MathApp, PlayActivity.java

package com.example.mathapplication;

import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;

public class PlayActivity extends AppCompatActivity {
    TextView tvNo1, tvOp, tvNo2, tvScore;
    EditText etAnswer;
    ImageView imgImage;
    Exercise ex;

    int countAll = 0, countRight = 0;

    Handler handler = new Handler();

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

        tvNo1 = findViewById(R.id.tvNo1);
        tvNo2 = findViewById(R.id.tvNo2);
        tvOp = findViewById(R.id.tvOp);
        etAnswer = findViewById(R.id.etAnswer);
        imgImage = findViewById(R.id.imgImage);
        tvScore = findViewById(R.id.tvScore);
        showEx();
    }

    private void showEx() {
        ex = new Exercise();
        tvNo1.setText(ex.getN1() + "");
        tvOp.setText(ex.getOp());
        tvNo2.setText(ex.getN2() + "");
        etAnswer.setText("");
        imgImage.setVisibility(View.INVISIBLE);
    }

    public void checkAnswer(View view) {
        imgImage.setVisibility(View.VISIBLE);
        int answer = Integer.parseInt(etAnswer.getText().toString());
        if (answer == ex.getValue()) {
            // right answer
            imgImage.setImageResource(R.drawable.right);
            countAll++;
            countRight++;
            tvScore.setText(countRight + " /" + countAll);


            handler.postDelayed(new Runnable() {
                @Override
                public void run() {
                    showEx();
                }
            }, 3000);

        } else {
            // wrong answer
            imgImage.setImageResource((R.drawable.error));
            countAll++;
            tvScore.setText(countRight + " /" + countAll);
        }


    }
}