Companion objects as static alternatives 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 which behave like static members in Java. It belongs to the class, not to instances.

beginner

How do you declare a companion object inside a Kotlin class?

Use the keyword <code>companion object</code> inside the class. For example:<br><pre>class MyClass {
  companion object {
    val constant = 42
  }
}</pre>

beginner

Why use companion objects instead of static members in Kotlin?

Kotlin does not have static members. Companion objects provide a way to have class-level members that can be accessed without creating an instance, similar to static in Java.

beginner

How do you access a member inside a companion object?

You access it using the class name, like <code>MyClass.constant</code>. You don't need to create an instance of the class.

intermediate

Can a companion object implement interfaces in Kotlin?

Yes, a companion object can implement interfaces, allowing it to have behavior and be passed around as an object.

What keyword is used to declare a companion object in Kotlin?

A companion object

B static

C object static

D class companion

In Kotlin, the keyword companion object declares a companion object inside a class.

How do you access a property named count inside a companion object of class Counter?

A Counter.count

B count.Counter

C Counter().count

D companion.count

Members of a companion object are accessed using the class name, like Counter.count.

Which of these is true about companion objects?

A They are the same as Java static methods

B They are created for each instance

C They cannot have functions

D They belong to the class, not instances

Companion objects belong to the class and are shared, unlike instance members.

Can a companion object implement an interface?

A No

B Yes

C Only if the class implements it

D Only in Java

Companion objects can implement interfaces to provide behavior.

Why does Kotlin use companion objects instead of static members?

A Static members are deprecated

B Companion objects are faster

C Kotlin does not support static members

D Companion objects are only for constants

Kotlin does not have static members; companion objects provide a similar feature.

Explain what a companion object is and how it acts as a static alternative in Kotlin.

Think about how you would use static members in Java.

Describe how to declare and access a property inside a companion object.

Remember the keyword and how you call it.