为什么凌空课使用单例模式设计？

Question

to use the volley library i found the following code:

public class AppController extends Application {

public static final String TAG = AppController.class
        .getSimpleName();

private RequestQueue mRequestQueue;

private static AppController mInstance;

@Override
public void onCreate() {
    super.onCreate();
    mInstance = this;
}

public static synchronized AppController getInstance() {
    return mInstance;
}

public RequestQueue getRequestQueue() {

    //baraye avalin bar ejra mishe
    if (mRequestQueue == null) {
        mRequestQueue = Volley.newRequestQueue(getApplicationContext());
    }

    return mRequestQueue;
}

public <T> void addToRequestQueue(Request<T> req, String tag) {
    // set the default tag if tag is empty
    req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);
    getRequestQueue().add(req);
}

public void cancelPendingRequests(Object tag) {
    if (mRequestQueue != null) {
        mRequestQueue.cancelAll(tag);
    }
}

}

i want to understand why this class uses Singleton pattern design?if we create two objects of the class what problems occur?

Answer 1

This is not the singleton pattern. The singleton pattern enforces that there can only ever be (at most) a single instance of a particular class. That's not true here. You could have many RequestQueue s across many AppController s.

What this is, in fact, is lazy initialisation . It is probably employed because instantiating a RequestQueue is somewhat of a large operation, and you don't want to instantiate it unnecessarily if you don't have to.