Inner class access to outer members in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the outer class property from the inner class.

Kotlin

class Outer {
    val greeting = "Hello"
    inner class Inner {
        val greeting = "inner"
        fun greet() = [1]
    }
}

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

[e-mail]

Bthis.greeting

COuter.greeting

Dgreeting

Attempts:

3 left

💡 Hint

Common Mistakes

Using just 'greeting' or 'this.greeting' accesses inner class scope, not outer.

In Kotlin, to access an outer class member from an inner class, use this@Outer.

2fill in blank

medium

Complete the code to call the outer class function from the inner class.

Kotlin

class Outer {
    fun outerFun() = "Outer Function"
    inner class Inner {
        fun outerFun() = "inner"
        fun callOuter() = [1]
    }
}

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

AouterFun()

Bthis.outerFun()

[e-mail]()

DOuter.outerFun()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling outerFun() directly calls the inner class function.

Use [e-mail]() to call the outer class function from the inner class.

3fill in blank

hard

Fix the error in accessing the outer property inside the inner class method.

Kotlin

class Outer {
    val number = 42
    inner class Inner {
        val number = 0
        fun printNumber() {
            println([1])
        }
    }
}

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

Anumber

[e-mail]

Cthis.number

DOuter.number

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or this.number accesses the inner class property.

Use [e-mail] to correctly access the outer class property.

4fill in blank

hard

Fill both blanks to access outer class property and call its function from inner class.

Kotlin

class Outer {
    val text = "Hi"
    fun outerFun() = "Called"
    inner class Inner {
        val text = "inner"
        fun outerFun() = "inner func"
        fun combined() = [1] + " " + [2]()
    }
}

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

[e-mail]

Btext

[e-mail]

DouterFun

Attempts:

3 left

💡 Hint

Common Mistakes

Using unqualified names causes errors or accesses inner class members.

Use [e-mail] to access the property and [e-mail]() to call the function.

5fill in blank

hard

Fill all three blanks to create a map with outer property keys and inner class method results as values.

Kotlin

class Outer {
    val prefix = "Key"
    inner class Inner {
        fun suffix() = "Value"
        fun createMap() = mapOf([1] to [2].[3]())
    }
}

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

Aprefix

Bthis@Outer

Cthis

Dsuffix

Attempts:

3 left

💡 Hint

Common Mistakes

Using this@Outer to call inner methods causes errors.

The map key is the outer property prefix. The value calls the inner class method suffix() using this.suffix().

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'