What is the right way of using “greater than”, “less than” comparison on nullable integers in Kotlin?

Question

var _age: Int? = 0

public var isAdult: Boolean? = false
   get() = _age?.compareTo(18) >= 0 

This still gives me a null-safety, compile error,

Answer 1

A generic and flexible solution would be:

infix fun <T : Comparable<T>> T?.isGreaterThan(other: T?): Boolean? =
    if (this != null && other != null) this > other else null

infix fun <T : Comparable<T>> T?.isGreaterThanOrEqual(other: T?): Boolean? =
    if (this != null && other != null) this >= other else null

infix fun <T : Comparable<T>> T?.isLessThan(other: T?): Boolean? =
    if (this != null && other != null) this < other else null

infix fun <T : Comparable<T>> T?.isLessThanOrEqual(other: T?): Boolean? =
    if (this != null && other != null) this <= other else null

Depending on your needs, you could use it as:

public var isAdult: Boolean? = false
    get() = _age isGreaterThanOrEqual 18

or:

public var isAdult: Boolean = false
    get() = _age isGreaterThanOrEqual 18 == true

Answer 2

var age : Int? = 0

public val isAdult : Boolean?
    get() = age?.let { it >= 18 }

The other solution would be using delegates:

var age : Int by Delegates.notNull()
public val isAdult : Boolean
    get () = age >= 18

So if you try to get age or check isAdult before age was actually assigned then you'll get exception instead of null.

Anyway I believe age = 0 is some kind magic that one day may lead to issue (even prod issue).

Answer 3

I used the null coalescing operator to convert from nullable Int? to non-nullable Int:

var age: Int? = 0

public var isAdult: Boolean? = null
   get() = if(age == null) null else (age ?: 0 >= 18)

Answer 4

Kotlin could sure use an extension function on Int for this, but until they do:

fun Int?.isGreaterThan(other: Int?) =
    this != null && other != null && this > other

fun Int?.isLessThan(other: Int?) =
    this != null && other != null && this < other

My methods returns false, not null if either operand is null. That makes more sense to me.

Answer 5

You could possibly use the built-in function compareValue:

fun <T : Comparable<*>> compareValues(a: T?, b: T?): Int

public var isAdult: Boolean = false
   get() = compareValues(_age, 18) >= 0

or

public var isAdult: Boolean = false
   get() = compareValues(18, _age) <= 0

Note however that null is considered less than any value, which may suit your case, but may lead to undesired behavior in other cases. E.g., think of var grantMinorsDiscount.

Answer 6

Also can try this:

var position = 100

    mArrayList?.let {

                if (it.size > 0 && position >= 0 ) 
                     return true

            }