1. Classes and Objects:

In Java, classes act as templates for creating objects. They define the properties (attributes) and behaviors (methods) that objects of the class will possess.
Objects are instances of classes. They represent real-world entities and interact with each other by invoking methods and accessing attributes.
2. Encapsulation:

Encapsulation refers to the bundling of data and methods that operate on the data within a single unit, i.e., a class.
In Java, encapsulation is achieved through access modifiers such as private, protected, and public, which control the visibility of class members.
3. Inheritance:

Inheritance allows a class (subclass) to inherit attributes and methods from another class (superclass).
Java supports single inheritance, where a subclass can extend only one superclass, and multilevel inheritance, where a subclass can inherit from a superclass that is also a subclass of another class.
4. Polymorphism:

Polymorphism enables objects to take on different forms or behave differently based on their context.
In Java, polymorphism is achieved through method overriding (inherited methods are redefined in the subclass) and method overloading (multiple methods with the same name but different parameters).
5. Abstraction:

Abstraction involves hiding the complex implementation details of a class and exposing only the essential features to the outside world.
Abstract classes and interfaces are used in Java to achieve abstraction. Abstract classes cannot be instantiated and may contain abstract methods, while interfaces define a contract for classes to implement