Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which may contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods. A feature of objects is that an object's procedures can access and often modify the data fields of the object with which they are associated (objects have a notion of "this" or "self"). There is a significant diversity of OOP languages, but the most popular ones (C++/Java) are class-based, meaning that objects are instances of classes, which typically also determine their type.