In Kotlin, what is the idiomatic way to deal with nullable values, referencing or converting them

问题

If I have a nullable type Xyz?, I want to reference it or convert it to a non-nullable type Xyz. What is the idiomatic way of doing so in Kotlin?

For example, this code is in error:

val something: Xyz? = createPossiblyNullXyz()
something.foo() // Error: \"Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type Xyz?\"

But if I check null first it is allowed, why?

val something: Xyz? = createPossiblyNullXyz()
if (something != null) {
    something.foo() 
}

How do I change or treat a value as not null without requiring the if check, assuming I know for sure it is truly never null? For example, here I am retrieving a value from a map that I can guarantee exists and the result of get() is not null. But I have an error:

val map = mapOf(\"a\" to 65,\"b\" to 66,\"c\" to 67)
val something = map.get(\"a\")
something.toLong() // Error: \"Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type Int?\"

The method get() thinks it is possible that the item is missing and returns type Int?. Therefore, what is the best way to force the type of the value to be not nullable?

Note: this question is intentionally written and answered by the author (Self-Answered Questions), so that the idiomatic answers to commonly asked Kotlin topics are present in SO. Also to clarify some really old answers written for alphas of Kotlin that are not accurate for current-day Kotlin.

回答1:

First, you should read all about Null Safety in Kotlin which covers the cases thoroughly.

In Kotlin, you cannot access a nullable value without being sure it is not null (Checking for null in conditions), or asserting that it is surely not null using the !! sure operator, accessing it with a ?. Safe Call, or lastly giving something that is possibly null a default value using the ?: Elvis Operator.

For your 1st case in your question you have options depending on the intent of the code you would use one of these, and all are idiomatic but have different results:

val something: Xyz? = createPossiblyNullXyz()

// access it as non-null asserting that with a sure call
val result1 = something!!.foo()

// access it only if it is not null using safe operator, 
// returning null otherwise
val result2 = something?.foo()

// access it only if it is not null using safe operator, 
// otherwise a default value using the elvis operator
val result3 = something?.foo() ?: differentValue

// null check it with `if` expression and then use the value, 
// similar to result3 but for more complex cases harder to do in one expression
val result4 = if (something != null) {
                   something.foo() 
              } else { 
                   ...
                   differentValue 
              }

// null check it with `if` statement doing a different action
if (something != null) { 
    something.foo() 
} else { 
    someOtherAction() 
}

For the "Why does it work when null checked" read the background information below on smart casts.

For your 2nd case in your question in the question with Map, if you as a developer are sure of the result never being null, use !! sure operator as an assertion:

val map = mapOf("a" to 65,"b" to 66,"c" to 67)
val something = map.get("a")!!
something.toLong() // now valid

or in another case, when the map COULD return a null but you can provide a default value, then Map itself has a getOrElse method:

val map = mapOf("a" to 65,"b" to 66,"c" to 67)
val something = map.getOrElse("z") { 0 } // provide default value in lambda
something.toLong() // now valid

---

Background Information:

Note: in the examples below I am using explicit types to make the behavior clear. With type inference, normally the types can be omitted for local variables and private members.

More about the !! sure operator

The !! operator asserts that the value is not null or throws an NPE. This should be used in cases where the developer is guaranteeing that the value will never be null. Think of it as an assert followed by a smart cast.

val possibleXyz: Xyz? = ...
// assert it is not null, but if it is throw an exception:
val surelyXyz: Xyz = possibleXyz!! 
// same thing but access members after the assertion is made:
possibleXyz!!.foo()

read more: !! Sure Operator

---

More about null Checking and Smart Casts

If you protect access to a nullable type with a null check, the compiler will smart cast the value within the body of the statement to be non-nullable. There are some complicated flows where this cannot happen, but for common cases works fine.

val possibleXyz: Xyz? = ...
if (possibleXyz != null) {
   // allowed to reference members:
   possiblyXyz.foo()
   // or also assign as non-nullable type:
   val surelyXyz: Xyz = possibleXyz
}

Or if you do a is check for a non-nullable type:

if (possibleXyz is Xyz) {
   // allowed to reference members:
   possiblyXyz.foo()
}

And the same for 'when' expressions that also safe cast:

when (possibleXyz) {
    null -> doSomething()
    else -> possibleXyz.foo()
}

// or

when (possibleXyz) {
    is Xyz -> possibleXyz.foo()
    is Alpha -> possibleXyz.dominate()
    is Fish -> possibleXyz.swim() 
}

Some things do not allow the null check to smart cast for the later use of the variable. The example above uses a local variable that in no way could have mutated in the flow of the application, whether val or var this variable had no opportunity to mutate into a null. But, in other cases where the compiler cannot guarantee the flow analysis, this would be an error:

var nullableInt: Int? = ...

public fun foo() {
    if (nullableInt != null) {
        // Error: "Smart cast to 'kotlin.Int' is impossible, because 'nullableInt' is a mutable property that could have been changed by this time"
        val nonNullableInt: Int = nullableInt
    }
}

The lifecycle of the variable nullableInt is not completely visible and may be assigned from other threads, the null check cannot be smart cast into a non-nullable value. See the "Safe Calls" topic below for a workaround.

