Only safe or non null assserted calls are allowed

问题:

Just started using kotlin for android development.My arraylist is declared like this-

var day1: ArrayList<DietPlanDetailModel>? = null

Now I am trying to access an element by its position

    val dietPlan= day1[position]

but i am getting below compile time error-

Only safe or non null assserted calls are allowed on a nullable receiver type of arraylist

Why am i getting this error and how can i resolve it?

回答1:

The problem is, that you defined the ArrayList as nullable. You have two options here:

1) don't define the variable nullable (this depends on your code): var day1: ArrayList<DietPlanDetailModel> = ArrayList()

2) access your data-structure with a null check: val dietPlan= day1?.get(position)

回答2:

As defined, day1 can be null but you're invoking a function by doing [], which is basically the same as calling day1.get(index).

This can throw a NullpointerException, which the Kotlin compiler tries to prevend. Thus, only safe calls like this are allowed: day1?.get().

回答3:

You told compiler that your variable can be null (and assigned null to it).

day1[position] is essentially day1.get(position) which will crash with NPE if day1 is null -> null.get(position)

If you can guarantee that day1 will be initialized id recommend lateinit or just straight up assigning new Arraylist with declaration. Of course, simple day1?.get(position) works fine.