MyMath, Java file

package com.example.mymath;

import androidx.appcompat.app.AppCompatActivity;

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

import java.util.Random;

public class MainActivity extends AppCompatActivity {
    private Random rnd;
    private TextView tvExercise;
    private EditText etAnswer;
    private Button btnCheck;
    private int answer;

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

        tvExercise = findViewById(R.id.tvExercise);
        etAnswer = findViewById(R.id.etAnswer);
        btnCheck = findViewById(R.id.btnCheck);

        rnd = new Random();

        showExercise();
    }

    public void showExercise() {
        int n1, n2;
        int op;
        String operation = "";
        n1 = rnd.nextInt(11);
        n2 = rnd.nextInt(11);

        op = rnd.nextInt(4);
        if (op == 0) {
            // +
            operation = " + ";
            answer = n1 + n2;
        } else if (op == 1) {
            // -
            operation = " - ";
            answer = n1 - n2;
        } else if (op == 2) {
            // *
            operation = " * ";
            answer = n1 * n2;
        } else if (op == 3) {
            // /
            operation = " / ";
            answer = n1 / n2;
        }

        tvExercise.setText(n1 + operation + n2);
        etAnswer.setText("");
    }


    public void checkAnswer(View view) {
        int userAnswer;

        userAnswer = Integer.parseInt(etAnswer.getText().toString());

        if (userAnswer == answer) {
            // right answer
            Toast.makeText(this, "good work! It's the right answer", Toast.LENGTH_LONG).show();
            showExercise();
        } else {
            // wrong answer
            Toast.makeText(this, "OOOh . . .try again", Toast.LENGTH_LONG).show();
        }
    }
}