How to create folder in sdcard in android

I want to make folders in my sdcard and I have used the code below:

public class Screen extends Activity {

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

        operateOnFirstUsage();
    }

    private void operateOnFirstUsage() {

        String state = Environment.getExternalStorageState();
        Log.d("Media State", state);

        if (Environment.MEDIA_MOUNTED.equals(state)) {
            File appDirectory = new File(
                    Environment.getExternalStorageDirectory().getAbsolutePath() + "/MyApp/");

            Log.d("appDirectroyExist", appDirectory.exists() + "");
            if (!appDirectory.exists()) 
                Log.d("appDir created: ", appDirectory.mkdir() + "");

            File dbDirectory = new File(
                    Environment.getExternalStorageDirectory().getAbsolutePath() + "/MyApp/Database/");

            Log.d("dbDirectroyExist", dbDirectory.exists() + "");
            if (!dbDirectory.exists())
                Log.d("dbDir created: ", dbDirectory.mkdirs() + "");


            File themesDirectory = new File(
                    Environment.getExternalStorageDirectory().getAbsolutePath() + "/MyApp/Themes/");
            Log.d("themesDirectroyExist", themesDirectory.exists() + "");
            if (!themesDirectory.exists()) 
                Log.d("themesDir created: ", themesDirectory.mkdirs() + "");

        }
    }
}

Also, I have set the sdcard write permission:

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

I've run the application several times and every time I get the LogCat output:

01-09 21:38:13.701: D/Media State(15363): mounted
01-09 21:38:13.701: D/appDirectroyExist(15363): false
01-09 21:38:13.701: D/appDir created:(15363): false
01-09 21:38:13.701: D/dbDirectroyExist(15363): false
01-09 21:38:13.701: D/dbDir created:(15363): false
01-09 21:38:13.701: D/themesDirectroyExist(15363): false
01-09 21:38:13.701: D/themesDir created:(15363): false

I have read similar question, but nothing useful to get. What should I do to get the code running? What is my problem?

I don't know why the application acted that way, but finally I overcame this problem by removing and resetting the sdcard write permission in the Android manifest file. Thanks all of you for your kind help and apologize for taking your time.