my first application: java file - MainActivity.java

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;
import android.widget.Toast;

import java.util.Random;

public class MainActivity extends AppCompatActivity {
    // 1 - define variables
    TextView tvQuestion, tvScore;
    RadioButton rbtn1, rbtn2, rbtn3;
    RadioGroup rgOptions;
    Button btnCheck, btnStartAgain;

    Random rnd;
    int n1, n2, answer;
    int score;

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

        // 2 - bind each variable to a view from the layout
        tvQuestion = findViewById(R.id.tvQuestion);
        tvScore = findViewById(R.id.tvScore);
        rbtn1 = findViewById(R.id.rbtn1);
        rbtn2 = findViewById(R.id.rbtn2);
        rbtn3 = findViewById(R.id.rbtn3);
        rgOptions = findViewById(R.id.rgOptions);
        btnCheck = findViewById(R.id.btnCheck);
        btnStartAgain = findViewById(R.id.btnStartAgain);

        // 3 - set a listener for each view event
        btnCheck.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                checkAnswer();
            }
        });
        btnStartAgain.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                reStart();
            }
        });
        // 4 - select a new question and update ui
        rnd = new Random();

        reStart();

    }

    private void checkAnswer() {
        int userAnswer;
        switch (rgOptions.getCheckedRadioButtonId()) {
            case R.id.rbtn1:
                userAnswer = Integer.parseInt(rbtn1.getText().toString());
                break;
            case R.id.rbtn2:
                userAnswer = Integer.parseInt(rbtn2.getText().toString());
                break;
            case R.id.rbtn3:
                userAnswer = Integer.parseInt(rbtn3.getText().toString());
                break;
            default:
                userAnswer = -1;
        }

        if (userAnswer == -1) {
            Toast.makeText(MainActivity.this, "choose an answer first!", Toast.LENGTH_SHORT).show();
        } else {
            if (answer == userAnswer) {
                // update and show score
                score++;
                tvScore.setText("Score: " + score);
                Toast.makeText(MainActivity.this, "good work!", Toast.LENGTH_SHORT).show();

                // Show new question
                newQuestion();
            } else {
                // stop currrent game
                btnCheck.setEnabled(false);
                rbtn1.setEnabled(false);
                rbtn2.setEnabled(false);
                rbtn3.setEnabled(false);

                // display restart button
                btnStartAgain.setVisibility(View.VISIBLE);
            }
        }
    }

    private void reStart() {
        btnStartAgain.setVisibility(View.GONE);
        score = 0;
        tvScore.setText("Score: " + score);
        btnCheck.setEnabled(true);
        rbtn1.setEnabled(true);
        rbtn2.setEnabled(true);
        rbtn3.setEnabled(true);
        newQuestion();
    }

    private void newQuestion() {
        n1 = rnd.nextInt(50);
        n2 = rnd.nextInt(50);
        answer = n1 + n2;

        tvQuestion.setText("  " + n1 + " + " + n2 + "  ");

        int opt1, opt2;
        do {
            opt1 = rnd.nextInt(100);
        } while (opt1 == answer);
        do {
            opt2 = rnd.nextInt(100);
        } while (opt2 == answer || opt2 == opt1);

        switch (rnd.nextInt(3)) {
            case 0:
                rbtn1.setText(answer + "");
                rbtn2.setText(opt1 + "");
                rbtn3.setText(opt2 + "");
                break;
            case 1:
                rbtn1.setText(opt1 + "");
                rbtn2.setText(answer + "");
                rbtn3.setText(opt2 + "");
                break;
            case 2:
                rbtn1.setText(opt1 + "");
                rbtn2.setText(opt2 + "");
                rbtn3.setText(answer + "");
                break;
        }

        // clear current selection
        rgOptions.clearCheck();
    }
}