Secondary constructors in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a secondary constructor in the class.

Kotlin

class Person {
    var name: String
    constructor(name: String) {
        this.name = [1]
    }
}

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

Aname

BPerson

Cthis.name

Dconstructor

Attempts:

3 left

💡 Hint

Common Mistakes

Using this.name on the right side instead of the parameter.

Forgetting to assign the parameter to the property.

The secondary constructor assigns the parameter name to the property name using this.name = name.

2fill in blank

medium

Complete the code to call the primary constructor from the secondary constructor.

Kotlin

class Person(val name: String) {
    constructor() : this([1]) {
        println("Secondary constructor called")
    }
}

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

APerson

B"Unknown"

Cthis.name

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use name without defining it.

Using this.name which is not valid here.

The secondary constructor calls the primary constructor with a default name "Unknown".

3fill in blank

hard

Fix the error in the secondary constructor call to the primary constructor.

Kotlin

class Person(val name: String) {
    constructor(age: Int) : this([1]) {
        println("Age is $age")
    }
}

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

A"Unknown"

B"age"

Cage

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Passing age directly which is an Int.

Passing "age" as a string literal which is not meaningful here.

The secondary constructor must call the primary constructor with a String. Passing "Unknown" is correct.

4fill in blank

hard

Fill both blanks to create a secondary constructor that calls the primary constructor with a default name and prints a message.

Kotlin

class Person(val name: String) {
    constructor() : this([1]) {
        println([2])
    }
}

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

A"Guest"

B"Secondary constructor called"

C"Hello"

D"Name not provided"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-string value in the first blank.

Printing the wrong message.

The secondary constructor calls the primary constructor with "Guest" and prints "Secondary constructor called".

5fill in blank

hard

Fill all three blanks to define a class with a primary constructor and two secondary constructors calling it with different default names.

Kotlin

class Person(val name: String) {
    constructor() : this([1]) {
        println([2])
    }
    constructor(age: Int) : this([3]) {
        println("Age is $age")
    }
}

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

A"Anonymous"

B"No name provided"

C"Unknown"

D"Default"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same string for both secondary constructors.

Printing a message that doesn't match the constructor.

The first secondary constructor calls the primary with "Default" and prints "No name provided". The second calls it with "Unknown".

Practice

1. What is the main purpose of a secondary constructor in Kotlin?

easy

A. To create static methods

B. To replace the primary constructor completely

C. To provide additional ways to create an object with different parameters

D. To define functions inside a class

Solution

1. Step 1: Understand constructors in Kotlin

   Kotlin classes can have a primary constructor and one or more secondary constructors to create objects.

2. Step 2: Role of secondary constructors

   Secondary constructors allow creating objects with different sets of parameters, providing flexibility.

3. Final Answer:

   To provide additional ways to create an object with different parameters -> Option C

4. Quick Check:

   Secondary constructors = multiple ways to create objects [OK]

Hint: Secondary constructors add extra ways to create objects [OK]

Common Mistakes:

• Thinking secondary constructors replace the primary constructor
• Confusing constructors with functions
• Assuming secondary constructors create static methods

2. Which of the following is the correct syntax for a secondary constructor in Kotlin?

easy

A. secondary constructor(name: String) { this.name = name }

B. fun constructor(name: String) { this.name = name }

C. init constructor(name: String) { this.name = name }

D. constructor(name: String) { this.name = name }

Solution

1. Step 1: Recall Kotlin secondary constructor syntax

   Secondary constructors use the keyword constructor followed by parameters and a body.

2. Step 2: Check each option

   constructor(name: String) { this.name = name } uses correct syntax: constructor(name: String) { ... }. Others use invalid keywords.

3. Final Answer:

   constructor(name: String) { this.name = name } -> Option D

4. Quick Check:

   Keyword 'constructor' starts secondary constructors [OK]

Hint: Look for 'constructor' keyword without fun or init [OK]

Common Mistakes:

• Adding 'fun' before constructor
• Using 'secondary' keyword
• Confusing with init blocks

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

class Person(val name: String) {
    var age: Int = 0
    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }
}

fun main() {
    val p = Person("Alice", 30)
    println("Name: ${p.name}, Age: ${p.age}")
}

medium

A. Name: Alice, Age: 0

B. Name: Alice, Age: 30

C. Name: , Age: 30

D. Compilation error

Solution

1. Step 1: Understand constructor calls

   The secondary constructor calls the primary constructor with this(name), initializing name.

2. Step 2: Assign age in secondary constructor

   The secondary constructor sets this.age = age, so age becomes 30.

3. Final Answer:

   Name: Alice, Age: 30 -> Option B

4. Quick Check:

   Secondary constructor sets age correctly [OK]

Hint: Secondary constructor calls primary, then sets extra properties [OK]

Common Mistakes:

• Assuming age stays 0
• Thinking name is empty
• Expecting compilation error

4. Identify the error in this Kotlin class with a secondary constructor:

class Car(val model: String) {
    constructor(model: String, year: Int) {
        println("Year: $year")
    }
}

medium

A. Secondary constructor must call the primary constructor

B. Cannot assign to val property 'model'

C. Missing return type in secondary constructor

D. println cannot be used inside constructor

Solution

1. Step 1: Check secondary constructor rules

   In Kotlin, a secondary constructor must delegate to the primary constructor using : this(...).

2. Step 2: Analyze given code

   The secondary constructor does not call this(model), so it violates the rule.

3. Final Answer:

   Secondary constructor must call the primary constructor -> Option A

4. Quick Check:

   Secondary constructors require delegation [OK]

Hint: Always delegate secondary constructors to primary with ': this(...)' [OK]

Common Mistakes:

• Forgetting to delegate to primary constructor
• Trying to assign val property directly
• Thinking println is disallowed

5. You want to create a Kotlin class Book with a primary constructor that takes title and a secondary constructor that also takes author. The secondary constructor should reuse the primary constructor and set author. Which code correctly implements this?

hard

A. class Book(val title: String) {
var author: String = "Unknown"
constructor(title: String, author: String) : this(title) {
this.author = author
}
}

B. class Book(val title: String, var author: String) {
constructor(title: String) {
this.title = title
this.author = "Unknown"
}
}

C. class Book(val title: String) {
constructor(title: String, author: String) {
this.title = title
this.author = author
}
}

D. class Book {
constructor(title: String) {
this.title = title
}
constructor(title: String, author: String) {
this.title = title
this.author = author
}
}

Solution

1. Step 1: Check primary constructor and property initialization

   class Book(val title: String) { var author: String = "Unknown" constructor(title: String, author: String) : this(title) { this.author = author } } defines primary constructor with title and a mutable author property with default value.

2. Step 2: Verify secondary constructor delegates properly

   The secondary constructor calls : this(title) to reuse primary constructor and sets author inside its body.

3. Final Answer:

   class Book(val title: String) { var author: String = "Unknown" constructor(title: String, author: String) : this(title) { this.author = author } } -> Option A

4. Quick Check:

   Secondary constructor delegates and sets extra property [OK]

Hint: Secondary constructors must delegate and set extra properties inside body [OK]

Common Mistakes:

• Not delegating secondary constructor to primary
• Trying to assign val properties
• Missing property initialization