Kotlin - Property initialization using by lazy vs lateinit

Question

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    Lazy Initialization   lazy   is a function that takes a lambda and returns an instance of Lazy lt T gt  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    quot Hello quot         So the first call and the subsequential calls  wherever it is  to myLazyString will return Hello  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

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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

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Here are the significant differences between lateinit var and by lazy         delegated property   lazy         delegate can only be used for val properties  whereas lateinit can only be applied to vars  because it can t be compiled to a final field  thus no immutability can be guaranteed   lateinit var has a backing field which stores the value  and by lazy         creates a delegate object in which the value is stored once calculated  stores the reference to the delegate instance in the class object and generates the getter for the property that works with the delegate instance  So if you need the backing field present in the class  use lateinit   In addition to vals  lateinit cannot be used for nullable properties or Java primitive types  this is because of null used for uninitialized value    lateinit var can be initialized from anywhere the object is seen from  e g  from inside a framework code  and multiple initialization scenarios are possible for different objects of a single class  by lazy          in turn  defines the only initializer for the property  which can be altered only by overriding the property in a subclass  If you want your property to be initialized from outside in a way probably unknown beforehand  use lateinit   Initialization by lazy         is thread-safe by default and guarantees that the initializer is invoked at most once  but this can be altered by using another lazy overload   In the case of lateinit var  it s up to the user s code to initialize the property correctly in multi-threaded environments   A Lazy instance can be saved  passed around and even used for multiple properties  On contrary  lateinit vars do not store any additional runtime state  only null in the field for uninitialized value    If you hold a reference to an instance of Lazy  isInitialized   allows you to check whether it has already been initialized  and you can obtain such instance with reflection from a delegated property   To check whether a lateinit property has been initialized  you can use property  isInitialized since Kotlin 1 2   A lambda passed to by lazy         may capture references from the context where it is used into its closure   It will then store the references and release them only once the property has been initialized  This may lead to object hierarchies  such as Android activities  not being released for too long  or ever  if the property remains accessible and is never accessed   so you should be careful about what you use inside the initializer lambda    Also  there s another way not mentioned in the question  Delegates notNull    which is suitable for deferred initialization of non-null properties  including those of Java primitive types

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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

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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

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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

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lateinit vs lazy  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   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

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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

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If you are using Spring container and you want to initialize non-nullable bean field  lateinit is better suited        Autowired     lateinit var myBean  MyBean

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

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