NullPointerException in class
<p>I have a class that contains 2 functions:</p>

<pre><code>public class FileHandler extends Activity  {
public void writeToFile(){
    String fileName = "lastDevice.txt";
    try {


        FileOutputStream fos = openFileOutput(fileName, MODE_PRIVATE); //Exception thrown here

        fos.write("some device id".getBytes());
        fos.close();

        Toast.makeText(this, "File updated", Toast.LENGTH_SHORT).show();
    } catch (FileNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}
public String readFromFile(){
    try {
        String fileName = "lastDevice.txt";

        FileInputStream fis = openFileInput(fileName); //Exception thrown here
        InputStreamReader isr = new InputStreamReader(fis);
        BufferedReader br = new BufferedReader(isr);
        String sLine = null;
        String data ="";
        while ((sLine = br.readLine())!=null) {
            data+= sLine;
        }
        return data;
    } catch (FileNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        return "FileNotFoundException";
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        return "IOException";
    } catch (NullPointerException e){
        // TODO Auto-generated catch block
        e.printStackTrace();
        return "Null Pointer Exception";
    }
}
</code></pre>

<p>these functions are called from my main activity as follows:</p>

<pre><code>@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    lvDevices = (ListView)findViewById(R.id.ListViewDevices);
    lastDeviceTxt = (TextView)findViewById(R.id.lastDeviceTxt);

    //get last connected device
    FileHandler fh = new FileHandler();
    String last = fh.readFromFile();

    lastDeviceTxt.setText(last);
}
</code></pre>

<p>but i keep getting a <code>NullPointerException</code> from both functions.<br>
when running the functions from my <code>MainActivity</code> (I copied them to my main activity) they work fine.
What am I doing wrong? (please remember that I'm very new to <code>android</code> development).</p>
