Multiple type parameters in Kotlin - Mini Project: Build & Apply

Using Multiple Type Parameters in Kotlin

📖 Scenario: Imagine you are building a simple app to store pairs of related information, like a person's name and their age, or a product and its price.

🎯 Goal: You will create a generic class in Kotlin that uses two type parameters to hold pairs of values of different types. Then, you will create an instance of this class and print its contents.

📋 What You'll Learn

Create a generic class called PairBox with two type parameters: A and B

Add two properties to PairBox: first of type A and second of type B

Create a variable called person that is an instance of PairBox holding a String and an Int

Print the values of person.first and person.second in a friendly sentence

💡 Why This Matters

🌍 Real World

Generic classes with multiple type parameters are useful when you want to store or work with pairs or groups of related data of different types, like key-value pairs or coordinate points.

💼 Career

Understanding generics and multiple type parameters is important for writing reusable and type-safe code in Kotlin, which is widely used in Android app development and backend services.

1

Create the generic class with two type parameters

Create a generic class called PairBox with two type parameters A and B. Inside the class, declare two properties: first of type A and second of type B.

Kotlin

# Your code here

Hint

Use the syntax class PairBox(val first: A, val second: B) to define the class.

2

Create an instance of PairBox

Create a variable called person and assign it an instance of PairBox with String and Int as type arguments. Use the values "Alice" for first and 30 for second.

Kotlin

class PairBox(val first: A, val second: B)

# Your code here

Hint

Use val person = PairBox("Alice", 30) to create the instance.

3

Access the properties of the PairBox instance

Write a variable called description that uses string interpolation to create a sentence: "Name: person.first, Age: person.second".

Kotlin

class PairBox(val first: A, val second: B)

val person = PairBox("Alice", 30)

# Your code here

Hint

Use val description = "Name: ${person.first}, Age: ${person.second}" to create the sentence.

4

Print the description

Write a println statement to print the variable description.

Kotlin

class PairBox(val first: A, val second: B)

val person = PairBox("Alice", 30)

val description = "Name: ${person.first}, Age: ${person.second}"

# Your code here

Hint

Use println(description) to show the sentence on the screen.

Practice

1. What does it mean when a Kotlin function has <T, U> before its name?

easy

A. The function is limited to only one type parameter named T or U.

B. The function can only accept types T or U but not both at the same time.

C. The function uses two different type parameters, T and U, to work with multiple types.

D. The function automatically converts types T and U to String.

Solution

1. Step 1: Understand type parameters in Kotlin

   Type parameters like <T, U> allow a function to accept multiple types, making it flexible.

2. Step 2: Interpret the meaning of <T, U>

   Having <T, U> means the function can work with two different types, T and U, independently.

3. Final Answer:

   The function uses two different type parameters, T and U, to work with multiple types. -> Option C

4. Quick Check:

   Multiple type parameters = flexible function types [OK]

Hint: Multiple type parameters mean multiple flexible types [OK]

Common Mistakes:

• Thinking it limits to only one type at a time
• Assuming automatic type conversion happens
• Confusing type parameters with actual types

2. Which of the following is the correct syntax to declare a Kotlin function with two type parameters named T and U?

easy

A. fun myFunction(param1: T, param2: U) {}

B. fun myFunction(param1: T, param2: U) {}

C. fun myFunction(param1: T, param2: U) {}

D. fun myFunction(param1: T, param2: U) {}

Solution

1. Step 1: Recall Kotlin generic function syntax

   Type parameters are declared immediately after the fun keyword inside angle brackets, separated by commas.

2. Step 2: Check each option's syntax

   fun myFunction(param1: T, param2: U) {} correctly uses <T, U> after fun and before the function name, matching Kotlin syntax.

3. Final Answer:

   fun <T, U> myFunction(param1: T, param2: U) {} -> Option A

4. Quick Check:

   Type parameters go after fun with commas [OK]

Hint: Type parameters come right after fun, separated by commas [OK]

Common Mistakes:

• Missing comma between type parameters
• Placing type parameters after function name
• Putting type parameters after parameter list

3. What is the output of this Kotlin code?

fun <T, U> pairToString(pair: Pair<T, U>): String {
    return "${pair.first} and ${pair.second}"
}

fun main() {
    val result = pairToString(Pair(10, "cats"))
    println(result)
}

medium

A. Compilation error

B. Pair(10, cats)

C. cats and 10

D. 10 and cats

Solution

1. Step 1: Understand the function behavior

   The function takes a Pair with types T and U, then returns a string combining first and second values.

2. Step 2: Trace the main function call

   pairToString is called with Pair(10, "cats"), so pair.first = 10 and pair.second = "cats".

3. Final Answer:

   10 and cats -> Option D

4. Quick Check:

   pair.first + " and " + pair.second = "10 and cats" [OK]

Hint: Read pair.first and pair.second values carefully [OK]

Common Mistakes:

• Swapping first and second values
• Expecting Pair's default toString output
• Thinking it causes a compilation error

4. Identify the error in this Kotlin function declaration:

fun <T, U> combine(a: T, b: U): T {
    return b
}

medium

A. Return type T does not match returned value b of type U.

B. Missing comma between T and U in type parameters.

C. Function must return type U instead of T.

D. No error, the function is correct.

Solution

1. Step 1: Check function return type and return statement

   The function declares return type T but returns b which is of type U.

2. Step 2: Understand type mismatch error

   Returning U where T is expected causes a type mismatch compilation error.

3. Final Answer:

   Return type T does not match returned value b of type U. -> Option A

4. Quick Check:

   Return type must match returned value type [OK]

Hint: Return type must match the returned value type [OK]

Common Mistakes:

• Ignoring type mismatch between T and U
• Assuming Kotlin auto-converts types
• Missing comma in type parameters (not the case here)

5. You want to create a Kotlin generic class Box that can hold two different types of items, T and U. Which of the following class declarations correctly uses multiple type parameters and allows storing and retrieving both items?

hard

A. class Box<T>(val first: T, val second: T) {
fun getFirst(): T = first
fun getSecond(): T = second
}

B. class Box(val first: T, val second: U) {
fun getFirst(): T = first
fun getSecond(): U = second
}

C. class Box {
val first: T
val second: U
}

D. class Box(val first: T, val second: U) {
fun getFirst(): T = first
fun getSecond(): U = second
}

Solution

1. Step 1: Check correct multiple type parameter declaration

   class Box(val first: T, val second: U) { fun getFirst(): T = first fun getSecond(): U = second } declares class Box with <T, U> and constructor parameters of types T and U, which is correct.

2. Step 2: Verify methods to retrieve both items

   class Box(val first: T, val second: U) { fun getFirst(): T = first fun getSecond(): U = second } provides getFirst() returning T and getSecond() returning U, matching stored types.

3. Final Answer:

   class Box<T, U>(val first: T, val second: U) { fun getFirst(): T = first fun getSecond(): U = second } -> Option B

4. Quick Check:

   Multiple type parameters with matching properties and methods [OK]

Hint: Use and matching properties for both types [OK]

Common Mistakes:

• Using only one type parameter for both items
• Not declaring type parameters in class header
• Using undefined types without generics