Secondary constructors in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a secondary constructor in Kotlin?

A secondary constructor is an additional constructor in a Kotlin class that allows creating objects with different sets of parameters besides the primary constructor.

beginner

How do you declare a secondary constructor in Kotlin?

Use the keyword <code>constructor</code> inside the class body, followed by parameters and an optional call to the primary constructor using <code>this()</code>.

intermediate

Why would you use a secondary constructor instead of just a primary constructor with default parameters?

Secondary constructors let you provide different ways to create an object when default parameters are not enough or when you want to run different initialization code.

intermediate

What must a secondary constructor do if the class has a primary constructor?

It must delegate to the primary constructor either directly or indirectly by calling this() with appropriate arguments.

beginner

Example: What does this Kotlin code do?<br><pre>class Person(val name: String) {
  constructor(name: String, age: Int) : this(name) {
    println("Age is $age")
  }
}</pre>

It defines a class Person with a primary constructor taking name. The secondary constructor takes name and age, calls the primary constructor with name, and prints the age.

In Kotlin, how do you declare a secondary constructor?

A Using the keyword <code>init</code>

B Using the keyword <code>constructor</code> inside the class body

C Using the keyword <code>fun</code>

D Using the keyword <code>override</code>

Secondary constructors are declared with the constructor keyword inside the class body.

What must a secondary constructor do if the class has a primary constructor?

A Override the primary constructor

B Call the superclass constructor using <code>super()</code>

C Do nothing special

D Call the primary constructor using <code>this()</code>

A secondary constructor must delegate to the primary constructor using this().

Which of these is a reason to use a secondary constructor?

A To provide an alternative way to create an object with different parameters

B To replace the primary constructor completely

C To define a function inside the class

D To override a method

Secondary constructors provide alternative ways to create objects with different parameters.

Can a Kotlin class have multiple secondary constructors?

A Yes, multiple secondary constructors are allowed

B No, only one secondary constructor is allowed

C Only if there is no primary constructor

D Only if the class is abstract

Kotlin allows multiple secondary constructors in a class.

What happens if a secondary constructor does not delegate to the primary constructor in a class that has one?

A The primary constructor is called automatically

B The program runs but with warnings

C Compilation error occurs

D The secondary constructor is ignored

Kotlin requires secondary constructors to delegate to the primary constructor; otherwise, it causes a compilation error.

Explain what a secondary constructor is in Kotlin and when you might use it.

Think about creating objects with different parameters or initialization.

Describe the rules for delegation between secondary and primary constructors in Kotlin.

Focus on how constructors connect inside the class.

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