Inner classes and nested classes in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested vs inner class access

What is the output of this Kotlin code?

Kotlin

class Outer {
    private val outerVal = "Outer"
    class Nested {
        fun nestedFun() = "Nested"
    }
    inner class Inner {
        fun innerFun() = outerVal
    }
}

fun main() {
    val nested = Outer.Nested()
    val inner = Outer().Inner()
    println(nested.nestedFun())
    println(inner.innerFun())
}

A

Outer
Outer

B

Nested
Outer

C

Nested
Nested

D

Outer
Nested

💡 Hint

Remember that nested classes do not have access to outer class members, but inner classes do.

The Nested class is static and cannot access outerVal. It returns "Nested". The Inner class is an inner class and can access outerVal, so it returns "Outer".

🧠 Conceptual

intermediate

Difference between inner and nested classes

Which statement correctly describes the difference between inner and nested classes in Kotlin?

A Inner classes can access members of the outer class, nested classes cannot.

B Nested classes can access members of the outer class, inner classes cannot.

C Both inner and nested classes can access members of the outer class.

D Neither inner nor nested classes can access members of the outer class.

💡 Hint

Think about which class type holds a reference to the outer class instance.

Inner classes hold a reference to an instance of the outer class and can access its members. Nested classes are static and cannot access outer class members.

❓ Predict Output

advanced

Output of accessing outer class property from inner class

What will be printed when running this Kotlin code?

Kotlin

class Container(val name: String) {
    inner class Content {
        fun printName() = println(name)
    }
}

fun main() {
    val container = Container("Box")
    val content = container.Content()
    content.printName()
}

A Runtime exception

B Content

C Compilation error

D Box

💡 Hint

Inner classes can access outer class properties directly.

The inner class Content can access the name property of Container because it holds a reference to the outer class instance.

❓ Predict Output

advanced

Error type when accessing outer class property from nested class

What error will this Kotlin code produce?

Kotlin

class Example {
    private val value = 10
    class Nested {
        fun getValue() = value
    }
}

fun main() {
    val nested = Example.Nested()
    println(nested.getValue())
}

A Compilation error: Cannot access private property

B Runtime exception: NullPointerException

C Compilation error: Unresolved reference: value

D No error, prints 10

💡 Hint

Nested classes do not have access to outer class instance members.

The nested class is static and cannot access the instance property value. This causes a compilation error for unresolved reference.

🧠 Conceptual

expert

Memory implications of inner vs nested classes

Which statement about memory usage is true regarding Kotlin inner and nested classes?

A Inner classes hold a reference to the outer class instance, potentially increasing memory usage.

B Nested classes hold a reference to the outer class instance, increasing memory usage.

C Both inner and nested classes do not hold references to the outer class instance.

D Neither inner nor nested classes affect memory usage related to the outer class.

💡 Hint

Consider which class type keeps a pointer to the outer class object.

Inner classes keep a reference to the outer class instance, which can increase memory usage if many inner class instances exist. Nested classes are static and do not hold such references.