Android Unit test simple write/read String file

Question

I spend last few days trying to get a solid grasp of android testing. Its hard to find good explanations/full examples. Could anyone help me with writing simple test to the class below? I know how to make an instrumental test for such class, but I believe that it could be easily tested using Mockito and Junit.

public class Storage {
private final static String FILENAME = "data_storage.txt";

private static boolean isExternalStorageWritable() {
    String state = Environment.getExternalStorageState();
    return Environment.MEDIA_MOUNTED.equals(state);
}

public static String readDataFromFile(File file) {
    FileInputStream fis = null;
    final StringBuilder storedString = new StringBuilder();

    try {
        fis = new FileInputStream(file);
        DataInputStream dataIO = new DataInputStream(fis);
        String strLine = null;

        if ((strLine = dataIO.readLine()) != null) {
            storedString.append(strLine);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }

    return storedString.toString();
}

public static void saveDataToFile(String data, File file) {
    FileOutputStream fileOutput = null;
    try {

        fileOutput = new FileOutputStream(file);
        OutputStreamWriter outputStreamWriter = new OutputStreamWriter(fileOutput);
        outputStreamWriter.write(data);
        outputStreamWriter.flush();
        fileOutput.getFD().sync();
        outputStreamWriter.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}


public static void saveData(Context context, String data) {
    File file = null;
    if (isExternalStorageWritable())
        file = new File(Environment.getExternalStorageDirectory(), FILENAME);
    else
        file = new File(context.getCacheDir(), FILENAME);
    saveDataToFile(data, file);
}

public static String readData(Context context) {
    File file = null;
    if (isExternalStorageWritable())
        file = new File(Environment.getExternalStorageDirectory(), FILENAME);
    else
        file = new File(context.getCacheDir(), FILENAME);
    return readDataFromFile(file);

}
}

here is an example of what I tried to do:

public class StorageTest {
File file = Mockito.mock(File.class);

@Mock Context mockContext;

@Before
public void setup() {
    MockitoAnnotations.initMocks(this);
    when(file.getPath()).thenReturn("/tmp/");

}

@Test
public void shouldWriteData()
{
    assertNotNull(file);
    String data = "mockito data test";
    Storage.saveDataToFile(data,file);

}
}

that line

fileOutput = new FileOutputStream(file);

throws a null pointer exception. How should I mock that kind of code, how should I mock files and input/output streams? IS it possible to test it using Junit, or only instrumental tests?

Cheers guys