The Java Language Environment

3.1 Object Technology in Java

To be truly considered "object oriented", a programming language should support at a minimum four characteristics:

• Encapsulation--implements information hiding and modularity (abstraction)
• Polymorphism--the same message sent to different objects results in behavior that's dependent on the nature of the object receiving the message
• Inheritance--you define new classes and behavior based on existing classes to obtain code re-use and code organization
• Dynamic binding--objects could come from anywhere, possibly across the network. You need to be able to send messages to objects without having to know their specific type at the time you write your code. Dynamic binding provides maximum flexibility while a program is executing

Java meets these requirements nicely, and adds considerable run-time support to make your software development job easier.

3.2 What Are Objects?

At its simplest, object technology is a collection of analysis, design, and programming methodologies that focuses design on modelling the characteristics and behavior of objects in the real world. True, this definition appears to be somewhat circular, so let's try to break out into clear air.

What are objects? They're software programming models. In your everyday life, you're surrounded by objects: cars, coffee machines, ducks, trees, and so on. Software applications contain objects: buttons on user interfaces, spreadsheets and spreadsheet cells, property lists, menus, and so on. These objects have state and behavior. You can represent all these things with software constructs called objects, which can also be defined by their state and their behavior.

In your everyday transportation needs, a car can be modelled by an object. A car has state (how fast it's going, in which direction, its fuel consumption, and so on) and behavior (starts, stops, turns, slides, and runs into trees).

You drive your car to your office, where you track your stock portfolio. In your daily interactions with the stock markets, a stock can be modelled by an object. A stock has state (daily high, daily low, open price, close price, earnings per share, relative strength), and behavior (changes value, performs splits, has dividends).

After watching your stock decline in price, you repair to the cafe to console yourself with a cup of good hot coffee. The espresso machine can be modelled as an object. It has state (water temperature, amount of coffee in the hopper) and it has behavior (emits steam, makes noise, and brews a perfect cup of java).