Question

Look at image below and notice JRadioButtonMenuItem do not extends JRadioButton, even not JToggleButton. My guess is that they repeat code. :P Or perhaps they have a intermediary class inside these toggle component?

Answer 1

The thing is that Java does not support multiple inheritance, otherwise you could have considered having JRadioButtonMenuItem extending both JMenuItem and JRadioButton (although it is not clear if it would have been a good implementation choice).

So this means that you have to choose one or the other, in this case, the choice has been made to extend JMenuItem.

Now, if you take a look, you see that JMenuItem and JRadioButton have a common ancestor which is AbstractButton which actually encapsulates a ButtonModel where everything related to the current state of a button is stored (armed, selected, etc…).

In the case of a JRadioButtonMenuItem it uses a ToggleButtonModel and so here you have how the same model is shared by JRadioButton and JRadioButtonMenutItem.

Regarding the view, this is all delegated into the BasicRadioButtonMenuItemUI and especially more into BasicMenuItemUI (which is the parent of BasicRadioButtonMenuItemUI). Eventually it relies on the current Look-and-feel to provide the different icons, styles, borders, fonts, etc…

And this is how the whole thing goes.