How can I find text files by file name as a serial number and get the file content in Android?

I created two buttons: "next" and "back" for opened and read text files arranged serially (textfile_1.txt, textfile_2.txt, textfile_3.txt...etc) and show them in the xml Text-view layout.

I tried this method for the next button but it resulted in an error. Can you please assist me in finding a solution?

public void readnextbtn(){
increment++;
TextView textView = (TextView)findViewById(R.id.TextView);
String data = readTextFile(this, R.raw.textfile."+".increment);
textView.setText(data);
}