Any type as universal base in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of casting Any to specific types

What is the output of this Kotlin code snippet?

Kotlin

fun main() {
    val value: Any = 42
    when (value) {
        is String -> println("String: $value")
        is Int -> println("Int: $value")
        else -> println("Unknown type")
    }
}

A String: 42

B Int: 42

C Unknown type

D Compilation error

💡 Hint

Remember that 'value' is assigned an Int but typed as Any.

The variable 'value' is of type Any but holds an Int value 42. The 'when' expression checks the actual type at runtime. Since 'value' is an Int, it matches the 'is Int' branch and prints 'Int: 42'.

🧠 Conceptual

intermediate

Why is Any the universal base type in Kotlin?

Which statement best explains why Kotlin uses 'Any' as the universal base type?

A Because all Kotlin classes inherit from 'Any', allowing any value to be stored in variables of type 'Any'.

B Because 'Any' is used only for nullable types.

C Because 'Any' is an interface that all classes implement explicitly.

D Because 'Any' can hold only primitive types like Int and Double.

💡 Hint

Think about inheritance and type hierarchy in Kotlin.

'Any' is the root of the Kotlin class hierarchy. All classes inherit from 'Any' implicitly, so variables of type 'Any' can hold any non-null object.

🔧 Debug

advanced

Identify the runtime error with Any casting

What runtime error will this Kotlin code produce?

Kotlin

fun main() {
    val obj: Any = "Hello"
    val num: Int = obj as Int
    println(num)
}

A Prints 'Hello' without error

B Compilation error due to invalid cast

C ClassCastException at runtime

D Prints 0 due to default conversion

💡 Hint

Casting a String to Int directly is unsafe.

The code tries to cast a String object to Int using 'as'. This compiles but fails at runtime with ClassCastException because the actual object is not an Int.

📝 Syntax

advanced

Which code snippet correctly checks type of Any?

Which of the following Kotlin snippets correctly checks if a variable of type Any holds a String and prints it?

A if (obj is String()) println(obj) else println("Not a String")

B if (obj instanceof String) println(obj) else println("Not a String")

C if (obj.type == String) println(obj) else println("Not a String")

D if (obj is String) println(obj) else println("Not a String")

💡 Hint

Remember Kotlin uses 'is' for type checking.

Kotlin uses 'is' keyword to check type at runtime. Option D uses correct syntax. Option D uses Java syntax 'instanceof' which is invalid in Kotlin. Option D and D are invalid syntax.

🚀 Application

expert

Result of storing different types in a List

What will be the output of this Kotlin program?

Kotlin

fun main() {
    val items: List<Any> = listOf(10, "Kotlin", 3.14, true)
    for (item in items) {
        when (item) {
            is Int -> print("I")
            is String -> print("S")
            is Double -> print("D")
            is Boolean -> print("B")
            else -> print("?")
        }
    }
}

A ISDB

B IDSB

C ISD?

D Compilation error

💡 Hint

Check the type of each element in the list and the corresponding print letter.

The list contains Int(10), String("Kotlin"), Double(3.14), and Boolean(true). The when expression prints 'I' for Int, 'S' for String, 'D' for Double, and 'B' for Boolean. So the output is 'ISDB'.

Practice

1. In Kotlin, which type is the universal base type that can hold any kind of value?

easy

A. Universal

B. Object

C. Base

D. Any

Solution

1. Step 1: Understand Kotlin's type hierarchy

   Kotlin uses Any as the root of its type system, similar to Object in Java.

2. Step 2: Identify the universal base type

   Any can hold any non-null value, making it the universal base type in Kotlin.

3. Final Answer:

   Any -> Option D

4. Quick Check:

   Universal base type = Any [OK]

Hint: Remember: Kotlin's root type is called Any [OK]

Common Mistakes:

• Confusing Any with Object (Java's base type)
• Thinking Base or Universal are Kotlin types
• Assuming Any can hold null (it cannot)

2. Which of the following is the correct way to declare a variable of type Any in Kotlin?

easy

A. val x: Any = 10

B. val x = Any(10)

C. val x: Any() = 10

D. val x: any = 10

Solution

1. Step 1: Review Kotlin variable declaration syntax

   Variables are declared with val or var, followed by a colon and the type.

2. Step 2: Check correct usage of Any type

   Any is a type, not a constructor, so you assign values directly without parentheses.

3. Final Answer:

   val x: Any = 10 -> Option A

4. Quick Check:

   Declare with colon and type name = val x: Any = 10 [OK]

Hint: Use colon for type, no parentheses for Any [OK]

Common Mistakes:

• Trying to call Any as a constructor
• Using lowercase 'any' instead of 'Any'
• Adding parentheses after type name

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

fun main() {
    val a: Any = "Hello"
    val b: Any = 123
    println(a)
    println(b)
}

medium

A. Hello
123

B. Any
Any

C. Compilation error

D. null
null

Solution

1. Step 1: Understand variable assignments

   Variable a holds a string "Hello" and b holds an integer 123, both typed as Any.

2. Step 2: Analyze the print statements

   Printing a and b calls their toString() method, outputting "Hello" and "123" respectively.

3. Final Answer:

   Hello 123 -> Option A

4. Quick Check:

   Any holds values, prints their string form [OK]

Hint: Any prints actual value's string, not type name [OK]

Common Mistakes:

• Expecting type name 'Any' to print
• Thinking it causes compile error
• Assuming null prints for Any

4. Identify the error in this Kotlin code snippet:

fun main() {
    val x: Any = null
    println(x)
}

medium

A. Missing semicolon after println

B. println cannot print Any type

C. Cannot assign null to a variable of type Any

D. Variable x must be declared as var, not val

Solution

1. Step 1: Check type nullability

   Any in Kotlin is non-nullable by default, so it cannot hold null.

2. Step 2: Identify assignment error

   Assigning null to x of type Any causes a compile-time error.

3. Final Answer:

   Cannot assign null to a variable of type Any -> Option C

4. Quick Check:

   Any is non-nullable, null assignment fails [OK]

Hint: Any disallows null; use Any? for nullable [OK]

Common Mistakes:

• Thinking println can't print Any
• Expecting semicolons in Kotlin
• Confusing val and var for nullability

5. Given a list of mixed types, how can you create a new list containing only the string representations of elements using Any as the base type?

val mixedList: List<Any> = listOf(1, "two", 3.0, true)

Which Kotlin code correctly produces List<String> with string forms of all elements?

hard

A. val stringList = mixedList.filterIsInstance<String>()

B. val stringList = mixedList.map { it.toString() }

C. val stringList = mixedList.map { it as String }

D. val stringList = mixedList.filter { it is String }

Solution

1. Step 1: Understand the goal

   We want a list of strings representing all elements, not just the strings themselves.

2. Step 2: Analyze each option

   val stringList = mixedList.map { it.toString() } maps each element to its string form, producing List<String>. val stringList = mixedList.filterIsInstance<String>() filters only strings, losing other elements. val stringList = mixedList.map { it as String } tries unsafe cast causing runtime error. val stringList = mixedList.filter { it is String } filters strings but returns List<Any>, not List<String>.

3. Final Answer:

   val stringList = mixedList.map { it.toString() } -> Option B

4. Quick Check:

   Map all to string form for List<String> [OK]

Hint: Use map with toString() to convert all elements [OK]

Common Mistakes:

• Using filterIsInstance<String> loses non-string elements
• Casting with 'as String' causes runtime errors
• Filtering returns List<Any>, not List<String>