How to focus view(Navigator or Explorer) by keyboard in Eclipse

You can use <Ctrl><F7>,<Ctrl><Shift><F7> to cycle through the different views.

Via the menus: Window > Preferences, then General -> Keys
(or just Ctrl-Shift-L twice)

Then you can search in the type filter text field for "view: navigator" or target id: navigator

If there's not a key binding already, you can add one in the Binding: text field below.