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.
Related
I was following the google codelab. There I came across the following code:
class Calculator: Fragment() {
private var _binding: FragmentCalculatorBinding? = null
private val binding get() = _binding!!
}
What is the need for the get()?
We can do this in the following way:
class Calculator: Fragment() {
private var _binding: FragmentCalculatorBinding? = null
private val binding = _binding!!
}
The explaination given there was:
Here, get() means this property is "get-only". That means you can get the value, but once assigned (as it is here), you can't assign it to something else.
but I don't understand it. Please help me with this.
The whole point in having a second property here is to allow the first property to be set back to null. (The second property is for convenience and should only be used when the fragment is known to be attached to an Activity.) Using a getter means it does the evaluation _binding!! each time it is accessed. Without get() it evaluates it once when the class is instantiated and assigns the result to a backing field. Since _binding is null at class instantiation time, this would be guaranteed to fail. And even if it didn’t fail, it would have an outdated reference if the fragment got detached and reattached.
Your description of what “get only” means is inverted. Either the code lab got their explanation backwards or you paraphrased it backwards.
In this case, you don't. It's very messy in my opinion.
Simply use:
private lateinit var binding: FragmentCalculatorBinding
It's does the same thing as your existing code - throws exception if you use the variable before instantiate it.
Keep in mind that you must instantiate it before using it (the whole point of lateinit var).
If you use it like this, you have a very explicit way of setting binding - you have to do it using _binding (which would probably just happen once in onCreateView(). For all the places where you actually use it, you would use binding, which both checks if _banding has actually been set, and cannot accidentally be changed as it is immutable, thereby saving you from potential bugs.
That being said - I would probably go with a lateinit var as well, and lose all the null checking, but that is a matter of personal taste I think.
using get() after the variable name makes it assign to the value when we each time trying to access this variable
as mentioned above without get() the variable will be assigned immediately when this class instantiated
I was declared a binding object in a fragment as lateinit var binding: EditInsuranceDialogBinding but a colleague said "This is a bad practice and Binding object should be Optional . "
So in the declaration will be instead like this: var binding: EditInsuranceDialogBinding? = null, initialised in the onCreateContentView and make it null in the onDestroyView
I want to know what is the best to choose the type of Binding (optional or not)? and does lateinit cost a lot in the compiler and memory ? When we should NOT choose lateinit and when we should use it?
but a colleague said "This is a bad practice and Binding object should be Optional . "
It is possible that your colleague really meant that the binding object should be wrapped in Optional.
I want to know what is the best to choose the type of Binding (optional or not)?
lateinit var is not evil. However, it is not ideal for all circumstances.
In this case, a binding object has a specific lifecycle, and we need to stop using it after onDestroyView(). If you declare the property as:
private lateinit var binding: EditInsuranceDialogBinding
...then you have no way of setting it to something after onDestroyView() that says "we do not have a valid binding". It is easy to wind up having code in a fragment that runs after onDestroyView(), and that code needs to know that it is not safe to use the binding. There is no way to create an EditInsuranceDialogBinding instance that represents the "not safe to use the binding" state.
The alternative that you chose is reasonable:
private var binding: EditInsuranceDialogBinding? = null
...where you set binding to null in onDestroyView().
You could also go with:
private var binding: Optional<EditInsuranceDialogBinding> = Optional.empty()
...where you set binding back to Optional.empty() in onDestroyView(). There are also custom binding delegates, such as this one, that you could use.
and does lateinit cost a lot in the compiler and memory ?
No.
When we should NOT choose lateinit and when we should use it?
I try to only use lateinit when I am very certain that I will initialize it before use.
This one is not bad approach. We use lateinit when we guarantee the initialization before using it. lateinit doesn't allocate the memory until it is initialized.
It is initialized only once. Next time when you use it, you get the value from memory where it is used for the first time.
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.
Why can't we use lateinit with nullable variables?
lateinit var v: String?
lateinit modifier is not allowed on properties of nullable types
lateinit is only for avoid null checks in future, that's why lateinit modifier is not allowed on properties of nullable types.
If you want it to be nullable then simply you can use like var b: String? = null
If you want to make a variable of nullable type then you don't need late init . The doc says
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 nonnull
initializer in the constructor, but you still want to avoid null checks when referencing the
property inside the body of a class.
So late init is intended to be used when you intend to initialize a variable somewhere not in the constructor and also want to avoid null checks.
As stated in the documentation, lateinit specializes on non-nullable properties:
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.
Also, if you take a look at the byte code of such a lateinit property, you see that the compiler adds a code block to ensure that this property has been initialized when being accessed. For lateinit properties, null indicates the properties initial but invalid state.
class WithLateInit {
lateinit var something : String
}
becomes
public final class WithLateInit {
#NotNull
public String something;
#NotNull
public final String getSomething() {
String var10000 = this.something;
if (var10000 == null) { // <- here you can see the lateinit check
Intrinsics.throwUninitializedPropertyAccessException("something");
}
return var10000;
}
//setter
}
Kotlin's type system is aimed at eliminating the danger of null references from code
so both represent compromises. With lateinit you ensure that the variable will eventually be initialized to non-null. If you cannot even guarantee that, you can use nullable.
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.
That's why it doesn't support null.
So, if you indicate any var as lateinit meaning compiler simply ignores it for initialization and mark it as non-null type that would be initialized in nearer future and is why it doesn't support nullable type to avoid runtime ambiguities.
One of the main feature in kotlin is Null Safe.
By default it won't allow u to create a null value. You have explicitly defined
var a: String? = null
, If u don't want to initialise any value to the variable there come "lateinit". While using lateinit variable You do pre-check whether it is initialised or not
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.