public class MainActivity extends AppCompatActivity {

    private DatabaseReference mFirebaseDatabase;
    private FirebaseDatabase mFirebaseInstance;

    //Create an object of DatabaseReference to create second table
    private DatabaseReference mFirebaseDatabase1;

    private String userId;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mFirebaseInstance = FirebaseDatabase.getInstance();

        // get reference to 'RepositoryName' node
        mFirebaseDatabase = mFirebaseInstance.getReference("RepositoryName");

        // get reference to 'Bookmarks' node
        mFirebaseDatabase1 = mFirebaseInstance.getReference("Bookmarks");

        // Save / update the user
        btnSave.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String name = inputName.getText().toString();
                String email = inputEmail.getText().toString();
                String password = inputPassword.getText().toString();
                String confirmPassword = inputConfirmPassword.getText().toString();

                // Check for already existed userId
                if (TextUtils.isEmpty(userId)) {
                    createUser(name, email, password, confirmPassword);
                }
            }
        });
    }

    /**
     * Creating new user node under 'users'
     */
    private void createUser(String name, String email, String password, String confirmPassword) {
        // TODO
        // In real apps this userId should be fetched
        // by implementing firebase auth
        if (TextUtils.isEmpty(userId)) {
                /*userId store the unique key like KYNGnlMMIf3w11VukqD
                in this key store usename and email as JSON format in firebase cloud database*/

            // "mFirebaseDatabase" is for table1(i.e RepositoryName)
            //userId = mFirebaseDatabase.push().getKey();

            // "mFirebaseDatabase1" is for table2(i.e Bookmarks)
            userId = mFirebaseDatabase1.push().getKey();
        }

        User user = new User(name, email, password, confirmPassword);

        //insert data in firebase database RepositoryName
        mFirebaseDatabase.child(userId).setValue(user);

        //insert data in firebase database Bookmarks
        mFirebaseDatabase1.child(userId).setValue(user);
    }
}