How to append or write data into text file in android?

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.