1. activity_main.xmlコード

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

    <Button
        android:id="@+id/btnNewActivity"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="새 화면 열기" />

</androidx.appcompat.widget.LinearLayoutCompat>

2. activity_second.xmlコード

<androidx.appcompat.widget.LinearLayoutCompat
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/purple_200"
    tools:context=".SecondActivity">

    <Button
        android:id="@+id/btnReturn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="돌아가기" />

</androidx.appcompat.widget.LinearLayoutCompat>

3. MainActivity.JAvaコード

public class MainActivity extends AppCompatActivity {

    private Button btnNewActivity;

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

        init();
        initLr();
    }

    public void init(){
        btnNewActivity = findViewById(R.id.btnNewActivity);
    }

    public void initLr(){
        btnNewActivity.setOnClickListener(v -> {
            Intent intent = new Intent(getApplicationContext(), SecondActivity.class);
            startActivity(intent);
        });
    }
}

4. SecondActivity.JAvaコード

public class SecondActivity extends AppCompatActivity {

    private Button btnReturn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
        init();
        initLr();
    }

    public void init(){
        btnReturn = findViewById(R.id.btnReturn);
    }

    public void initLr(){
        btnReturn.setOnClickListener(v -> {
            finish();
        });
    }
}