Classes and Objects In Java

A class is a blueprint for a part of an application; it holds methods and variables that make up that component. Many individual working copies of a given class can exist while an application is active. These individual incarnations are called instances of the class, or objects. Two instances of a given class may contain different data, but they always have the same methods. As an example, consider a Button class. There is only one Button class, but an application can create many different Button objects, each one an instance of the same class. Furthermore, two Button instances might contain different data, perhaps giving each a different appearance and performing a different action. In this sense, a class can be considered a mold for making the object it represents. As you’ll see later, there’s a bit more to it than that—a class can in fact share information among its instances—but this explanation suffices for now. The term object is very general and in some other contexts is used almost interchangeably with class. Objects are the abstract entities that all object-oriented languages refer to in one form or another. We will use object as a generic term for an instance of a class. We might, therefore, refer to an instance of the Button class as a button, a Button object, or, indiscriminately, as an object. The main() method in the previous example creates a single instance of the JLabel class and shows it in an instance of the JFrame class. You could modify main() to create many instances of JLabel, perhaps each in a separate window.