presenter.java

     DatabaseReference countRef =  databaseReference.child("users").child(uid);

        countRef.runTransaction(new Transaction.Handler() {
            @Override
            public Transaction.Result doTransaction(MutableData mutableData) {
                User u = mutableData.getValue(User.class);
                if (u == null) {
                    Log.e("NULL","N");
                    return Transaction.success(mutableData);
                }

                if (u.followers.containsKey(currentUser.getUid())) {
                    Log.e("complete","contains");
                    u.follower_count = u.follower_count - 1;
                    u.followers.remove(currentUser.getUid());
                } else {
                    Log.e("complete","!contains");
                    u.follower_count = u.follower_count + 1;
                    u.followers.put(currentUser.getUid(), true);
                }

                mutableData.setValue(u);
                return Transaction.success(mutableData);
            }

            @Override
            public void onComplete(DatabaseError databaseError, boolean b,
                                   DataSnapshot dataSnapshot) {
                // Transaction completed
                Log.e("complete","casc");

            }
        });

User.java

    public String uid;
    public String email;
    public String name;
    public String surname;
    public String interests;
    public String avatar_url;
    public long following_count;
    public long follower_count;
    public Map<String, Boolean> followers = new HashMap<>();
    public Map<String, Boolean> following = new HashMap<>();