Kotlin

What is the difference between val and var in Kotlin?

• Variables declared with var are final, those with val are not.

• Variables declared with val are final, those with var are not.

• Variables declared with val can only access const members

• var is scoped to the nearest function block and val is scoped to the nearest enclosing block

Answer explanation

https://kotlinlang.org/docs/reference/basic-syntax.html#defining-variables

What does a data class not offer?

• A generated copy(.. .) method, to create copies of instances

• Automatic conversion from/to JSON

• Auto-generated hashCode() and equals() methods

• An auto-generated toString() method

Answer explanation

Data classes are a very handy feature of Kotlin to create classes whose main purpose is to hold data, with the Kotlin compiler automatically creating methods like equals()/hashCode(), toString(), copy(...) and more. Automatic conversion to/from JSON is not (yet) part of this.

Which of these targets does Kotlin currently not support?

• .NET CLR

• LLVM

• JavaScript

• JVM

Answer explanation

Kotlin can currently be compiled to both JVM bytecode, JavaScript and to native code (LLVM). The .NET CLR is not (yet) officially supported.

What are Kotlin coroutines?

• These are functions which accept other functions as arguments or return them

• They provide asynchronous code without thread blocking

• That's how the automatically generated methods hashCode() and equals() in data classes are called

• It's Kotlin's term for class methods

Answer explanation

Coroutines are a way to run code asynchronously without the usual thread blocking.

https://kotlinlang.org/docs/reference/coroutines.html

What is 'to' in the example below:

• An infix extension function creating a Pair(33, 42)

• A syntax error

• A Kotlin keyword to create a Range from 33 to 42

• A Kotlin keyword to create a Pair(33, 42)

Answer explanation

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/to.html

What is the correct way to declare a variable of integer type in Kotlin?

• int i = 42

• var i : int = 42

• let i = 42

• var i : Int = 42

Answer explanation

Variables in Kotlin are declared with the var (variable) or val (value) keywords, the latter making it final, followed by a : and the type. As Kotlin does not have primitive types, Int is correct while int leads to an error. When also defining a variable as in the example, you may often omit the type and let the compiler infer it. So var i = 42 would be fine, too.

Which is true for the following simple class declaration?

• It has a private property "name"

• It can be extended by other classes

• It is public

• It is package-private

Answer explanation

Classes in Kotlin are public and final by default. To allow other classes to extend it, the open modifier is used. Declared properties are public by default.

What is the correct syntax to convert the String "42" to a Long in Kotlin?

• 
• 
• 
• 

Answer explanation

Thanks to extension functions, Kotlin is able to extend the String class with the toLong() (and similar) functions, which internally just calls java.lang.Long.parseLong(this).

Does Kotlin have primitive data types, such as int, long, float?

• No, Kotlin does not have nor use primitive data types

• Yes, Kotlin is similar to Java in this respect

• Yes, but Kotlin internally always converts them to their non-primitive counterparts

• No, not at language level. But the Kotlin compiler makes use of JVM primitives for best performance

Answer explanation

Kotlin gets rid of the difference between Java's primitives and their object counterparts at language level. Internally, the Kotlin compiler creates byte code which does make use of primitives where possible for best performance.

What does the following code print?

• Unit

• Nothing, there is a compile error.

• true

• [1, 2, 3, 4]

Answer explanation

Just like in Java, the add method of the List interface returns a boolean, indicating whether the Collection has been changed by the add operation - which always is true for Lists. One might have guessed that Kotlin instead returns a new List with the element appended, which is not the case. Due to type inference this is not obvious. If you would rather append an element to a list, you can write this val listB = listA + 4, making use of the extension + operator of Collection.