1. activity_main.xml 코드

<androidx.appcompat.widget.LinearLayoutCompat
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <EditText
        android:id="@+id/et1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:hint="숫자 1" />
    <EditText
        android:id="@+id/et2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:hint="숫자 2" />
    <Button
        android:id="@+id/btnAdd"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:text="더하기" />
    <Button
        android:id="@+id/btnMinus"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:text="빼기" />
    <Button
        android:id="@+id/btnDup"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:text="곱하기" />
    <Button
        android:id="@+id/btnDivide"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:text="나누기" />
    <TextView
        android:id="@+id/tvResult"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="30dp"
        android:textColor="@android:color/holo_red_light"
        android:layout_margin="10dp"
        android:text="계산 결과 : " />

</androidx.appcompat.widget.LinearLayoutCompat>

2. MainActivity.java 코드

public class MainActivity extends AppCompatActivity{

    private EditText et1, et2;
    private Button btnAdd, btnMinus, btnDup, btnDivide;
    private TextView tvResult;
    private String str1, str2;
    private Integer result;

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

        init();
        initLr();
    }

    @Override
    public void init() {
        setTitle("초간단 계산기");
        et1 = findViewById(R.id.et1);
        et2 = findViewById(R.id.et2);
        btnAdd = findViewById(R.id.btnAdd);
        btnMinus = findViewById(R.id.btnMinus);
        btnDup = findViewById(R.id.btnDup);
        btnDivide = findViewById(R.id.btnDivide);
        tvResult = findViewById(R.id.tvResult);
    }

    @Override
    public void initLr() {
        btnAdd.setOnClickListener(v -> {
            str1 = et1.getText().toString();
            str2 = et2.getText().toString();
            result = Integer.parseInt(str1) + Integer.parseInt(str2);
            tvResult.setText("계산 결과 : " + result.toString());
        });
        btnMinus.setOnClickListener(v -> {
            str1 = et1.getText().toString();
            str2 = et2.getText().toString();
            result = Integer.parseInt(str1) - Integer.parseInt(str2);
            tvResult.setText("계산 결과 : " + result.toString());
        });
        btnDup.setOnClickListener(v -> {
            str1 = et1.getText().toString();
            str2 = et2.getText().toString();
            result = Integer.parseInt(str1) * Integer.parseInt(str2);
            tvResult.setText("계산 결과 : " + result.toString());
        });
        btnDivide.setOnClickListener(v -> {
            str1 = et1.getText().toString();
            str2 = et2.getText().toString();
            result = Integer.parseInt(str1) / Integer.parseInt(str2);
            tvResult.setText("계산 결과 : " + result.toString());
        });
    }
}