class a extends JFrame inherits all the JFrame functionality
you can overwrite functions,...

For example if you want to make a FullScreenFrame you can
extend JFrame and when the JFrame is created you set
initial values and make it fullScreen without knowing how it does it.



JFrame f = new JFrame("test")

can only be modified by calling functions on it.

f.setVisible(true)




Does that awnser your question?