Kotlin's Classes and Objects Flashcards

Study Notes

Object Keyword in Kotlin

Used for Object Expression, Object Declaration, and Companion Object.
Object Expression defines an anonymous class with no explicit name.
Object Declaration creates a Singleton instance easily, initialized lazily on first access.
Companion Objects are associated with a class, initialized when the class is loaded, similar to Java's static initializers.

Sealed Keyword

Defines restricted class hierarchies, controlling inheritance.
All subclasses of a sealed class are known at compile time, preventing unknown subclasses in external modules.
Similar to enum classes, but subclasses of sealed classes can have multiple instances and states.
Sealed classes cannot be instantiated directly and must have a restricted set of subclasses, with private constructors by default.

Data Class

Main purpose is to hold data, with compiler-generated functions: equals(), toString(), componentN(), and copy().
Requirements include at least one primary constructor and parameters must be marked as val or var.
Cannot be abstract, open, sealed, or inner.

Inheritance in Data Classes

Data classes can extend other classes, but only one at a time.
Recommended to extend a sealed class for better structure and control.

Open vs Abstract

Classes and methods are final by default in Kotlin; using the open keyword makes them overridable.
Abstract classes cannot be instantiated and must be subclassed; methods in abstract classes must be overridden by subclasses.

Default Nullability

Kotlin uses Not Null By Default, meaning variables must explicitly be declared nullable.

Inner Keyword

Marks a nested class that can access members of its outer class.
Inner class has a reference to an instance of the outer class.

Benefit of Final Classes

Facilitates smart casting, allowing the compiler to automatically handle type casting.
Reduces risk of misuse of base classes, ensuring subclasses function correctly.

Scope Functions

Blocks of code that execute within an object's context, allowing access without naming the object.
Types include:
- let
- run
- apply
- also
- with
Each has unique characteristics regarding reference access using this or it, and governs the result of the expression, with return behavior differing among the types.

Description

Test your knowledge on Kotlin's functionality regarding classes and objects with these flashcards. Understand key concepts such as object expressions, declarations, and companion objects. Perfect for Kotlin learners looking to solidify their understanding of object-oriented programming.