An object has an interface

Bruce Eckel's Thinking in Java Contents | Prev | Next

Aristotle was probably the first to begin a careful study of the concept of type. He was known to speak of “the class of fishes and the class of birds.” The concept that all objects, while being unique, are also part of a set of objects that have characteristics and behaviors in common was directly used in the first object-oriented language, Simula-67, with its fundamental keyword class that introduces a new type into a program (thus class and type are often used synonymously [3]).

Simula, as its name implies, was created for developing simulations such as the classic “bank teller problem.” In this, you have a bunch of tellers, customers, accounts, transactions, etc. The members (elements) of each class share some commonality: every account has a balance, every teller can accept a deposit, etc. At the same time, each member has its own state; each account has a different balance, each teller has a name. Thus the tellers, customers, accounts, transactions, etc. can each be represented with a unique entity in the computer program. This entity is the object, and each object belongs to a particular class that defines its characteristics and behaviors.

So, although what we really do in object-oriented programming is create new data types, virtually all object-oriented programming languages use the “class” keyword. When you see the word “type” think “class” and vice versa.

Once a type is established, you can make as many objects of that type as you like, and then manipulate those objects as the elements that exist in the problem you are trying to solve. Indeed, one of the challenges of object-oriented programming is to create a one-to-one mapping between the elements in the problem space (the place where the problem actually exists) and the solution space (the place where you’re modeling that problem, such as a computer).

But how do you get an object to do useful work for you? There must be a way to make a request of that object so it will do something, such as complete a transaction, draw something on the screen or turn on a switch. And each object can satisfy only certain requests. The requests you can make of an object are defined by its interface, and the type is what determines the interface. The idea of type being equivalent to interface is fundamental in object-oriented programming.

A simple example might be a representation of a light bulb:

Light lt = new Light();
lt.on();

The name of the type/class is Light, and the requests that you can make of a Light object are to turn it on, turn it off, make it brighter or make it dimmer. You create a “handle” for a Light simply by declaring a name ( lt) for that identifier, and you make an object of type Light with the new keyword, assigning it to the handle with the = sign. To send a message to the object, you state the handle name and connect it to the message name with a period (dot). From the standpoint of the user of a pre-defined class, that’s pretty much all there is to programming with objects.


[3] Some people make a distinction, stating that type determines the interface while class is a particular implementation of that interface.



Comments

  • There are no comments yet. Be the first to comment!

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Live Event Date: December 11, 2014 @ 1:00 p.m. ET / 10:00 a.m. PT Market pressures to move more quickly and develop innovative applications are forcing organizations to rethink how they develop and release applications. The combination of public clouds and physical back-end infrastructures are a means to get applications out faster. However, these hybrid solutions complicate DevOps adoption, with application delivery pipelines that span across complex hybrid cloud and non-cloud environments. Check out this …

  • VMware vCloud® Government Service provided by Carpathia® is an enterprise-class hybrid cloud service that delivers the tried and tested VMware capabilities widely used by government organizations today, with the added security and compliance assurance of FedRAMP authorization. The hybrid cloud is becoming more and more prevalent – in fact, nearly three-fourths of large enterprises expect to have hybrid deployments by 2015, according to a recent Gartner analyst report. Learn about the benefits of …

Most Popular Programming Stories

More for Developers

RSS Feeds