Why generics provide type safety in Kotlin - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic class that holds a value of any type.

Kotlin

class Box<[1]>(val value: T)

Drag options to blanks, or click blank then click option'

AType

BAny

CT

DValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete type like Any instead of a generic parameter.

The generic type parameter is declared using T inside angle brackets.

2fill in blank

medium

Complete the function signature to make it generic over type T.

Kotlin

fun <[1]> printValue(value: T) { println(value) }

Drag options to blanks, or click blank then click option'

AT

BValue

CAny

DType

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to declare the generic type parameter.

Generic functions declare their type parameter before the function name using <T>.

3fill in blank

hard

Fix the error by specifying the correct generic type in the list declaration.

Kotlin

val numbers: List<[1]> = listOf(1, 2, 3)

Drag options to blanks, or click blank then click option'

ANumber

BString

CAny

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using String or Any which do not match the list elements.

The list contains integers, so the generic type must be Int for type safety.

4fill in blank

hard

Fill both blanks to create a generic function that returns the first element of a list.

Kotlin

fun <[1]> firstElement(list: List<[2]>): [1]? = list.firstOrNull()

Drag options to blanks, or click blank then click option'

AT

BInt

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types for the list and return value.

The function uses the same generic type T for the parameter and return type to ensure type safety.

5fill in blank

hard

Fill all three blanks to create a generic class with a method that returns the stored value.

Kotlin

class Container<[1]>(private val item: [2]) {
    fun getItem(): [3] = item
}

Drag options to blanks, or click blank then click option'

AT

DAny

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types for the property and return type.

The generic type T is used consistently to ensure the class stores and returns the same type safely.

Practice

1. Why do generics in Kotlin help provide type safety?

easy

A. They automatically convert types at runtime.

B. They allow any type without restrictions.

C. They make the program run faster.

D. They check types at compile time to prevent errors.

Solution

1. Step 1: Understand generics role in type checking

   Generics allow specifying the type a class or function works with, so the compiler can check if the correct type is used.

2. Step 2: Identify how this prevents errors

   By checking types early, generics prevent mixing incompatible types, avoiding bugs before running the program.

3. Final Answer:

   They check types at compile time to prevent errors. -> Option D

4. Quick Check:

   Generics = Compile-time type checks [OK]

Hint: Generics catch wrong types before running code [OK]

Common Mistakes:

• Thinking generics allow any type without checks
• Believing generics speed up runtime
• Confusing compile-time and runtime type checks

2. Which of the following is the correct syntax to declare a generic class in Kotlin?

easy

A. class Box { var value: T }

B. class Box<T> { var value: T }

C. class Box<T> { var value: Int }

D. class Box<T> (value: T) : T

Solution

1. Step 1: Review generic class syntax

   In Kotlin, generic classes declare a type parameter inside angle brackets after the class name, like class Box<T>.

2. Step 2: Check property type usage

   The property inside should use the generic type T, not a fixed type like Int.

3. Final Answer:

   class Box<T> { var value: T } -> Option B

4. Quick Check:

   Generic class syntax = class Box<T> { var value: T } [OK]

Hint: Generic type goes in angle brackets after class name [OK]

Common Mistakes:

• Using fixed types instead of generic type parameter
• Omitting the generic type parameter declaration
• Incorrect inheritance syntax with generics

3. What will be the output of this Kotlin code?

fun main() {
    val list: List<String> = listOf("apple", "banana")
    val first: String = list[0]
    println(first.length)
}

medium

A. Syntax error

B. Runtime error

C. 5

D. 0

Solution

1. Step 1: Understand the list type and access

   The list is declared as List<String>, so it only holds strings. Accessing list[0] returns "apple".

2. Step 2: Calculate the length of the first string

   "apple" has 5 characters, so first.length is 5, which is printed.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   "apple" length = 5 [OK]

Hint: Check type and value before output [OK]

Common Mistakes:

• Confusing list element type causing errors
• Expecting runtime error due to generics
• Miscounting string length

4. Identify the error in this Kotlin code using generics:

fun printFirst(list: List<Int>) {
    val first = list[0]
    println(first.length)
}

medium

A. Cannot call length on Int type

B. List<Int> cannot be indexed

C. Missing generic type declaration

D. Function return type is missing

Solution

1. Step 1: Analyze the list element type

   The list holds Int values, so first is an integer.

2. Step 2: Check the method called on the element

   Calling length on an integer is invalid because Int does not have a length property.

3. Final Answer:

   Cannot call length on Int type -> Option A

4. Quick Check:

   Int has no length property [OK]

Hint: Check methods valid for the generic type used [OK]

Common Mistakes:

• Assuming all types have length property
• Thinking indexing List<Int> is invalid
• Ignoring type mismatch errors

5. How do generics prevent mixing wrong types in Kotlin collections? Consider this code:

val numbers: MutableList<Number> = mutableListOf(1, 2.5, 3)
numbers.add(4.0f)
numbers.add("five")

What happens and why?

hard

A. Compilation error on adding "five" because it's not a Number

B. All values added successfully, no errors

C. Runtime error when adding 4.0f

D. Compilation error on adding 4.0f because it's a Float

Solution

1. Step 1: Understand the collection type

   The list is declared as MutableList<Number>, so it accepts any subtype of Number like Int, Double, Float.

2. Step 2: Check the added elements

   Adding 4.0f (a Float) is allowed because Float is a Number subtype. Adding "five" (a String) causes a compile-time error because String is not a Number.

3. Final Answer:

   Compilation error on adding "five" because it's not a Number -> Option A

4. Quick Check:

   Generics block wrong types at compile time [OK]

Hint: Only add types matching generic parameter [OK]

Common Mistakes:

• Thinking all types can be added to Number list
• Confusing runtime and compile-time errors
• Assuming Float is not a Number subtype