Java Object-Oriented Programming (OOP) is a programming paradigm that organizes software design around objects, rather than functions and logic. It is built on four main principles: encapsulation, which involves bundling data and methods that operate on the data within a single unit (class); inheritance, which allows one class to inherit fields and methods from another; polymorphism, which enables objects to be treated as instances of their parent class while allowing them to behave differently; and abstraction, which hides complex implementation details and exposes only essential features. This approach promotes modularity, reusability, and maintainability, making Java a popular language for building large-scale, complex software systems.