Kotlin - no_image

lazy vs lateinit

lazy()

lazy() is a function that takes a lambda and returns an instance of Lazy<T>, which can serve as a delegate for implementing a lazy property. The first call to get() executes the lambda passed to lazy() and remembers the result. Subsequent calls to get() simply return the remembered result.

val lazyValue: String by lazy {
    println("computed!")
    "Hello"
}

fun main() {
    println(lazyValue)
    println(lazyValue)
}

Output:

computed!
Hello
Hello

By default, the evaluation of lazy properties is synchronized: the value is computed only in one thread, but all threads will see the same value.¹

lateinit

The lateinit keyword in Kotlin enables the declaration of non-nullable properties without immediate initialization. This is beneficial when dealing with properties that necessitate a context or extensive initialization that cannot be done during object creation. However, it is crucial to initialize lateinit properties before accessing them, as failing to do so will result in an UninitializedPropertyAccessException. Let’s examine a code sample to better understand this concept:

class DemoClass {
    lateinit var userAge: UserAge

    fun init() {
        userAge = UserAge(27)
    }

    fun printAge() {
        if (::age.isInitialized) {
            println("Age is : ${userAge.age}")
        } else {
            println("Age not initialized")
        }
    }
}

In the provided code snippet, a property called age is declared using the lateinit keyword. The init() function is responsible for initializing this property. The code then checks if the property has been initialized using the isInitialized property reference. If it is initialized, the value of age is printed; otherwise, a message indicating that it has not been initialized yet is displayed.²

Differences

| lazy | lateinit | |—|—| | Can be used with primitive types | Can’t be used with primitive types | | Can be used for nullable property | Can’t be used for nullable property | | Can only be used for val | Can only be used for var | | Thread safe by default. Guarantees that the initializer is invoked at most once | Non thread safe. User is responsible for initialize in multi-thread inveronments | | Can only be initialized from the initializer lambda | Can be initialized from anywhere |

Links

Delegated properties

Kotlin: lateinit vs lazy

Property initialization using by lazy vs. lateinit

Further reading

Initializing lazy and lateinit variables in Kotlin

credit goes to @swayangjit