Android : Cache not getting cleared of Android Application programmatically?

In my app because I use Bitmap factory to get bitmap image object from http url lot of data is getting cached. So i wrote following code to clear cache data everytime application loads. Its similar to what we do manually by clicking Clear Cache button in Setting -> Apps -> YOUR_APP.

However following code i wrote is not working. Cache size is as it is.

public void clearApplicationData() {
        File cache = getCacheDir();
        File appDir = new File(cache.getParent());
        if (appDir.exists()) {
            String[] children = appDir.list();
            for (String s : children) {
                if (!s.equals("lib") && !s.equals("shared_prefs")) {
                    deleteDir(new File(appDir, s));
                }
            }
        }
    }

    public static boolean deleteDir(File dir) {
        if (dir != null && dir.isDirectory()) {
            String[] children = dir.list();
            for (int i = 0; i < children.length; i++) {
                boolean success = deleteDir(new File(dir, children[i]));
                if (!success) {
                    return false;
                }
            }
        }

        return dir.delete();
    }

I also added following permission in AndroidManifest.xml of my application:

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

Please help. And how we use Bitmap factory in efficient way to reduce performance and improve efficiency.