What is the relation between ContentPane and JPanel?

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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.

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