Use of Boolean? in if expression

Question

You can compare nullable boolean with true, false or null using equality operator:

var b: Boolean? = null
if (b == true) {
    // b was not null and equal true
} 
if (b == false) {
   // b is false 
}
if (b != true) { 
   // b is null or false 
}

If you want to cleanly check whether a Boolean? is true or false you can do:

when(b) {
    true -> {}
    false -> {}
}

If you want to check if it's null you can add that (or else) as a value in the when:

when(b) {
    true -> {}
    false -> {}
    null -> {}
}

when(b) {
    true -> {}
    false -> {}
    else-> {}
}

Kotlin will statically analyze your null checks. This is fine:

val b: Boolean? = null
if (b != null && b) {
    println(b)
}

Even though this fails with type error:

val b: Boolean? = null
if (b == null && b) {
    println(b)
}

For more see: http://kotlinlang.org/docs/reference/null-safety.html

You can also use "null coalescing operator" (which will work for mutable variables):

val b: Boolean? = null
if (b ?: false) {
    println(b)
}

From what I've seen the Boolean? is a result of a method that returns Boolean on an object that is nullable

val person: Person? = null
....
if(person?.isHome()) { //This won't compile because the result is Boolean?
  //Do something
}

The solution I've been using is to use the let function to remove the possible returned null value like so

person?.let {
  if(it.isHome()) {
    //Do something
  }
}

In Kotlin, you can do like this:

val b: Boolean? = true
if (b == true) { // if b is null, this should be null == true
    /* Do something */
} else {
    /* Do something else */
}

first, add the custom inline function below:

inline fun Boolean?.ifTrue(block: Boolean.() -> Unit): Boolean? {
    if (this == true) {
        block()
    }
    return this
}

inline fun Boolean?.ifFalse(block: Boolean?.() -> Unit): Boolean? {
    if (null == this || !this) {
        block()
    }

    return this
}

then you can write code like this:

val b: Boolean? = ...
b.ifTrue {
   /* Do something in true case */
}

//or

b.ifFalse {
   /* Do something else in false case */
}