Everything is an object, Part 1

Learn to write your first Java program with these Java basics

By Bruce Eckel, JavaWorld.com, 09/08/00

TEXTBOX: TEXTBOX_HEAD: Everything is an object: Read the whole series!

• Part 1. Learn to write your first Java program with these Java basics

• Part 2. Build your first Java program
• :END_TEXTBOX
  
  

The Java language assumes that you want to do only object-oriented programming. This means that before you can begin you must shift your mindset into an object-oriented world (unless it's already there). The benefit of this initial effort is the ability to program in a language that is simpler to learn and to use than many other OOP languages. In this [two-part article] we'll see the basic components of a Java program and we'll learn that everything in Java is an object, even a Java program.

You manipulate objects with references

Each programming language has its own means of manipulating data. Sometimes the programmer must be constantly aware of what type of manipulation is going on. Are you manipulating the object directly, or are you dealing with some kind of indirect representation (a pointer in C or C++) that must be treated with a special syntax?

All this is simplified in Java. You treat everything as an object, so there is a single consistent syntax that you use everywhere. Although you treat everything as an object, the identifier you manipulate is actually a "reference" to an object. You might imagine this scene as a television (the object) with your remote control (the reference). As long as you're holding this reference, you have a connection to the television, but when someone says "change the channel" or "lower the volume," what you're manipulating is the reference, which in turn modifies the object. If you want to move around the room and still control the television, you take the remote/reference with you, not the television.

Also, the remote control can stand on its own, with no television. That is, just because you have a reference doesn't mean there's necessarily an object connected to it. So if you want to hold a word or sentence, you create a String reference:

String s;

But here you've created only the reference, not an object. If you decided to send a message to s at this point, you'll get an error (at run-time) because s isn't actually attached to anything (there's no television). A safer practice, then, is always to initialize a reference when you create it:

String s = "asdf";

However, this uses a special Java feature: strings can be initialized with quoted text. Normally, you must use a more general type of initialization for objects.

You must create all the objects

When you create a reference, you want to connect it with a new object. You do so, in general, with the new keyword. new says, "Make me a new one of these objects." So in the above example, you can say: