Kotlin: Classes, Objects, and Inheritance

• Lecture 3 covers the Kotlin programming language
• It reviews classes, objects, inheritance interfaces and more

Classes and Objects

• Primary constructor, default values and object properties are re-iterated with examples
• Instance variables can be accessed using objectReference.fieldName
• Methods are accessed with objectReference.methodName(argumentList) or objectReference.methodName()
• Anonymous objects are created directly, like Bicycle().gear
• If constructors are not defined, the compiler provides a default constructor, calling the superclass's default constructor
• Object creation is handled similarly to other languages

Garbage Collection

• Kotlin uses garbage collection to deallocate objects no longer in use
• An object is removed when no references point to it
• References are removed when their block of code completes (anonymous or named)
• All references to an object must be removed before it can be garbage collected

Inheritance

• Kotlin classes are final by default, meaning they cannot be subclassed unless declared as open
• Methods in Kotlin are also final by default and cannot be overridden unless opened
• Methods are overridden using the override keyword

Method Overriding

• Kotlin uses the override keyword to override methods, as shown in the Point class example regarding toString
• The this keyword refers to the current object instance
• It distinguishes instance variables from constructor or method parameters with the same name
• Used to invoke other constructors within the same class

Interfaces

• Contain only variables and signature methods, implicitly public
• They cannot be instantiated
• Interfaces can extend other interfaces
• Classes implement interfaces directly
• A reference can be of an interface type, assigned an object of a class implementing that interface
• Kotlin allows inheritance of interfaces and implementation of multiple interfaces

Parameters

• Identifiers for parameters must be unique within their scope
• A parameter name can match an instance variable name, shadowing the instance variable (accessed via this)
• Shadowing makes code harder to read and is generally used in constructors or methods setting instance variable values.
• Kotlin passes parameters by value and copies the values provided

Passing by Value

• Objects are passed by value, meaning the reference is copied, but the original object remains unchanged outside the method
• Modifications to an object's state within a method are visible outside the method

Overloading

• Kotlin uses static type checking, enabling overloading
• Overloading involves calling method, that are contextually independent precluding declaring 2 methods with same signature, returning different types

Overriding

• Kotlin also supports runtime polymorphism, enabling overriding
• Kotlin can apply covariant return types to enable function returns
• All methods are virtual
• The method visibility determines access, overriding methods in derived classes can widen the visibility
• Prekrita overrides

Any Class

• Includes implicit type parameters (parametric polymorphism)
• Generics can be implemented using the Any class
• This approach can lead to type-related issues
• Any is the superclass for all types
• They cannot hold null values unless nullable Any? is used

Type Casting

• Conversion between object types involves the use of casting
• Implicit casting (assigning a MountainBike object to Any type)
• Explicit casting with potential runtime checks(obj as MountainBike)
• Safe casting using as?

Unit Class

• It is returned from functions that do not return any meaningful value
• It is similar the void type in Java
• Such functions alter program state, producing side effects
• Only one possible value for unit which is the unit object

Nothing Class

• Indicates the function never returns normally
• It either throws an exception or enters a loop for infinity
• Code after a function call is normally unreachable
• This contrasts with Nothing which holds exactly the null value type

"is" Keyword

• This is used to verify the type for example object is Parent or object !is Parent

Data Classes

• Key purpose is to hold data with reserved word data
• The compiler automatically generates equals(), toString() and more
• They cannot be abstract, sealed or inner
• Primary constructor requires a parameter, marked with val or var

Enums

• Enable the use of type-safe enumerations
• For the Day enum class, a variable of type Day can only be one of the predefined constants

Sealed Classes

• These are used to restrict class hierarchies in Kotlin
• sealed class and it's sub classes must be defined in the same file

Abstract Classes

• Can be defined in Kotlin using keyword abstract
• This cannot create type object of abstract classes
• Override key word necessary within abstract class

Extension Function (Kotlin)

• Adds function to existing classes
• Example of adding function markInPercentage to ExamMark classes

Object Declaration

• They are instances of object types are singletons
• Kotlin instantiates an object with the key word object without a constructor

Companion Object Declaration

• This is used to implement static methods
• Are declared inside class body

Lambda Functions

• Provide a concise way to represent simple functions
• Enable the implementor to write very short ad hoc function
• Compiler can infer the type variable

Higher-Order Functions

• Accept functions as parameters, creating higher-order functions
• Enable generic algorithms by accepting functions

Inline Functions

• Improve performance by inserting the function body directly into the calling code, indicated by the inline keyword
• This optimizes higher-order functions

JIT Compilation

• JIT allows Kotlin to do following
• Prevents the need for packages
• Removal of redundant coding

Exceptions

• Solve issues with programs using specific functions
• They have a block try-catch
• Must "throw" error message
• Can annotate

Generics

• Generics or (Generics.ang) allows for creation of params for classes or functions
• Type var can be any referencing type

Packages In Kotlin

• A package is a way of logically sorting classes
• Usually contains directory of complied classes
• Keyword used" package", and contains either demo project or libraries files

Jar files

• Jar contain file/directories for all class and java files. Similar .exe file/directory in C++

Access and Modules

• Members include public, private internal, and protected which are accessibility features.
• Moduels are a type of file that are interdependent and must be called so it prevant malfunction
• This must be done through Mavan and Gradle

Compilation

• Compilation allows java, class to all convert with into a format code which is done to save space

"Več na"

• Provide external resources that implement or allow for more research in project