Named companion objects in Kotlin

Introduction

Named companion objects let you give a special name to the companion object inside a class. This helps you organize code and call its members clearly.

When you want to group related functions or constants inside a class.

When you want to call companion object members with a clear name to avoid confusion.

When you want to give the companion object a descriptive name like 'Factory' or 'Builder'.

When you want to improve code readability by naming the companion object.

Syntax

Kotlin

class ClassName {
    companion object Name {
        // members here
    }
}

The keyword companion object creates a singleton object inside the class.

Giving it a name is optional but useful for clarity.

Examples

Companion object without a name. You call members as MyClass.greet().

Kotlin

class MyClass {
    companion object {
        fun greet() = "Hello"
    }
}

Named companion object called Factory. You call members as MyClass.Factory.create().

Kotlin

class MyClass {
    companion object Factory {
        fun create() = MyClass()
    }
}

Sample Program

This program defines a class Car with a named companion object Builder. The build function creates a new Car. In main, we call Car.Builder.build() to create a car and print it.

Kotlin

class Car {
    companion object Builder {
        fun build() = Car()
    }
}

fun main() {
    val myCar = Car.Builder.build()
    println("Car created: $myCar")
}

Important Notes

The printed Car@<hashcode> is the default string representation of the object.

You can omit the name if you don't need to distinguish the companion object.

Named companion objects help clarify code and indicate the purpose of the companion object.

Summary

Named companion objects give a clear name to the singleton object inside a class.

They help organize and call companion members clearly.

Use them to improve code readability and avoid confusion.

Practice

1. What is the main purpose of naming a companion object in Kotlin?

easy

A. To give a clear and specific name to the singleton object inside a class

B. To create multiple instances of the companion object

C. To make the companion object inherit from another class

D. To prevent the companion object from being accessed outside the class

Solution

1. Step 1: Understand companion objects

   Companion objects are singletons inside a class used to hold members related to the class itself.

2. Step 2: Purpose of naming companion objects

   Giving a name to a companion object helps to clearly identify and access its members, improving code readability.

3. Final Answer:

   To give a clear and specific name to the singleton object inside a class -> Option A

4. Quick Check:

   Named companion object = clear singleton name [OK]

Hint: Named companion objects clarify singleton usage [OK]

Common Mistakes:

• Thinking companion objects create multiple instances
• Confusing naming with inheritance
• Assuming naming restricts access

2. Which of the following is the correct syntax to declare a named companion object in Kotlin?

easy

A. class MyClass { companionobject Named { fun greet() = "Hi" } }

B. class MyClass { companion Named object { fun greet() = "Hi" } }

C. class MyClass { companion object Named { fun greet() = "Hi" } }

D. class MyClass { object companion Named { fun greet() = "Hi" } }

Solution

1. Step 1: Recall correct companion object syntax

   The correct syntax is 'companion object Name { ... }' inside the class.

2. Step 2: Check each option

   class MyClass { companion object Named { fun greet() = "Hi" } } matches the correct syntax exactly. Others have misplaced keywords or wrong order.

3. Final Answer:

   class MyClass { companion object Named { fun greet() = "Hi" } } -> Option C

4. Quick Check:

   companion object Name { } syntax [OK]

Hint: Remember: 'companion object Name' order is fixed [OK]

Common Mistakes:

• Swapping 'object' and 'companion' keywords
• Writing 'companionobject' as one word
• Placing the name before 'companion'

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

class Box {
    companion object Factory {
        fun create() = "Box created"
    }
}
fun main() {
    println(Box.Factory.create())
}

medium

A. Factory created

B. Box created

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand named companion object usage

   The companion object named 'Factory' holds the function 'create' which returns "Box created".

2. Step 2: Analyze the main function call

   Calling 'Box.Factory.create()' accesses the named companion object and calls 'create', printing "Box created".

3. Final Answer:

   Box created -> Option B

4. Quick Check:

   Box.Factory.create() prints "Box created" [OK]

Hint: Call named companion with ClassName.Name.method() [OK]

Common Mistakes:

• Trying to call create() without Factory prefix
• Expecting 'Factory created' output
• Confusing syntax causing compile errors

4. Identify the error in this Kotlin code snippet:

class Sample {
    companion object Companion {
        fun info() = "Info"
    }
}
fun main() {
    println(Sample.companion.info())
}

medium

A. Incorrect companion object name usage; should be 'Companion' not 'companion'

B. Missing 'object' keyword in companion declaration

C. Function 'info' must be static

D. No error; code runs fine

Solution

1. Step 1: Check companion object name and usage

   The companion object is named 'Companion' with uppercase C.

2. Step 2: Analyze the call in main()

   The code calls 'Sample.companion.info()' with lowercase 'companion', which does not match the name and causes a compile error.

3. Final Answer:

   Incorrect companion object name usage; should be 'Companion' not 'companion' -> Option A

4. Quick Check:

   Use exact companion object name when calling [OK]

Hint: Match companion object name exactly (case sensitive) [OK]

Common Mistakes:

• Using lowercase 'companion' instead of named object
• Forgetting 'object' keyword
• Assuming companion functions must be static

5. You want to create a Kotlin class Logger with a named companion object Factory that provides a method create() returning a new Logger instance. Which code correctly implements this?

hard

A. class Logger {
companion Factory object {
fun create() = Logger()
}
}

B. class Logger {
companion object {
fun create() = Logger()
}
}

C. class Logger {
object Factory companion {
fun create() = Logger()
}
}

D. class Logger {
companion object Factory {
fun create() = Logger()
}
}

Solution

1. Step 1: Recall correct syntax for named companion object

   The correct syntax is 'companion object Factory { ... }' inside the class.

2. Step 2: Check method implementation

   The method 'create()' returns a new instance of Logger using 'Logger()'. class Logger { companion object Factory { fun create() = Logger() } } matches both syntax and logic.

3. Final Answer:

   class Logger { companion object Factory { fun create() = Logger() } } -> Option D

4. Quick Check:

   Named companion object with factory method = class Logger { companion object Factory { fun create() = Logger() } } [OK]

Hint: Use 'companion object Name' and return new instance inside [OK]

Common Mistakes:

• Swapping 'object' and 'companion' keywords
• Omitting the companion object name
• Incorrect order of keywords