lecture 5:programming language

Classes and Objects:
The class is the essence of Java. It is the foundation upon which the entire Java language is built because the class defines the nature of an object. As such, the class forms the basis for object oriented programming in Java. Within a class are defined data and code that acts upon that data. The code is contained in methods.
Class Fundamentals:
A class is a template that defines the form of an object. It specifies both the data and the code that will operate on that data. Java uses a class specification to construct objects. Objects are instances of a class. Thus, a class is essentially a set of plans that specify how to build an object. It is important to be clear on one issue: a class is a logical abstraction. It is not until an object of that class has been created that a physical representation of that class exists in memory.
One other point: recall that the methods and variables that constitute a class are called members of the class. The data members are also referred to as instance variables.
The General Form of a Class
When you define a class, you declare its exact form and nature. You do this by specifying the instance variables that it contains and the methods that operate on them. Although very simple classes might contain only methods or only instance variables, most real-world classes contain both.
A class is created by using the keyword class. The general form of a class definition is