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Why we can't declare variable as "var" for lazy initialization [duplicate]

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<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
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?

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.

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 property 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 property can only be val.
vi) The property can be of any type (including primitives and nullables, which are not allowed with lateinit).

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.

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.

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.

Diff btw lateinit and lazy
lateinit
Use only with mutable variable i.e. var and non-nullable data types
lateinit var name: String //Allowed with non-nullable
You are telling the 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
Lazy initialization was designed to prevent unnecessary initialization of objects.
Your variable will not be initialized unless you use it.
It is initialized only once. Next time when you use it, you get the value from cache memory.
It is thread safe.
The variable can only be val and non-nullable.
Cheers :)

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

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.

Nullable var with `?` vs. lateinit var

What is the best way to define global variables in a Kotlin/Android activity/fragment?
What are the different scenarios when you should use these 2 methods for defining a global variable:
var viewpager: CustomViewPager? = null
or
lateinit var viewpager: CustomViewPager
?
If I use the former, I won't have to check for null in my code. For example if I used lateinit for the following:
viewpager = activity?.findViewById<CustomViewPager>(R.id.viewpager) then I would have to check for null.

using lateinit, you are saying that you absolutely will make sure that an instance of that variable is created somewhere (otherwise, your application will throw an exception if a lateinit has not been initialized) and then that variable also will not be null throughout the rest of your project, compared to using null, it means that this object potentially could be null somewhere in your code for the rest of the project and you will have to deal with nullability throughout.
If you are positive that you are not going to make a variable null and you require an instance of it always, use lateinit
Ask yourself this question :
Am I 100% sure that I will be using an instance of this variable somewhere in this class ?
If the answer to that is Yes, you should probably be using lateinit, as lateinit forces you to create an instance of it.
If the answer is No, you should probably be using a nullable field instead.
Taken from here : https://www.kotlindevelopment.com/lateinit-kotlin/
The lateinit keyword stands for late initialization. Lateinit comes very handy when a non-null initializer cannot be supplied in the constructor, but the developer is certain that the variable will not be null when accessing it, thus avoiding null checks when referencing it later.

i would recommend using the first method, it's better cause it eliminates app crashes if the code is trying to access the viewPager while it was not initialized, you can also use this in order to access the viewPager in the first method to make sure your app won't crash
viewPager?.let{"it"
}
this will create an instance of the viewPager if it was already initialized and use it as a val instead of a var, the instance is called "it" and u can rename it as anything by doing this after the first {
viewPager?.let{viewPager ->
}

If you make sure that variable will never be null in any place of the code then use lateinit. Here you have to initialize it before using it (in your case you can initialize it inside onCreate(..). Make sure that the variable will never become null later ELSE a null pointer exception will be fired.
In this way, you can directly use the variable without checking it if it's null or not.
Also, you can detect if the variable has initialized or not using:
if (:: viewpager.isInitialized) { .... }
On the other hand, you should use the nullable option using ?
In this case, you should check the variable before using it if it's null or not.
you can use ?. for that. You also can combine that with let for example:
viewPager?.let {
....
}

Kotlin - local variables vs global variables

You can declare a local variable without initializing or adding lateinit, but same is not true for global variable, my question is why ? why compiler do not give error for local variable too?
for example :
class A{
var abc : String // this is not allowed by compiler
fun myOwnedFun(){
var abcd : String // this is allowed
}
}
I understand one thing is allowed and the other is not but am curious as to why.

The scope of a local variable is the function where it's defined. The compiler has no problem verifying that a local variable has been initialized inside the function before it's accessed for the first time.
As for a public class field, the scope is infinite, hence the compiler has no way to ensure the field will be initialized before it's accessed. To prevent the program from getting into a bad state by using an uninitialized variable the compiler raises an error.

class A{
var abc : String // throws compile time error
fun myOwnedFun(){
var abcd : String // throws compile time error while accessing it
abcd. // throws error now, suggests you to initialize it.
}
}
This is because Kotlin is built as a null safe language, which means all the variables must be initialized before using it, either as nullable which should be suffixed with ? and assigned to null or with default constructor for any other classes, or use a lateinit var if you are sure that you will be assigning it somewhere before accessing it first.
In Android Kotlin, lateinit var is widely used for global variables which will be assigned in the lifecycle methods such as onCreate() etc.,

Managing Objects within Activities to avoid using Null

One of the benefits of using Kotlin is its Null safety. However when I have been programming Android Apps using it, I have found myself needing to use null. When declaring my UI elements such as TextViews and Buttons etc. I need to create private variables that are initialised to each object during the onCreate, but this means i need to explicitly allow null on each reference. This kind of defeats one of the purposes of using Kotlin. Is there a better solution, to creating instances of UI Objects within my activities in Android.
This is how I am doing it, at this moment.
var messageView: TextView? = null
var firstNameView: EditText? = null
var lastNameView: EditText? = null
var ageView: EditText? = null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
messageView = findViewById<TextView>(R.id.message)
firstNameView = findViewById<EditText>(R.id.firstName)
lastNameView = findViewById<EditText>(R.id.lastName)
ageView = findViewById<EditText>(R.id.age)
findViewById<Button>(R.id.showMessage).setOnClickListener(this)
findViewById<Button>(R.id.update).setOnClickListener(this)
}

Try defining these as lateinit, it should get you past the need to have them nullable if you can guarantee that you'll provide values before they are read.
lateinit var messageView: TextView
lateinit var firstNameView: EditText
lateinit var lastNameView: EditText
lateinit var ageView: EditText
From the documentation for lateinit:
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.

Kotlin - lateinit VS Any? = null

In Kotlin there appears to be two method of declaring a variable inside an object that can be null and instantiated after the object is created.
var myObject : Any? = null
or
var lateinit myObject : Any
I am confused about why the lateinit keyword is needed if we can just make the var nullable and assign it later. What are the pros and cons of each method and in what situation should each one be used?

Here is how I see the difference according to my current knowledge in Kotlin.
First one:
var myObject1 : Any? = null
Here myObject1 is a property that is nullable. That means you can assign null to it.
Second one:
lateinit var myObject2 : Any
Here myObject2 is a non-null property. That means you cannot assign null to it. Usually if a property is non-null you have to initialize it at the declaration. But adding the keyword lateinit allows you to postpone the initialization. If you try to access the lateinit property before initializing it then you get an exception.
In short the main difference is that myObject1 is a nullable and myObject2 is a non-null. The keyword lateinit provide you a convenience mechanism to allow a non-null property to be initialize at a later time rather than initializing it at the declaration.
For more info check this.

lateinit keyword is used on a field to avoid null checks when referencing the field inside of your object. The keyword is mainly used when you are using dependency injection to initialize the variable, or initializing the variable in the setup method of a unit test
? is used on a field when the field will be initialized later in your program either by a setter or inside of a method of the object, this is to enforce you to check for null or use null safety(?.) when referencing the field

If your property should not be null, but just isn't set after some point in the future, it is safer to declare it with a lateinit keyword. That guarantees that, if you access it before it is set, you get an exception explaining exactly that.
The traditional Java way is to throw a generic NullPointerException with no explanation about it. If you wrote the code yourself, you might have a clue, but if someone else catches the error, it won't be clear why that particular variable is null.