What do you expect from these functions?
If you want to switch interface/implementation sections there is "Toggle section" function (Crtl+Shift+Down by default). As well as icons on the left gutter.
Goto declaration navigates to the location of identifier declaration. Goto implementation searches for implementations of a method, class or interface.
Maybe this is my miss understanding, no argue here. For me function definition and declaration are obvious but nvm.
You have right about "Toggle section", but funny thing. There is shortcut Ctrl+Shift+Down/Up showing in menu, but in my editor this shortcut moving current line up and down. I have to check it, because I am sure, I never change any default shortcuts.