[properties] Kotlin - Property initialization using "by lazy" vs. "lateinit"

In Kotlin if you don't want to initialize a class property inside the constructor or in the top of the class body, you have basically these two options (from the language reference):

1. Lazy Initialization

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.

Example

public class Hello {

   val myLazyString: String by lazy { "Hello" }

}

So the first call and the subsequential calls, wherever it is, to myLazyString will return Hello

2. Late Initialization

Normally, properties declared as having a non-null type must be initialized in the constructor. However, fairly often this is not convenient. For example, properties can be initialized through dependency injection, or in the setup method of a unit test. In this case, you cannot supply a non-null initializer in the constructor, but you still want to avoid null checks when referencing the property inside the body of a class.

To handle this case, you can mark the property with the lateinit modifier:

public class MyTest {
   
   lateinit var subject: TestSubject

   @SetUp fun setup() { subject = TestSubject() }

   @Test fun test() { subject.method() }
}

The modifier can only be used on var properties declared inside the body of a class (not in the primary constructor), and only when the property does not have a custom getter or setter. The type of the property must be non-null, and it must not be a primitive type.

So, how to choose correctly between these two options, since both of them can solve the same problem?

If you use an unchangable variable, then it is better to initialize with by lazy { ... } or val. In this case you can be sure that it will always be initialized when needed and at most 1 time.

If you want a non-null variable, that can change it's value, use lateinit var. In Android development you can later initialize it in such events like onCreate, onResume. Be aware, that if you call REST request and access this variable, it may lead to an exception UninitializedPropertyAccessException: lateinit property yourVariable has not been initialized, because the request can execute faster than that variable could initialize.

Very Short and concise Answer

lateinit: It initialize non-null properties lately

Unlike lazy initialization, lateinit allows the compiler to recognize that the value of the non-null property is not stored in the constructor stage to compile normally.

lazy Initialization

by lazy may be very useful when implementing read-only(val) properties that perform lazy-initialization in Kotlin.

by lazy { ... } performs its initializer where the defined property is first used, not its declaration.

lateinit vs lazy

1. lateinit

   i) Use it with mutable variable[var]

    lateinit var name: String       //Allowed
    lateinit val name: String       //Not Allowed
   

ii) Allowed with only non-nullable data types

    lateinit var name: String       //Allowed
    lateinit var name: String?      //Not Allowed

iii) It is a promise to compiler that the value will be initialized in future.

NOTE: If you try to access lateinit variable without initializing it then it throws UnInitializedPropertyAccessException.

2. lazy

   i) Lazy initialization was designed to prevent unnecessary initialization of objects.

ii) Your variable will not be initialized unless you use it.

iii) It is initialized only once. Next time when you use it, you get the value from cache memory.

iv) It is thread safe(It is initialized in the thread where it is used for the first time. Other threads use the same value stored in the cache).

v) The variable can only be val.

vi) The variable can only be non-nullable.

If you are using Spring container and you want to initialize non-nullable bean field, lateinit is better suited.

    @Autowired
    lateinit var myBean: MyBean

Everything is correct above, but one of facts simple explanation LAZY----There are cases when you want to delay the creation of an instance of your object until its first usage. This technique is known as lazy initialization or lazy instantiation. The main purpose of lazy initialization is to boost performance and reduce your memory footprint. If instantiating an instance of your type carries a large computational cost and the program might end up not actually using it, you would want to delay or even avoid wasting CPU cycles.

Additionnally to hotkey's good answer, here is how I choose among the two in practice:

lateinit is for external initialisation: when you need external stuff to initialise your value by calling a method.

e.g. by calling:

private lateinit var value: MyClass

fun init(externalProperties: Any) {
   value = somethingThatDependsOn(externalProperties)
}

While lazy is when it only uses dependencies internal to your object.

In addition to all of the great answers, there is a concept called lazy loading:

Lazy loading is a design pattern commonly used in computer programming to defer initialization of an object until the point at which it is needed.

Using it properly, you can reduce the loading time of your application. And Kotlin way of it's implementation is by lazy() which loads the needed value to your variable whenever it's needed.

But lateinit is used when you are sure a variable won't be null or empty and will be initialized before you use it -e.g. in onResume() method for android- and so you don't want to declare it as a nullable type.