Question

Is there a difference between them and are there any conditions in which one should be used instead of the other?

Answer 1

From what I understand from Javadocs, JFrame.add calls the latter. It is a convenience method to get around the incompatibility between AWT’s frame and Swings JFrame.

From javadocs for JFrame:

The JFrame class is slightly incompatible with Frame. Like all other
JFC/Swing top-level containers, a JFrame contains a JRootPane as its
only child. The content pane provided by the root pane should, as a
rule, contain all the non-menu components displayed by the JFrame.
This is different from the AWT Frame case. As a conveniance add and
its variants, remove and setLayout have been overridden to forward to
the contentPane as necessary. This means you can write:

   `frame.add(child);`

And the child will be added to the
contentPane. The content pane will always be non-null. Attempting to
set it to null will cause the JFrame to throw an exception. The
default content pane will have a BorderLayout manager set on it. Refer
to RootPaneContainer for details on adding, removing and setting the
LayoutManager of a JFrame.