Data types and type inference in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with type inference holding the number 10.

Android Kotlin

val number = [1]

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

A10

B"10"

C10.0

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Putting 10 in quotes makes it a string, not a number.

Using 10.0 makes it a Double, not an Int.

The number 10 is an integer, so just writing 10 lets Kotlin infer the type Int.

2fill in blank

medium

Complete the code to explicitly declare a variable of type Double with value 3.14.

Android Kotlin

val pi: [1] = 3.14

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

AInt

BDouble

CString

DBoolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using Int will cause a type mismatch error.

Using String or Boolean is incorrect for numbers.

The value 3.14 is a decimal number, so the type should be Double.

3fill in blank

hard

Fix the error in the code by choosing the correct type for the variable holding true.

Android Kotlin

val isActive: [1] = true

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

AInt

BString

CBoolean

DDouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using Int or Double causes type mismatch errors.

Using String is incorrect because true is not text.

The value true is a boolean, so the variable type must be Boolean.

4fill in blank

hard

Fill both blanks to declare a mutable variable of type String with value "Hello".

Android Kotlin

var greeting: [1] = [2]

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

AString

B"Hello"

CInt

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Int or Boolean types for text variables.

Not putting the string value in quotes.

The variable greeting should be a String and assigned the text "Hello".

5fill in blank

hard

Fill all three blanks to create a read-only variable with type inferred as Int and value 42.

Android Kotlin

val [1] = [2] // inferred type is [3]

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

Aanswer

B42

CInt

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string or boolean value instead of a number.

Choosing a variable name that is not descriptive.

The variable name is answer, assigned the number 42, which Kotlin infers as type Int.

Practice

1. Which keyword in Kotlin declares a variable whose value cannot be changed after assignment?

easy

A. val

B. var

C. let

D. const

Solution

1. Step 1: Understand Kotlin variable keywords

   Kotlin uses val for immutable variables and var for mutable ones.

2. Step 2: Identify the immutable keyword

   val means the value cannot be changed after assignment.

3. Final Answer:

   val -> Option A

4. Quick Check:

   Immutable variable keyword = val [OK]

Hint: Remember: val = fixed, var = changeable [OK]

Common Mistakes:

• Confusing val and var
• Using let which is not a Kotlin keyword
• Thinking const is used like val

2. Which of the following is the correct way to declare an integer variable with value 10 using type inference in Kotlin?

easy

A. val number: Int = 10

B. val number = 10.0

C. var number = "10"

D. val number = 10

Solution

1. Step 1: Understand type inference in Kotlin

   Kotlin can guess the type from the assigned value, so val number = 10 infers Int.

2. Step 2: Check each option

   val number: Int = 10 declares type explicitly, which is correct but not type inference. val number = 10 uses type inference correctly. var number = "10" assigns a String, not Int. val number = 10.0 assigns a Double, not Int.

3. Final Answer:

   val number = 10 -> Option D

4. Quick Check:

   Type inference assigns Int from 10 [OK]

Hint: Omit type to let Kotlin infer it [OK]

Common Mistakes:

• Adding explicit type when not needed
• Assigning wrong data type
• Confusing Int and Double

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

val x = 5
val y = 2
val result = x / y
println(result)

medium

A. 2.0

B. 2.5

C. 2

D. Compilation error

Solution

1. Step 1: Identify variable types and operation

   Both x and y are Int by type inference. Integer division truncates decimals.

2. Step 2: Calculate integer division

   5 / 2 = 2 (decimal part discarded).

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Integer division truncates decimals = 2 [OK]

Hint: Integer division drops decimals in Kotlin [OK]

Common Mistakes:

• Expecting floating point result
• Using variables as Double without declaration
• Thinking println shows decimal automatically

4. Identify the error in this Kotlin code snippet:

val name = "Alice"
name = "Bob"

medium

A. Missing semicolon after first line

B. Cannot reassign a val variable

C. String values must use single quotes

D. Variable name is a reserved keyword

Solution

1. Step 1: Check variable declaration

   val name = "Alice" declares an immutable variable.

2. Step 2: Check reassignment

   Trying to assign "Bob" to name causes error because val cannot be reassigned.

3. Final Answer:

   Cannot reassign a val variable -> Option B

4. Quick Check:

   val variables are immutable [OK]

Hint: val means no reassignment allowed [OK]

Common Mistakes:

• Thinking val allows reassignment
• Confusing quotes for strings
• Assuming semicolons are mandatory

5. Given this Kotlin code, what is the inferred type of result?

val a = 3
val b = 4.5
val result = a + b

hard

A. Double

B. Int

C. Float

D. String

Solution

1. Step 1: Identify types of operands

   a is Int, b is Double (4.5 is a Double literal).

2. Step 2: Understand type promotion in arithmetic

   When adding Int and Double, Int is promoted to Double, so result is Double.

3. Final Answer:

   Double -> Option A

4. Quick Check:

   Int + Double results in Double [OK]

Hint: Mixed numeric types promote to the wider type [OK]

Common Mistakes:

• Assuming result is Int
• Confusing Float and Double
• Thinking result is String