There are two specific examples in the Android Floating Action Button guidelines that explain how the button can morph into either new buttons, or a toolbar. 


  If the hallmark of the app is adding file types, a floating action
  button can morph into related actions after it is first touched.





  If a floating action button morphs into a toolbar, that toolbar should
  contain related actions. In this example, the button lets the user
  select the media type to add.




After the button has morphed, how does the user revert back to the previous single button mode?

As I understand it, the rest of the UI is not blocked after these buttons have morphed, so it's not clear how the user can hide the new buttons or revert the morphed shape if they decide not to go through with the new action.

Update

This is more commentary than anything else: I just took a look at Drive to find implementation examples and found an Bottom sheet instead of a FAB morph.




Question: Do black holes have charges?
Answer: These are two separate questions. It's better if you don't try to combine two questions into one.

In answer to the first question, yes, a black hole can have a measurable charge. You measure it the same way you'd measure any other charge. This is all purely theoretical, however. Many real-life black holes have been observed and characterized by their masses, but there is no realistic chance that any black hole will ever have enough charge to be measurable.

The way we know that black holes can be charged is that there exist solutions to the Einstein field equations such as the Reissner–Nordström metric, in which a charged test particle experiences a force not experienced by a neutral test particle. The no-hair theorems for black holes explicitly allow electric charge as an observable type of "hair" that it is possible for a black hole to have.

In answer to the second question, Maxwell's equations can be expressed in tensor form, and in this form, they work equally well in general relativity.