Object-Oriented Programming (OOP) is a programming paradigm that Java is known for and heavily based on. OOP promotes the organization of code around objects, which are instances of classes. In Java, OOP is fundamental, and nearly all Java programs are designed using OOP principles. Here are the core concepts of OOP in Java:

1. Classes and Objects: A class is a blueprint or template for creating objects. Objects are instances of classes, and they encapsulate data (attributes) and behaviors (methods).

2. Encapsulation: Encapsulation is the concept of bundling data (attributes) and methods that operate on that data into a single unit, called a class. It restricts direct access to some of the object’s components and prevents unintended interference and misuse.

3. Inheritance: Inheritance allows you to create a new class (subclass or derived class) based on an existing class (superclass or base class). The subclass inherits the attributes and methods of the superclass, promoting code reuse and hierarchical organization.

4. Polymorphism: Polymorphism allows objects of different classes to be treated as objects of a common superclass. This simplifies code and allows for flexibility and extensibility. Polymorphism is achieved through method overriding and interfaces in Java.

5. Abstraction: Abstraction is the process of simplifying complex reality by modeling classes based on the essential properties and behaviors while ignoring unnecessary details. Abstract classes and interfaces are used to define abstractions in Java.