Need some help to understand what is done in the second line of code?

Question

 public <T> void addToRequestQueue(Request<T> req, String tag) {
    req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);
    getRequestQueue().add(req);
}

what is done in this second line? I can't understand what is this (tag) ?TAG :tag.

Here is the full code

{
import android.app.Application;
import android.text.TextUtils;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.toolbox.Volley;

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() {
        if (mRequestQueue == null) {
            mRequestQueue = Volley.newRequestQueue(getApplicationContext());
        }

        return mRequestQueue;
    }

    public <T> void addToRequestQueue(Request<T> req, String tag) {
        req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);
        getRequestQueue().add(req);
    }

    public <T> void addToRequestQueue(Request<T> req) {
        req.setTag(TAG);
        getRequestQueue().add(req);
    }

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

here is the full code now tell me and please explain if you can that what is done in that method second line

Answer 1

TAG must be some constant of type String representing the default value of tag, whatever it is - something like this:

public static final String TAG = "<default-tag>";

The idea here is to tag your request with the value passed as the second parameter, ie String tag . Conditional operator ?: is used to implement the check without using an if / else statement.

When the tag string passed in the call to addToRequestQueue happens to be empty, the method uses the value of TAG instead.

Answer 2

Translation of the second line

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

public <T> void addToRequestQueue(Request<T> req, String tag) {
    req.setTag("is tag empty?" yes, so use `TAG` : no, so use `tag`);
    getRequestQueue().add(req);
} 

In another words:

if(tag.isEmpty()){
  res.setTag(TAG)

} else {
  res.setTag(tag)

}