Q:

How to choose the name of key in Firebase Realtime Database?

How do I choose the name of the key in Firebase Realtime Database?
this is my database now
The key here is automatically generated (-LiNph8fnLlcDTkLqq_K). How can I choose its name?
FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference myRef = database.getReference();
myRef.child("Data").push().setValue("some value");

A:

push() method will generate a random id. If you want to choose your own id then you can do the following:
FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference myRef = database.getReference();
myRef.child("Data").child("firstData").child("value").setValue("some value");

Then you would have:
Data
  firstData
      value : some value

But it is better if you use push() since you cannot have the same key twice in your database.
Find the docs for saving data here.

