Private state

In programming languages, encapsulation is used to refer to one of two related but distinct notions, and sometimes to the combination thereof:

Some programming language researchers and academics use the first meaning alone or in combination with the second as a distinguishing feature of object-oriented programming, while some programming languages that provide lexical closures view encapsulation as a feature of the language orthogonal to object orientation.

The second definition is motivated by the fact that in many of the OOP languages hiding of components is not automatic or can be overridden; thus, information hiding is defined as a separate notion by those who prefer the second definition.

The features of encapsulation are supported using classes in most object-oriented programming languages, although other alternatives also exist.

Encapsulation is one of the fundamentals of OOP (object-oriented programming). It refers to the bundling of data with the methods that operate on that data. Encapsulation is used to hide the values or state of a structured data object inside a class, preventing unauthorized parties' direct access to them. Publicly accessible methods are generally provided in the class (so-called getters and setters) to access the values, and other client classes call these methods to retrieve and modify the values within the object.

This mechanism is not unique to object-oriented programming. Implementations of abstract data types, e.g. modules, offer a similar form of encapsulation. This similarity stems from the fact that both notions rely on the same mathematical fundamental of an existential type.

Encapsulation can be used to hide data members and members function. Under this definition, encapsulation means that the internal representation of an object is generally hidden from view outside of the object's definition. Typically, only the object's own methods can directly inspect or manipulate its fields. Some languages like Smalltalk and Ruby only allow access via object methods, but most others (e.g. C++, C#, Delphi or Java) offer the programmer a degree of control over what is hidden, typically via keywords like public and private. ISO C++ standard refers to protected, private and public as "access specifiers" and that they do not "hide any information". Information hiding is accomplished by furnishing a compiled version of the source code that is interfaced via a header file.

...
Wikipedia