Another case that cannot be trusted by a smart cast to not mutate is a val property on an object that has a custom getter. In this case, the compiler has no visibility into what mutates the value and therefore you will get an error message:

class MyThing {
    val possibleXyz: Xyz? 
        get() { ... }
}

// now when referencing this class...

val thing = MyThing()
if (thing.possibleXyz != null) {
   // error: "Kotlin: Smart cast to 'kotlin.Int' is impossible, because 'p.x' is a property that has open or custom getter"
   thing.possiblyXyz.foo()
}

read more: Checking for null in conditions

---

More about the ?. Safe Call operator

The safe call operator returns null if the value to the left is null, otherwise continues to evaluate the expression to the right.

val possibleXyz: Xyz? = makeMeSomethingButMaybeNullable()
// "answer" will be null if any step of the chain is null
val answer = possibleXyz?.foo()?.goo()?.boo()

Another example where you want to iterate a list but only if not null and not empty, again the safe call operator comes in handy:

val things: List? = makeMeAListOrDont()
things?.forEach {
    // this loops only if not null (due to safe call) nor empty (0 items loop 0 times):
}

In one of the examples above we had a case where we did an if check but have the chance another thread mutated the value and therefore no smart cast. We can change this sample to use the safe call operator along with the let function to solve this:

var possibleXyz: Xyz? = 1

public fun foo() {
    possibleXyz?.let { value ->
        // only called if not null, and the value is captured by the lambda
        val surelyXyz: Xyz = value
    }
}

read more: Safe Calls

---

More about the ?: Elvis Operator

The Elvis operator allows you to provide an alternative value when an expression to the left of the operator is null:

val surelyXyz: Xyz = makeXyzOrNull() ?: DefaultXyz()

It has some creative uses as well, for example throw an exception when something is null:

val currentUser = session.user ?: throw Http401Error("Unauthorized")

or to return early from a function:

fun foo(key: String): Int {
   val startingCode: String = codes.findKey(key) ?: return 0
   // ...
   return endingValue
}

read more: Elvis Operator

---

Null Operators with Related Functions

Kotlin stdlib has a series of functions that work really nicely with the operators mentioned above. For example:

// use ?.let() to change a not null value, and ?: to provide a default
val something = possibleNull?.let { it.transform() } ?: defaultSomething

// use ?.apply() to operate further on a value that is not null
possibleNull?.apply {
    func1()
    func2()
}

// use .takeIf or .takeUnless to turn a value null if it meets a predicate
val something = name.takeIf { it.isNotBlank() } ?: defaultName

val something = name.takeUnless { it.isBlank() } ?: defaultName

---

Related Topics

In Kotlin, most applications try to avoid null values, but it isn't always possible. And sometimes null makes perfect sense. Some guidelines to think about:

• in some cases, it warrants different return types that include the status of the method call and the result if successful. Libraries like Result give you a success or failure result type that can also branch your code. And the Promises library for Kotlin called Kovenant does the same in the form of promises.

• for collections as return types always return an empty collection instead of a null, unless you need a third state of "not present". Kotlin has helper functions such as emptyList() or emptySet() to create these empty values.

• when using methods which return a nullable value for which you have a default or alternative, use the Elvis operator to provide a default value. In the case of a Map use the getOrElse() which allows a default value to be generated instead of Map method get() which returns a nullable value. Same for getOrPut()

• when overriding methods from Java where Kotlin isn't sure about the nullability of the Java code, you can always drop the ? nullability from your override if you are sure what the signature and functionality should be. Therefore your overridden method is more null safe. Same for implementing Java interfaces in Kotlin, change the nullability to be what you know is valid.

• look at functions that can help already, such as for String?.isNullOrEmpty() and String?.isNullOrBlank() which can operate on a nullable value safely and do what you expect. In fact, you can add your own extensions to fill in any gaps in the standard library.

• assertion functions like checkNotNull() and requireNotNull() in the standard library.

• helper functions like filterNotNull() which remove nulls from collections, or listOfNotNull() for returning a zero or single item list from a possibly null value.

• there is a Safe (nullable) cast operator as well that allows a cast to non-nullable type return null if not possible. But I do not have a valid use case for this that isn't solved by the other methods mentioned above.

回答2:

The previous answer is a hard act to follow, but here's one quick and easy way:

val something: Xyz = createPossiblyNullXyz() ?: throw RuntimeError("no it shouldn't be null")
something.foo() 

If it really is never null, the exception won't happen, but if it ever is you'll see what went wrong.

来源：https://stackoverflow.com/questions/34498562/in-kotlin-what-is-the-idiomatic-way-to-deal-with-nullable-values-referencing-o