Here is demo for creating text file and append text to that file in JAVA. For this in Android you have to ask for WRITE_EXTERNAL_STORAGE permission.

you can check this :

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
    if (ContextCompat.checkSelfPermission(getBaseContext(), Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
        requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, WRITE_STORAGE_INT);
        return;
    }
}
Please handle this request in onRequestPermissionsResult.
 
Finally actual code to write text file in java is : 
try{ 
   FileWriter fileWriter = new FileWriter(Environment.getExternalStorageDirectory().getPath() + "/Android/data/com.StampWallet/" + "SBLog.txt", true); 
   fileWriter.write("Hello");
   fileWrite.close(); 
}catch(IOException e){ 
}
 
If you want to write new line in file then use this :  




if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
    fileWriter.write(System.lineSeparator());
} else {
    fileWriter.write(System.getProperty("line.separator"));
}
 




Thanks.