Object Oriented Programming

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Code examples are in C#; the same concept applies to most* languages

Polymorphism

Subtype Polymorphism

• Inheritance
• A reference variable of the superclass can reference objects of its subclass
• For instance, Apple would be a subclass of the superclass Fruit
  • Apple would have the behavior of Fruit, along with Apple specific behavior.
  • “All apples are fruits, but not all apples are fruits”

Parametric Polymorphism

• Generics
• Being able to pass the data type as an argument to a class
• For instance, List<string>

Ad-Hoc Polymorphism

• Overloading
• For instance, having an Add method that has two overloads, one for adding strings, and another for adding integers

public static int Add(int a, int b) => a + b;
public static string Add(string a, string b) => a + b;

Inheritance

• Generalization (parent/super/base) vs Specialization (child/sub/derived)
  • An Apple is a specialized Fruit
  • A Student is a specialized Person
• Subclasses don’t need to re-implement logic from the superclass—unless the methods are abstract

Code Example

public class Person(string name, DateOnly birthday)  
{  
    public string Name { get; set; } = name;  
    public DateOnly Birthday { get; set; } = birthday;  
  
    public override string ToString() => $"{Name}, {Birthday}";  
}  
  
public class Student(string name, DateOnly birthday, string major, string id)
	: Person(name, birthday)  
{  
	// inherits `Name`, `Birthday`, `ToString()` from `Person`
    public string Major { get; set; } = major;  
    public string Id { get; set; } = id;  
}

UML Representation

• Represented with a hollow arrow pointing from the subclass to the superclass

classDiagram
	class Person {
		+name: string
		+birthday: DateOnly
		+Person(name: string, birthday: DateOnly)
		+ToString() string
	}

	class Student {
		+major: string
		+id: string
		+Student(name: string, birthday: DateOnly, major: string, id: string)
		+ToString() string
	}

	Person <|-- Student

Keywords

• base → used to call a superclass’s member from a subclass
• override→ required with a subclass method to override the superclass
• virtual → required within the superclass method so the method can be overridden

Abstract Classes and Methods

Most of the time, you should use interfaces instead

Abstract Classes

• Cannot instantiate abstract classes
• Only serve as a superclass for other classes
• Difficult to implement general cases, but easier to implement it in a more specific case in a derived class
  • Shape may not be able to calculate its area, but a Rectangle can

public abstract class Shape
{
	public abstract double Area();
}
 
public class Square(double length) : Shape 
{
	public double Area() => length * length
}

Abstract Methods

• Methods that do not have implementation
• Only have the signature of the method

public abstract double Area();

Interfaces

• A contract between the interface and classes that implement it with a “has-a” relationship
• The class must implement the logic within the interface; similar to abstract class
• A class can implement multiple interfaces
• Interfaces usually start with I (in C#)

interface IEmployee  
{  
    public string Id { get; set; }  
    public double Salary { get; set; }  
}  
  
public class Employee(string name, DateOnly birthday, string id, double salary) 
	: Person(name, birthday), IEmployee  
{  
    public string Id { get; set; } = id;  
    public double Salary { get; set; } = salary;  
}

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