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?