How to check if a "lateinit" variable has been initialized?

There is a lateinit improvement in Kotlin 1.2 that allows to check the initialization state of lateinit variable directly:

lateinit var file: File    

if (this::file.isInitialized) { ... }

See the annoucement on JetBrains blog or the KEEP proposal.

UPDATE: Kotlin 1.2 has been released. You can find lateinit enhancements here:

Checking whether a lateinit var is initialized

Lateinit top-level properties and local variables

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Using .isInitialized property one can check initialization state of a lateinit variable.

if (::file.isInitialized) {
    // File is initialized
} else {
    // File is not initialized
}

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You can easily do this by:

::variableName.isInitialized

or

this::variableName.isInitialized

But if you are inside a listener or inner class, do this:

this@OuterClassName::variableName.isInitialized

Note: The above statements work fine if you are writing them in the same file(same class or inner class) where the variable is declared but this will not work if you want to check the variable of other class (which could be a superclass or any other class which is instantiated), for ex:

class Test {
    lateinit var str:String
}

And to check if str is initialized:

https://i.stack.imgur.com/f6wo6.png

What we are doing here: checking isInitialized for field str of Test class in Test2 class. And we get an error backing field of var is not accessible at this point. Check a question already raised about this.

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Try to use it and you will receive a UninitializedPropertyAccessException if it is not initialized.

lateinit is specifically for cases where fields are initialized after construction, but before actual use (a model which most injection frameworks use). If this is not your use case lateinit might not be the right choice.

EDIT: Based on what you want to do something like this would work better:

val chosenFile = SimpleObjectProperty<File?>
val button: Button

// Disables the button if chosenFile.get() is null
button.disableProperty.bind(chosenFile.isNull())

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If you have a lateinit property in one class and need to check if it is initialized from another class

if(foo::file.isInitialized) // this wouldn't work

The workaround I have found is to create a function to check if the property is initialized and then you can call that function from any other class.

Example:

class Foo() {

    private lateinit var myFile: File

    fun isFileInitialised() = ::file.isInitialized
}

 // in another class
class Bar() {

    val foo = Foo()

    if(foo.isFileInitialised()) // this should work
}

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This will work

if (::list.isInitialized) {
 //true
} 
else {
//false
}

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Accepted answer gives me a compiler error in Kotlin 1.3+, I had to explicitly mention the this keyword before ::. Below is the working code.

lateinit var file: File

if (this::file.isInitialized) {

    // file is not null
}

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To check if a lateinit var were initialised or not use a .isInitialized on the reference to that property:

if (foo::bar.isInitialized) {
    println(foo.bar)
}

This checking is only available for the properties that are accessible lexically, i.e. declared in the same type or in one of the outer types, or at top level in the same file.

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kotlin.UninitializedPropertyAccessException: lateinit property clientKeypair has not been initialized

Bytecode says...blah blah..

public final static synthetic access$getClientKeypair$p(Lcom/takharsh/ecdh/MainActivity;)Ljava/security/KeyPair;

`L0
LINENUMBER 11 L0
ALOAD 0
GETFIELD com/takharsh/ecdh/MainActivity.clientKeypair : Ljava/security/KeyPair;
DUP
IFNONNULL L1
LDC "clientKeypair"
INVOKESTATIC kotlin/jvm/internal/Intrinsics.throwUninitializedPropertyAccessException (Ljava/lang/String;)V
    L1
ARETURN

L2 LOCALVARIABLE $this Lcom/takharsh/ecdh/MainActivity; L0 L2 0 MAXSTACK = 2 MAXLOCALS = 1

Kotlin creates an extra local variable of same instance and check if it null or not, if null then throws 'throwUninitializedPropertyAccessException' else return the local object. Above bytecode explained here Solution Since kotlin 1.2 it allows to check weather lateinit var has been initialized or not using .isInitialized

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