FirebaseUser mFirebaseUser = mAuth.getCurrentUser();
if(mFirebaseUser != null) {
currentUserID = mFirebaseUser.getUid(); //Do what you need to do with the id
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// write this line under onCreate thanks me later
mAuth = FirebaseAuth.getInstance();
}