问如何使用getFilesDir()？

EN

   ContextWrapper c = new ContextWrapper(this);
   String FileDirectory =  c.getFilesDir().getPath();

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getFilesDir()' on a null object reference

public class MainActivity extends AppCompatActivity {
public static final int DIALOG_DOWNLOAD_PROGRESS = 0;
private ProgressDialog mProgressDialog;

ContextWrapper c = new ContextWrapper(this);

File dir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);
String FileDirectory =  c.getFilesDir().getPath();

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