How to create file not in onCreate method?

Question

Java

Write file creation and writing / reading in a separate class and call it in the button event handler. It works if you just write directly to the method. But how to put everything in a class and call a couple of methods from it in onCreate? Gives an error message :(

public class Own extends AppCompatActivity {

    TextView textView;
    Button button;
    private static final String LOG_TAG = "my_tag";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_own);

        textView = (TextView) findViewById(R.id.textView);
        button = (Button) findViewById(R.id.button);

button.setOnClickListener(new View.OnClickListener(){
            public void onClick(View view){

                FileWorker fileWorker = new FileWorker();
                fileWorker.fileProcess();

                textView.setText("Выполнено");
            }
        });

public class FileWorker extends Own {
    private static final String LOG_TAG = "my_tag";
    String string;
    FileWorker(){
        string = "hi";
    }
    void fileProcess() {
        File file = new File("myFile.txt");
        if(file.exists()) {
            Log.d(LOG_TAG, "Файл существует!");
        }
    }
}