Classes and Objects: A First Look
Classes are the single most important feature of Java. Everything in Java is either a class, a part of a class, or describes how a class behaves. Although classes will be covered in great detail in section four, they are so fundamental to an understanding of Java programs that a brief introduction is going to be given here.
All the action in Java programs takes place inside class blocks, in this case the HelloWorld class. In Java almost everything of interest is either a class itself or belongs to a class. Methods are defined inside the classes they belong to. This may be a little confusing to C++ programmers who are used to defining all but the simplest methods outside the class block, but this approach is really more sensible. C++ takes the road it does primarily out of desire to be compatible with C, not out of good object-oriented design. Both syntactically and logically everything in Java happens inside a class.
Even basic data primitives like integers often need to be incorporated into classes before you can do many useful things with them. The class is the fundamental unit of Java programs, not source code files like in C. For instance consider the following Java program:
class HelloWorld {
public static void main (String args[]) {
System.out.println("Hello World");
}
}
class GoodbyeWorld {
public static void main (String args[]) {
System.out.println("Goodbye Cruel World!");
}
}Save this code in a single file called hellogoodbye.java in your javahtml directory, and compile it with the command javac hellogoodbye.java. Then list the contents of the directory. You will see that the compiler has produced two separate class files, HelloWorld.class and GoodbyeWorld.class.
The second class is a completely independent program. Type java GoodbyeWorld and then type java HelloWorld. These programs run and execute independently of each other although they exist in the same source code file. Off the top of my head I can’t think of why you might want two separate programs in the same file, but if you do the capability is there.
It’s more likely that you’ll want more than one class in the same file. In fact you’ll see source code files with many classes and methods.
In fact there are a few statements that can, at least at first glance, appear outside a class. Import statements appear at the start of a file outside of any classes. However the compiler replaces them with the contents of the imported file which consists of, you guessed it, more classes.