OOP Concepts

Object − objects have states and behaviors.
Class – defines the grouping of data and code, the “type” of an object
Instance – a specific allocation of a class
Message – sent to objects to make them act
Method – a “function” that an object knows how to perform
Local Variables − variables defined inside methods, constructors or blocks
Instance Variables – variables within a class but outside any method (a specific piece of data belonging to an object)
Class Variables − variables declared within a class, outside any method, with the static keyword
Abstraction – show only “relevant” data and “hide” unnecessary details of object from the user
Encapsulation – keep implementation private and seperate from interface
Polymorphism – process objects differently based on their data type, using same interface
Inheritance – hierarchical organization, share code, customize or extend behaviors

Design Pattern

Tutorialspoint Design Patterns in Java Tutorial
https://www.tutorialspoint.com/design_pattern/index.htm

Javatpoint Design Patterns in Java Tutorial
https://www.javatpoint.com/design-patterns-in-java

DZone Design Patterns for Begineers with Java Examples
https://dzone.com/articles/design-patterns-for-beginners-with-java-examples

HowToDoInJava Design Patterns Tutorials
https://howtodoinjava.com/design-patterns/

W3spoint Java Design Patterns
https://www.w3spoint.com/java-design-patterns-tutorial-with-examples

SourceMaking Design Patterns
https://sourcemaking.com/design_patterns