Object-Oriented Programming Paradigm

Object-Oriented Programming (OOP)

• OOP is a programming paradigm that uses objects and classes to create programs.
• It mirrors real-world applications, where each object has its own data and methods.

Advantages of OOP

• Models real-world applications.
• Each programming module (object) can be changed independently of the other modules, making it flexible and scalable.
• Only allowed data (variables) and methods can be accessed by other modules, making it less error-prone.
• Each object can be modeled by an interface.

The Three Pillars of OOP

• Encapsulation: The concept of bundling data and methods that operate on that data within a single unit (a class).
• Inheritance: The ability of one class to inherit properties and behavior from another class.
• Polymorphism: The ability of an object to take on multiple forms.

Basics of OOP in Java

• Define objects à encapsulation.
• Create and use the objects.
• Two basic steps: define objects and create and use the objects.

Class File Structure

• Fields/attributes to hold data (variable declarations, typically private).
• Procedures/operations on the data (methods, typically public).

Example of a Simple Object-Oriented Program

• A Rectangle class with private instance variables (xpos, ypos, width, height) and public methods (setX, setY, setWidth, setHeight, getX, getY, getWidth, getHeight).

The this Parameter

• Refers to the current object.
• Used to distinguish instance variables from method parameters.

toString Method

• Gives a string representation of an object.
• Can be designed to print the object's attributes in a convenient form.

Method Overloading

• Can have multiple methods with the same name in a class.
• Provided their signatures are different (method name, type, and order of input parameters).
• Useful when we need several different ways to perform the same operation.

Garbage Collection in Java

• The memory space occupied by an object is reclaimed by the garbage collector when the object is no longer in use.
• Runs in the background, identifies objects without references, and reclaims the space used by the object.

Static Methods and Variables

• Static Methods: Can be used without an object.
• Static Variables: Only one copy exists, and individual objects do not get separate versions.

Examples of Static Methods

• main is a static method that can be included in any class.
• The Math class has multiple static methods (e.g., Math.pow, Math.random).### Static Variables and Methods
• A class can have both static and instance variables.
• A non-static method can access both instance and static variables.
• A non-static method can invoke both static and non-static methods.
• A static method can only access static variables and methods directly.
• Constants are shared by all objects of the class and should be declared final static.

Inheritance

• Inheritance is the process of deriving a new class from an existing class.
• The new class inherits the properties of the existing class.
• The existing class is called the superclass or base class.
• The new class is called the subclass or derived class.
• Inheritance is used to create a class hierarchy.
• The subclass inherits the attributes and methods of the superclass.

Class Hierarchy

• A class hierarchy is a tree-like structure where classes are arranged in a hierarchical order.
• The superclass is at the top of the hierarchy, and the subclasses are below.
• Inheritance is used to create a class hierarchy.
• The class hierarchy can be extended as required.

Programming Inheritance

• Inheritance is programmed using the "extends" keyword in the class declaration.
• The subclass inherits the properties of the superclass.
• The subclass can also add its own attributes and methods.

Superclass, Subclass, and UML Notation

• The superclass is the generalized class.
• The subclass is the specialized class.
• The UML notation is used to represent the inheritance relationship between classes.

Advantages of Inheritance

• Code modularization
• Code reusability
• Accessibility of methods by other classes
• Code extension by creating class hierarchies

Deciding on Inheritance

• If there is an "is-a" relationship between classes, then it is inheritance.
• Examples: Car is a Vehicle, Student is a Person, Monkey is an Animal.

Method Overriding

• A method in the subclass can have the same name and signature as a method in the superclass.
• The method in the subclass overrides the method in the superclass.

Abstract Class

• An abstract class is a class from which objects cannot be instantiated.
• An abstract class defines variables and methods.
• Methods can also be abstract, with just a header and no body.
• Subclasses must implement the abstract methods.

Interface

• An interface is a collection of method declarations with no data and no bodies.
• Any class that implements an interface guarantees to implement all the methods.
• An interface enforces an API in software design.

The Object Class

• In Java, every class inherits implicitly from the Object class.
• The Object class provides utility methods such as equals, toString, and clone.
• The Object class also provides methods for multi-threading.

Polymorphism

• Polymorphism means the ability of one object to be treated, or used, like another.
• Polymorphism works for objects in the inheritance hierarchy.
• A reference of a superclass type can be used to refer to an object of its subclass type.### Polymorphism
• Polymorphism allows you to declare an object of a superclass type and instantiate an object of a subclass using the same reference.
• Example: A obj; obj = new A(); obj = new B(); obj = new C(); where A is the superclass, and B and C are subclasses.
• This allows for flexibility in programming, as the same reference can be used to create objects of different types.

instanceof Operator

• The instanceof operator is used to check if an object is of a certain type or a subclass of that type.
• Example: obj1 instanceof Jedi returns true if obj1 is a Jedi object or a subclass of Jedi.
• It can be used to check the type of an object at runtime.

Casting

• Casting is used to convert an object of a superclass type to a subclass type.
• Example: GameEntity g1 = new Jedi(); Jedi j2 = (Jedi) g1; where g1 is a GameEntity object and j2 is a Jedi object.
• Casting can be used to work with objects of different types, but it can also lead to errors if the object is not of the correct type.

Generics

• Generics are a way to create classes that can work with objects of different types.
• Example: public class MyCollection { ... } where T is a type parameter.
• Generics allow for flexibility in programming, as the same class can be used to work with objects of different types.

Generics Example

• A Grade class is created that can work with either String objects or Integer objects.
• The Grade class has a constructor, get and set methods, and a toString method.
• The Grade class is used in a client program to create objects of different types.

Benefits of Generics

• Generics allow for flexibility in programming, as the same class can be used to work with objects of different types.
• Generics reduce code duplication, as the same class can be used to work with objects of different types.
• Generics make the code more reusable and easier to maintain.