swing skip what is behind it; false lets the parent show through.