Companion object with interfaces in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a companion object in Kotlin?

A companion object is a special object inside a class that allows you to define members that belong to the class itself, not to instances. It's like a static member in other languages.

intermediate

Can a companion object implement an interface in Kotlin?

Yes, a companion object can implement interfaces. This allows the companion object to provide specific behavior defined by the interface.

beginner

How do you call a function defined in a companion object that implements an interface?

You call it using the class name, like ClassName.functionName(), because companion object members behave like static members.

intermediate

Example: What does this code do?
<pre>interface Logger {
    fun log(message: String)
}

class MyClass {
    companion object : Logger {
        override fun log(message: String) {
            println("Log: $message")
        }
    }
}</pre>

This code defines an interface Logger with a log function. The companion object of MyClass implements Logger and provides the log function. You can call MyClass.log("Hello") to print "Log: Hello".

intermediate

Why use interfaces with companion objects?

Using interfaces with companion objects helps organize code by defining contracts for static-like behavior. It makes the companion object behave predictably and allows polymorphism.

What keyword defines a companion object inside a Kotlin class?

A static

B companion

C object

D interface

The keyword 'companion' defines a companion object inside a Kotlin class.

Can a companion object implement an interface in Kotlin?

A Yes, companion objects can implement interfaces

B Only in Kotlin versions before 1.0

C Only if the interface is marked as 'companion'

D No, companion objects cannot implement interfaces

Companion objects can implement interfaces to provide specific behavior.

How do you call a function defined in a companion object?

A Using the class name directly

B Using an instance of the class

C Using the 'new' keyword

D Using the 'static' keyword

Functions in companion objects are called using the class name, like ClassName.functionName().

What is the main benefit of implementing an interface in a companion object?

A To make the class abstract

B To allow the companion object to be instantiated

C To define a contract for static-like behavior

D To override instance methods

Implementing an interface in a companion object defines a contract for static-like behavior.

Which of these is a correct way to define a companion object implementing an interface Logger?

A companion object Logger { ... }

B object companion : Logger { ... }

C companion Logger object { ... }

D companion object : Logger { ... }

The correct syntax is 'companion object : Logger { ... }' to implement an interface.

Explain how a companion object can implement an interface in Kotlin and why this might be useful.

Think about how companion objects act like static members and how interfaces define behavior.

Write a simple Kotlin class with a companion object that implements an interface with one function, and show how to call that function.

Start with interface, then class with companion object, then call the function.