要检查用户是否在Firebase实时数据库中存在，然后在用户登录/ Android注册时添加一个用户：

  private void writeNewUser（final String userId，String [] names，String username，String email，String uid）{$ b $ final User user = new User用户名，名字[0]，名字[1]，电子邮件，uid）; 
 myUsers.addListenerForSingleValueEvent（new ValueEventListener（）{
 @Override $ b $ public void onDataChange（DataSnapshot dataSnapshot）{
 if（！dataSnapshot.hasChild（userId））{
 myUsers.child（userId）.setValue（user）; 
} 
} 
 @Override $ b $ public void onCancelled（DatabaseError databaseError）{} 
}）; 




 $ b我从这里和@囚徒的答案的指导。 
I am using Firebase Google authentication for login, and I had a list of data points associated with a user. The user populates their data by creating elements in a logged-in session, and these update a list in Firebase that is stored under their uid in a 'user' reference of the RTDB. 

Key points:

When I log out, the data persists.
When I log back in, the uid is the same in the RTDB
When I log back in, the user-specific list is deleted.

How can I make the data persist in the RTDB?

UserDataListActivity
@Override
public void onClick(View view) {
    switch (view.getId()) {
        case R.id.signout_button:
            FirebaseAuth.getInstance().signOut();
            Log.i(TAG, "User allegedly logged out.");
            Intent backToLogin = new Intent(UserDataListActivity.this, LoginActivity.class);
            startActivity(backToLogin);
            finish();
            break;
    }
}

LoginActivity

EDIT: I was under the impression that the write function would not overwrite the existing data. How can I add to the users reference a specific user's data upon Google login without overwriting whatever information they already have?
private void onAuthSuccess(FirebaseUser user) {
    String username = usernameFromEmail(user.getEmail());
    String[] names = firstAndLastNameFromDisplayName(user.getDisplayName());

    // Write new user
    writeNewUser(user.getUid(), names, username, user.getEmail(), user.getUid());

    // Go to MainActivity
    Intent startMainActivity = new Intent(LoginActivity.this, MainActivity.class);
    startActivity(startMainActivity);
    finish();
}
private String usernameFromEmail(String email) {
    if (email.contains("@")) {
        return email.split("@")[0];
    } else {
        return email;
    }
}
private String[] firstAndLastNameFromDisplayName(String fullName) {
    if (fullName != null) {
        if (fullName.contains(" ")) {
            return new String[]{fullName.split(" ")[0], fullName.split(" ")[1]};
        } else {
            return new String[]{fullName, "emptyLastName"};
        }
    } else {
        return new String[]{"defaultfirst","defaultlast"};
    }
}
private void writeNewUser(String userId, String[] names, String username, String email,String uid) {
    User user = new User(username,names[0],names[1], email, uid);
    myUsers.child(userId).setValue(user);
}

private boolean isEmailValid(String email) {
    return email.contains("@");
}

private boolean isPasswordValid(String password) {
    return (password.length() > 4) && !password.contains("\\");
}



To check if a user exists in Firebase Realtime Database before adding one upon user login/registration in Android:
private void writeNewUser(final String userId, String[] names, String username, String email,String uid) {
    final User user = new User(username,names[0],names[1], email, uid);
    myUsers.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (!dataSnapshot.hasChild(userId)) {
                myUsers.child(userId).setValue(user);
            }
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {}
    });

}

I got the tip from here and the guidance of @***er's answer.