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Object-Oriented Programming

In a class-based object-oriented programming language3, like Java, any object must be an instance of a class.

What is an Object?

An object represents an individual, identifiable item, unit, or entity, either real or abstract, with a well-defined role in the problem domain.2

What is a Class?

A class defines the attributes, structure and operations that are common to a set of objects, including how the objects are created.

A class is an abstraction or a model.

What is a model?

A model is a partial representation of something else (a concept, a system, a pattern, etc.). It helps us to visualize and understand the original and its role in a problem domain.

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It has been said that teaching object-oriented programming (OOP) to those with no programming background is easier than those with experience in (procedural or functional) programming.1 This is because experienced programmers get used to procedural (or functional) thinking and modeling. On the other hand, for non-programmers, the object-oriented way of decomposing a problem is similar to the way they are used to look at real life situations. Indeed, we live in a world made up of interacting objects.

When we model something (i.e. create an abstraction of it), we focus on some of its characteristics (that matter to the problem at hand) and ignore others (that are deemed irrelevant for solving the problem). For example, this class models (the idea, concept of) circle in Grade 3 (K5) Geometry:

public class Circle {
    private double radius;

    public Circle() { radius = 1; }
    public void setRadius (double r) { radius = r; }
    public double getRadius() { return radius; }
    public double getArea() { 
        return Math.PI * Math.pow(radius, 2); 
    }
}

And here we have a few different Circle objects (instances):

Circle c1 = new Circle();
Circle c2 = new Circle();
c2.setRadius(3);
Circle c3 = new Circle();

An object has state and behavior.

The state of an object (a.k.a. its properties or attributes) are its essential and distinctive characteristics. A class declares attributes of its objects through its data fields (instance variables). The state of an object is value of those data fields at any point in time. A Circle class, for example, has a data field radius, which is the property that characterizes a circle. A circle object with radius of 1 cm is an instance of the Circle class. If we update the radius of this circle to a new value, we have changed its state.

The behavior of an object is the set of operations, or responsibilities, it must fulfill. This includes the responsibility to provide and modify state information when asked by clients (other objects or services). A behavior of an object is defined by the instance method that implements that behavior. To invoke a method on an object is to ask the object to fulfill a behavior (to perform an action). For example, you may ask a circle object to provide you with its state by invoking the method getRadius() which is declared and implemented in the Circle class.

  1. There are also prototype-based object-oriented programming languages, like JavaScript. ↩
  2. Smith, M., and Tockey S. "An Integrated Approach to Software Requirement Definition Using Objects." Boeing Commercial Airplane Support Division, Seatle, WA, (1988).↩
  3. Alan Key, for example, discovered that children learned SmallTalk faster than experienced programmers; see Key A., "Microelectronics and Personal Computer." Scientific American 237(3):230-244 (1977)↩