Question

How can I create file.txt and insert data on file with content of some of variable on my code for example : population [][]; on Android, so there will be folder files on our package in file explorer (data/data/ourpackage/files/ourfiles.txt) Thank You

Answer 1

Using this code you can write to a text file in the SDCard.
Along with it, you need to set a permission in the Android Manifest.

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

This is the code :

public void generateNoteOnSD(Context context, String sFileName, String sBody) {
    try {
        File root = new File(Environment.getExternalStorageDirectory(), "Notes");
        if (!root.exists()) {
            root.mkdirs();
        }
        File gpxfile = new File(root, sFileName);
        FileWriter writer = new FileWriter(gpxfile);
        writer.append(sBody);
        writer.flush();
        writer.close();
        Toast.makeText(context, "Saved", Toast.LENGTH_SHORT).show();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Before writing files you must also check whether your SDCard is mounted & the external storage state is writable.

Environment.getExternalStorageState()