kotlin 返回布尔函数

Question

i did some research on boolean return type function and found out it's safer to use with null-able check. i have some understanding issue, compiler doesn't like what i did. i also tried with if statement and it somehow returns only false. can anyone please clear up my head? thanks!

    fun check(list:ArrayList<String>): Boolean {
    var test: Boolean?= null
    for (i in list) {
        when (i=="a") {
            true -> test == true
            false -> test == false
        }
    }
    return test!!
}

Answer 1

Since this function only returns a true or false, you shouldn't be making the variable test nullable. But the compiler will complain if you remove the ? and the = null because it can't guarantee the code in your for loop will ever be called (because the list could be empty).

As for why it's giving you the wrong result, it's because you're changing the value of test for every item in the list, so the final value depends only on the last element in the list. Also, if the list is empty, test will remain null and you'll get an exception when you use test!! .

I don't know if you're trying to see if any element of the list is “a” or if all elements are “a”, but for the sake of discussion I'll assume it is the first one. So the strategy here is to make false the default, and change it to true if you find any elements that are “a”. And once you find one, there's no reason to keep checking so you can break out of the loop.

var test = false // not nullable
for (i in list)
    if (i == "a") {
        test = true
        break
    }
return test

As a side note, Kotlin has a higher-order function for List that checks for any element that satisfies the lambda so the above code could be replaced with

return list.any { it == "a" }

Another side note: if you are setting a Boolean based on some criteria, a when statement is overkill and makes your code more complicated. You could have replaced your when statement with test = i == "a" and had equivalent behavior (although as you found, it wasn't right behavior to begin with).