Alan Kay summarized five basic characteristics of Smalltalk, the first successful object-oriented language and one of the languages upon which Java is based. This represents a pure approach to object-oriented programming: 1. Everything is an object. Think of an object as a fancy variable: it stores data, but you can also ask it to perform operations on itself by making requests. In theory, you can take any conceptual component in the problem youre trying to solve (dogs, buildings, services, etc.) and represent it as an object in your program. 2. A program is a bunch of objects telling each other what to do by sending messages. To make a request of an object, you send a message to that object. More concretely, you can think of a message as a request to call a function for a particular object. 3. Each object has its own memory made up of other objects. Or, you make a new kind of object by making a package containing existing objects. Thus, you can build up complexity in a program while hiding it behind the simplicity of objects. 4. Every object has a type. Using the parlance, each object is an instance of a class, where class is synonymous with type. The most important distinguishing characteristic of a class is what messages can you send to it? � 5. All objects of a particular type can receive the same messages. This is actually a very loaded statement, as you will see later: because an object of type circle is also an object of type shape, a circle is guaranteed to receive shape messages. This means you can write code that talks to shapes, and automatically handle anything that fits the description of a shape. This substitutability is one of the most powerful concepts in OOP. Some language designers have decided that object-oriented programming itself is not adequate to easily solve all programming problems, and advocate the combination of various approaches into multiparadigm programming languages 2 .