Use of Boolean? in if expression

问题:

If I have a nullable Boolean b, I can do the following comparison in Java:

Boolean b = ...;
if (b != null && b) {
   /* Do something */
} else {
   /* Do something else */
}

In Kotlin, I can achieve the same by using the !! operator:

val b: Boolean? = ...
if (b != null && b!!) {
   /* Do something */
} else {
   /* Do something else */
}

However, the use of !! feels a bit sketchy to me, circumventing the null safety system.

Is there a more elegant approach for this?

---

Edit It seems I oversimplicated a bit. For local variables, as Banthar shows, it does work. However, my Boolean b is actually a "property with a backing field" (I'm not really up to speed yet what this imposes). This is the result:

回答1:

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
}

回答2:

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)
}

回答3:

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-> {}
}

回答4:

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
  }
}

回答5:

In Kotlin, you can do like this:

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

回答6:

For Kotlin what i normally use is if (object?.booleanProperty ==true) { //do stuff } this would only work when the property is true and the object is not null. For the reverse: if (!object?booleanProperty !=true) { //do Stuff}