The fundamental programming unit of the Java programming language is the class. Classes provide the structure for objects and the mechanisms to manufacture objects from a class definition. Classes define methods: collections of executable code that are the focus of computation and that manipulate the data stored in objects. Methods provide the behavior of the objects of a class. Although you can compute using only primitive typesinteger, floating-point, and so onalmost any interesting program will create and manipulate objects.
Object-oriented programming strictly separates the notion of what is to be done from how it is done. "What" is described as a set of methods (and sometimes publicly available data) and their associated semantics. This combinationmethods, data, and semanticsis often described as a contract between the designer of the class and the programmer who uses it because it says what happens when certain methods are invoked on an object. This contract defines a type such that all objects that are instances of that type are known to honor that contract.
