What is the relation between ContentPane and JPanel?

Question

I found one example in which buttons are added to panels (instances of JPanel) then panels are added to the the containers (instances generated by getCont

Answer 1

It's not two competing mechanisms - a JPanel is a Container (just look at the class hierarchy at the top of the JPanel javadocs). JFrame.getContentPane() just returns a Container to place the Components that you want to display in the JFrame. Internally, it's using a JPanel (by default - you can change this by calling setContentPane()) As for why it's returning a Container instead of a JPanel - it's because you should program to an interface, not an implementation - at that level, all that you need to care about is that you can add Components to something - and even though Container is a class rather than an interface - it provides the interface needed to do exactly that.

As for why both JFrame.add() and JFrame.getContentPane().add() both do the same thing - JFrame.add() is overridden to call JFrame.getContentPane().add(). This wasn't always the case - pre-JDK 1.5 you always had to specify JFrame.getContentPane().add() explicitly and JFrame.add() threw a RuntimeException if you called it, but due to many complaints, this was changed in JDK 1.5 to do what you'd expect.