OOP with Java: Classes and Objects

Study Notes

OOP with Java

Classes and Objects

Class: A blueprint for creating objects. Defines properties (attributes) and methods (functions).
- Syntax:
```
class ClassName {
    // attributes
    // methods
}
```
Object: An instance of a class. Contains state and behavior defined by its class.
- Creation:
```
ClassName obj = new ClassName();
```
Attributes: Variables within a class. Can be private, public, or protected.
Methods: Functions defined in a class. Can manipulate attributes and define behaviors.
Encapsulation: Bundling of data and methods that operate on the data within one unit (class) and restricting access to some components.

Inheritance

Definition: Mechanism where one class (subclass/child class) inherits attributes and methods from another class (superclass/parent class).

Syntax:

class SubClass extends SuperClass {
    // additional attributes and methods
}

Types of Inheritance:
- Single Inheritance: A subclass inherits from one superclass.
- Multiple Inheritance: Java does not support multiple inheritance directly but allows it through interfaces.
Method Overriding: Redefining a method in a subclass that already exists in the superclass.
Super Keyword: Used to refer to the superclass and access its methods and constructors.

Interfaces

Definition: A reference type in Java, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types.
Purpose: To achieve abstraction and multiple inheritance.

Syntax:

interface InterfaceName {
    // abstract methods
}

Implementation: A class can implement an interface using the implements keyword.

class ClassName implements InterfaceName {
    // implementation of abstract methods
}

Default Methods: Introduced in Java 8, allows interfaces to provide default implementations for methods.
Static Methods in Interfaces: Can contain static methods, which can be called without creating an instance of the interface.
Multiple Interfaces: A class can implement multiple interfaces, providing a way to achieve multiple inheritance in Java.

Classes and Objects

A class serves as a blueprint for creating objects, defining their properties (attributes) and methods (functions).
Object creation syntax: ClassName obj = new ClassName(); which initializes an instance of a class.
Attributes are variables within a class, categorized as private, public, or protected, determining their visibility and accessibility.
Methods are functions within a class that can manipulate attributes and define behaviors.
Encapsulation involves bundling data and methods that act on the data within a single unit (class) while restricting access to selected components.

Inheritance

Inheritance enables a class (subclass or child class) to inherit attributes and methods from another class (superclass or parent class).
Subclass definition syntax: class SubClass extends SuperClass { /* additional attributes and methods */ }.
Single Inheritance allows a subclass to inherit from one superclass, while Multiple Inheritance is not directly supported in Java but can be achieved through interfaces.
Method Overriding allows a subclass to redefine a method that exists in its superclass.
The super keyword is utilized to reference the superclass, enabling access to its methods and constructors.

Interfaces

An interface is a reference type similar to a class that can include constants, method signatures, default methods, static methods, and nested types.
Interfaces are used to achieve abstraction and facilitate multiple inheritance in Java.
Interface implementation syntax: class ClassName implements InterfaceName { /* implementation of abstract methods */ }.
Default methods, introduced in Java 8, enable interfaces to have default method implementations, improving usability.
Interfaces can include static methods that can be called independently without instantiating the interface.
A class can implement multiple interfaces, offering a pathway to achieve multiple inheritance in Java.

Description

This quiz covers the fundamental concepts of Object-Oriented Programming (OOP) in Java, focusing specifically on classes and objects. You'll learn about the definitions, syntax, attributes, methods, and key principles like encapsulation and inheritance. Test your knowledge on how these components work together in Java.