Explicit type declaration in Kotlin - Mini Project: Build & Apply

Explicit type declaration

📖 Scenario: You are working on a simple Kotlin program that stores information about a book. You want to practice explicitly declaring variable types to make your code clear and easy to understand.

🎯 Goal: Create variables with explicit type declarations for a book's title, author, and number of pages. Then print these details.

📋 What You'll Learn

Create a variable title with type String and value "The Hobbit"

Create a variable author with type String and value "J.R.R. Tolkien"

Create a variable pages with type Int and value 310

Print the book details in the format: Title: The Hobbit, Author: J.R.R. Tolkien, Pages: 310

💡 Why This Matters

🌍 Real World

Explicit type declarations help make code easier to read and maintain, especially in larger projects where knowing the exact type of data is important.

💼 Career

Many Kotlin jobs require clear and maintainable code. Knowing how to declare explicit types is a fundamental skill for writing professional Kotlin applications.

1

Create variables with explicit types

Create a variable called title with type String and value "The Hobbit".

Kotlin

# Your code here

Hint

Use val to declare a read-only variable and specify the type after a colon.

2

Add more variables with explicit types

Create a variable called author with type String and value "J.R.R. Tolkien".
Create a variable called pages with type Int and value 310.

Kotlin

val title: String = "The Hobbit"
# Your code here

Hint

Remember to declare the type explicitly for both variables.

3

Create a formatted string with explicit type

Create a variable called details with type String that combines title, author, and pages into the format: "Title: The Hobbit, Author: J.R.R. Tolkien, Pages: 310".

Kotlin

val title: String = "The Hobbit"
val author: String = "J.R.R. Tolkien"
val pages: Int = 310
# Your code here

Hint

Use string templates with $ to insert variable values inside the string.

4

Print the book details

Print the variable details to display the book information.

Kotlin

val title: String = "The Hobbit"
val author: String = "J.R.R. Tolkien"
val pages: Int = 310
val details: String = "Title: $title, Author: $author, Pages: $pages"
# Your code here

Hint

Use println(details) to show the book details on the screen.

Practice

1. What does explicit type declaration in Kotlin do?

easy

A. It automatically converts types without programmer input.

B. It tells Kotlin exactly what type a variable or value has.

C. It removes the need to use val or var.

D. It makes variables changeable by default.

Solution

1. Step 1: Understand explicit type declaration

   Explicit type declaration means telling Kotlin the exact type of a variable, like Int or String.

2. Step 2: Compare with other options

   The other options describe behaviors that are not related to explicit type declaration.

3. Final Answer:

   It tells Kotlin exactly what type a variable or value has. -> Option B

4. Quick Check:

   Explicit type declaration = telling exact type [OK]

Hint: Explicit means you say the type clearly [OK]

Common Mistakes:

• Thinking Kotlin guesses types without declaration
• Confusing explicit type with variable mutability
• Believing explicit type removes need for val/var

2. Which of the following is the correct syntax for explicit type declaration of a variable in Kotlin?

easy

A. var age: Int = 25

B. val height = Int 170

C. val name String = "Alice"

D. var weight = 70: Int

Solution

1. Step 1: Recall Kotlin variable declaration syntax

   Kotlin uses val or var followed by variable name, colon, type, then assignment.

2. Step 2: Check each option

   var age: Int = 25 matches this syntax: var age: Int = 25. Others have syntax errors.

3. Final Answer:

   var age: Int = 25 -> Option A

4. Quick Check:

   Correct syntax uses colon before type [OK]

Hint: Use colon (:) before type in declaration [OK]

Common Mistakes:

• Omitting colon before type
• Placing type after value
• Using wrong order of keywords

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

val number: Int = 10
var text: String = "Hello"
text = "Hi"
println(number)
println(text)

medium

A. Error: Cannot reassign val

B. 10\nHello

C. 10\nHi

D. 10\nnull

Solution

1. Step 1: Analyze variable declarations and assignments

   number is a val (fixed) with value 10. text is a var (changeable) initially "Hello", then changed to "Hi".

2. Step 2: Determine printed output

   Printing number outputs 10. Printing text outputs the updated value "Hi".

3. Final Answer:

   10 Hi -> Option C

4. Quick Check:

   val fixed, var changeable = 10 and Hi [OK]

Hint: val is fixed, var can change value [OK]

Common Mistakes:

• Thinking val can be reassigned
• Confusing initial and updated values
• Expecting syntax error for reassignment of var

4. Identify the error in this Kotlin code snippet:

val count: Int = 5
count = 10

medium

A. Missing type declaration for count

B. Variable count should be declared with var

C. Incorrect type Int assigned to count

D. Cannot reassign a value to val variable

Solution

1. Step 1: Understand val and var difference

   val means the variable is read-only and cannot be reassigned after initialization.

2. Step 2: Check the reassignment

   The code tries to assign 10 to count which is declared as val, causing an error.

3. Final Answer:

   Cannot reassign a value to val variable -> Option D

4. Quick Check:

   val is fixed, reassignment causes error [OK]

Hint: val variables cannot be reassigned [OK]

Common Mistakes:

• Thinking val variables can be changed
• Confusing val and var keywords
• Ignoring reassignment rules

5. You want to declare a variable score that can change and must hold decimal numbers. Which explicit type declaration is correct in Kotlin?

hard

A. var score: Double = 95.5

B. var score: Int = 95.5

C. val score: Double = 95.5

D. val score: Float = "95.5"

Solution

1. Step 1: Identify mutability requirement

   The variable score must be changeable, so use var, not val.

2. Step 2: Choose correct type for decimal numbers

   Decimal numbers require Double or Float, but value 95.5 is a Double by default.

3. Step 3: Check options

   var score: Double = 95.5 uses var and Double with correct value. Others either use val, wrong type, or wrong value type.

4. Final Answer:

   var score: Double = 95.5 -> Option A

5. Quick Check:

   var + Double for changeable decimal [OK]

Hint: Use var for changeable, Double for decimals [OK]

Common Mistakes:

• Using val when variable should change
• Assigning decimal to Int type
• Putting string value for numeric type