Inner class access to outer members in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of inner class accessing outer class property

What is the output of this Kotlin code?

Kotlin

class Outer {
    val greeting = "Hello"
    inner class Inner {
        fun greet() = "$greeting from Inner"
    }
}

fun main() {
    val inner = Outer().Inner()
    println(inner.greet())
}

A Hello

B Error: Cannot access 'greeting' from Inner

C from Inner

D Hello from Inner

💡 Hint

Remember that inner classes have access to outer class members.

In Kotlin, an inner class can access members of its outer class instance. Here, 'greeting' is accessed directly inside the inner class method.

❓ Predict Output

intermediate

Output when inner class shadows outer property

What will be printed by this Kotlin program?

Kotlin

class Outer {
    val value = 10
    inner class Inner {
        val value = 20
        fun printValues() {
            println(value)
            println([e-mail])
        }
    }
}

fun main() {
    Outer().Inner().printValues()
}

A

10
20

B

20
10

C

20
20

D

10
10

💡 Hint

Use 'this@Outer' to refer to the outer class instance.

The inner class has its own 'value' property which shadows the outer class's 'value'. 'value' alone refers to inner's property, '[e-mail]' refers to outer's property.

🔧 Debug

advanced

Why does this code cause a compilation error?

This Kotlin code tries to access an outer class property from a nested class but fails. Why? class Outer { val name = "Outer" class Nested { fun printName() { println(name) } } } fun main() { Outer.Nested().printName() }

A Nested class is not inner, so it cannot access outer class members directly.

B The property 'name' is private and inaccessible.

C The syntax for accessing outer class members is incorrect.

D Nested classes cannot have functions in Kotlin.

💡 Hint

Check the difference between 'inner' and 'nested' classes in Kotlin.

In Kotlin, nested classes are static by default and do not hold a reference to the outer class instance. Only inner classes can access outer class members directly.

❓ Predict Output

advanced

Output of inner class accessing outer class function

What is the output of this Kotlin program?

Kotlin

class Outer {
    fun outerFun() = "Outer Function"
    inner class Inner {
        fun callOuter() = outerFun()
    }
}

fun main() {
    println(Outer().Inner().callOuter())
}

A Outer Function

B Error: Cannot call outerFun from Inner

C null

D Outer

💡 Hint

Inner classes can call outer class functions directly.

The inner class method 'callOuter' calls the outer class function 'outerFun' directly, so it returns 'Outer Function'.

🧠 Conceptual

expert

How to access outer class property when inner class has same property name?

In Kotlin, if an inner class has a property with the same name as the outer class, how can you access the outer class property inside the inner class?

A Use 'Outer.propertyName' directly without any qualifier.

B Use 'super.propertyName' inside the inner class.

C Use '[e-mail]' to refer to the outer class property.

D It's not possible to access outer class property if names clash.

💡 Hint

Kotlin provides a special syntax to refer to outer class instance.

The syntax 'this@Outer' refers to the instance of the outer class, allowing access to its members even if shadowed.

Practice

1. In Kotlin, what keyword must you use to allow an inner class to access members of its outer class?

easy

A. inner

B. outer

C. this

D. nested

Solution

1. Step 1: Understand inner class behavior

   By default, nested classes in Kotlin cannot access outer class members directly.

2. Step 2: Identify the keyword for access

   The keyword inner allows the nested class to hold a reference to the outer class and access its members.

3. Final Answer:

   inner -> Option A

4. Quick Check:

   Use inner for access [OK]

Hint: Remember: only 'inner' classes access outer members directly [OK]

Common Mistakes:

• Confusing 'inner' with 'nested' keyword
• Using 'this' instead of 'inner'
• Assuming all nested classes access outer members

2. Which of the following Kotlin class declarations correctly allows the inner class to access the outer class's property name?

easy

A. class Outer { val name = \"Kotlin\"; inner class Inner { fun printName() = println(name) } }

B. class Outer { val name = \"Kotlin\"; class Inner { fun printName() = println(name) } }

C. class Outer { val name = \"Kotlin\"; nested class Inner { fun printName() = println(name) } }

D. class Outer { val name = \"Kotlin\"; inner class Inner { fun printName() = println(this.name) } }

Solution

1. Step 1: Check class declaration syntax

   Only the class declared with inner keyword can access outer class properties directly.

2. Step 2: Analyze each option

   class Outer { val name = \"Kotlin\"; inner class Inner { fun printName() = println(name) } } uses inner and accesses name correctly. class Outer { val name = \"Kotlin\"; class Inner { fun printName() = println(name) } } lacks inner. class Outer { val name = \"Kotlin\"; nested class Inner { fun printName() = println(name) } } uses invalid nested keyword. class Outer { val name = \"Kotlin\"; inner class Inner { fun printName() = println(this.name) } } tries to access this.name which refers to inner class, not outer.

3. Final Answer:

   B -> Option A

4. Quick Check:

   Use inner and direct property name [OK]

Hint: Use 'inner' keyword and access outer property by name [OK]

Common Mistakes:

• Omitting 'inner' keyword
• Using 'nested' instead of 'inner'
• Using 'this.name' inside inner class incorrectly

3. What is the output of the following Kotlin code?

class Outer(val x: Int) {
    inner class Inner {
        fun printX() = println(x)
    }
}

fun main() {
    val outer = Outer(10)
    val inner = outer.Inner()
    inner.printX()
}

medium

A. Runtime exception

B. 0

C. Compilation error

D. 10

Solution

1. Step 1: Understand inner class access

   The inner class can access the outer class property x because it is declared with inner.

2. Step 2: Trace the code execution

   An Outer object is created with x=10. The inner class instance calls printX() which prints x value 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Inner class prints outer property value [OK]

Hint: Inner class prints outer property directly if 'inner' keyword used [OK]

Common Mistakes:

• Assuming inner class cannot access outer property
• Expecting 0 instead of 10
• Thinking code causes error

4. Identify the error in this Kotlin code snippet:

class Outer {
    val message = "Hello"
    class Inner {
        fun greet() = println(message)
    }
}

medium

A. Incorrect println syntax

B. Missing semicolon after 'message' declaration

C. Inner class cannot access 'message' without 'inner' keyword

D. Outer class must be declared as 'inner'

Solution

1. Step 1: Check nested class access rules

   Nested classes without inner keyword cannot access outer class properties.

2. Step 2: Identify cause of error

   The Inner class tries to print message but cannot see it, causing a compilation error.

3. Final Answer:

   Inner class cannot access 'message' without 'inner' keyword -> Option C

4. Quick Check:

   Use 'inner' to access outer members [OK]

Hint: Add 'inner' keyword to nested class to fix access error [OK]

Common Mistakes:

• Thinking semicolon is required in Kotlin
• Assuming println syntax is wrong
• Confusing outer class declaration with inner

5. Given this Kotlin code, what will be the output?

class Outer(val number: Int) {
    inner class Inner {
        fun multiply(factor: Int) = number * factor
    }
}

fun main() {
    val outer = Outer(5)
    val inner = outer.Inner()
    println(inner.multiply(3))
}

hard

A. 53

B. 15

C. Compilation error due to access

D. Runtime exception

Solution

1. Step 1: Understand inner class access and method

   The inner class can access the outer class property number because it is declared with inner. The method multiplies number by the given factor.

2. Step 2: Calculate the output

   Outer number is 5, factor is 3, so 5 * 3 = 15 is printed.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   Inner class method uses outer property correctly [OK]

Hint: Inner class can use outer properties in calculations [OK]

Common Mistakes:

• Concatenating instead of multiplying
• Expecting access error without 'inner'
• Confusing output with string '53'