Java：三元运算符，不能正常工作

Question

I have some sample code and it looks like it uses a ternery operator but it takes 3 items. So I am really confused, the ones I have seen in other languages take only 2, ie true and false.

Here is the code

        Tracker t = (trackerId == TrackerName.APP_TRACKER) ? analytics.newTracker(R.xml.app_tracker)
                : (trackerId == TrackerName.GLOBAL_TRACKER) ? analytics.newTracker(PROPERTY_ID)
                : analytics.newTracker(R.xml.ecommerce_tracker);

what i wish to do is remove the line

     : analytics.newTracker(R.xml.ecommerce_tracker);

So it would now be

        Tracker t = (trackerId == TrackerName.APP_TRACKER) ? analytics.newTracker(R.xml.app_tracker)
                : (trackerId == TrackerName.GLOBAL_TRACKER) ? analytics.newTracker(PROPERTY_ID);

but it states it is missing a : (colon) in the IDE. So maybe this isn't a ternery operator after all ?

THe ide I am using is Android Studio.

I know it must be something simple, but i can't figure it out.

Any ideas ?

Answer 1

What you have there is nested ternary operators. Think of them as the following (pseudo-code below)

  condition ? answerifTrue : secondternaryoperator;
  where secondternaryoperator =
        secondcondition ? answerifTrue : answerIfFalse;

Basically, this is two ternary operators combined. If the first condition is true it simply returns answerifTrue . But if the condition is false, it runs the second ternary operator to determine what to return based on a second condition. A singular ternary operator simply looks like this;

  condition ? answerifTrue : answerIfFalse;        

So if you want to create a single ternary operator from both you need to determine which answer you want if the first condition is false, for example;

  Tracker t = (trackerId == TrackerName.APP_TRACKER) ? analytics.newTracker(R.xml.app_tracker) : analytics.newTracker(PROPERTY_ID);

I imagine however, that what you need isn't well suited to ternary operators anyway and you might need to restructure the statement entirely into if else blocks or a switch statement.

Answer 2

Hm, as far as I understand that use case you have to do something like this:

 Tracker t = null; 

 if (trackerId == TrackerName.APP_TRACKER) {
    t = analytics.newTracker(R.xml.app_tracker);
 } else if (trackerId == TrackerName.GLOBAL_TRACKER) {
    t = analytics.newTracker(PROPERTY_ID);
 } else {
    t = analytics.newTracker(R.xml.ecommerce_tracker);
 }