CODEWITHSUNDEEP
kotlin
Ygor Reis
Ygor Reis

Reputation: 1

Why Kotlin required put "==true" inside "if" verification with nullables var?

folks!

We know, IF statement only works with true condition. The operator ? checks and guarantees firstName property is not null. So, if firstName isn't null and isEmpty() is also true, why "==true" is needed?

Shortly, why simple condition "firstName?.isEmpty()" is invalid?

class Person (firstName: String?) {
    init {
        if (firstName?.isEmpty() == true) {
            println("firstName is null")
        }else{
            println("The name is $firstName")
        }
    }
}

Upvotes: 0

Views: 90

Answers (5)

Angel Koh
Angel Koh

Reputation: 13505

Do note that the code does not work as intended (i.e. prints null if it is null or empty) https://pl.kotl.in/H7rBIT8H6

fun main() {
    var firstName:String? = null
    
    if(firstName?.isEmpty() ==true){
        //print("firstName is null")
        print("test1: TRUE. ")   //<<-- THIS WILL NOT BE PRINTED
    }else {
        print ("test1: FALSE. ")   // <<-- THIS IS RETURNED INSTEAD
    }
    
    firstName = ""
     if(firstName?.isEmpty() ==true){
        print("test2: TRUE. ") //<<--WORKS AS INTENDED
    }else {
        print ("test2: FALSE. ")
    }
    
    firstName = "Hello"
    if(firstName?.isEmpty() ==true){
        print("test1: TRUE. ")
    }else {
        print ("test1: FALSE. ") //<<--WORKS AS INTENDED
    }
}

The output is

test1: FALSE. test2: TRUE. test1: FALSE.

notice "test1" is FALSE?

what is happening is the code

"firstName?.isEmpty()" returns null if firstName is null. and consequently, "null == true" will return false

So instead, what you should do is either call

firstName.isNullOrEmpty()

or

firstName==null || firstName?.isEmpty()==true

Upvotes: 0

Naetmul
Naetmul

Reputation: 15552

It is because firstName?.isEmpty() can be true, false, or null.

if (null) does not make sense
UNLESS you are using a language that implicitly converts null to true or false.
Kotlin does not implicitly convert it.

Upvotes: 1

Михаил Нафталь
Михаил Нафталь

Reputation: 7882

The operator ? checks and guarantees firstName property is not null.

No, safe call operator doesn't guarantees this. It guarantees that you won't get NullPointerException if firstName is null, instead you will get null as a result of firstName?.isEmpty() expression.

So, the type of this expression is Boolean?, and if condition must be Boolean.

I believe, you'd better use firstName.isNullOrEmpty() here.

Upvotes: 3

Sergey Afinogenov
Sergey Afinogenov

Reputation: 2212

IsEmpty will be called in this case only if firstName is not null

Upvotes: 0

LenglBoy
LenglBoy

Reputation: 331

The issue here is that your list is nullable. In Kotlin you should always avoid having nullable values. I would suggest refactoring.

Anyway, the answer is:

  • Comparison needed because the list is nullable and you need a fallback because if the list is null, isEmpty() will not be called so you will not get a boolean value or condition which is needed for if.
  • use isNullOrEmpty() instead

Upvotes: -1

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