public abstract void setContentPane (Container contentPane)

The "contentPane" is the primary container for application specific components. Applications should add children to the contentPane, set its layout manager, and so on.

The contentPane may not be null.

Generally implemented with getRootPane().setContentPane(contentPane);

contentPane    the Container to use for the contents of this JRootPane

java.awt.IllegalComponentStateException    (a runtime exception) if the content pane parameter is null

See also:
JRootPane.getContentPane, getContentPane