Thread: Noob question: Why .class?

I'm using Eclipse as my IDE for Java coding, and noticed that all files are .class, even if they do not define classes within them... From what I know so far, classes are templates for objects, so code that say prints 'Hello World' does not fit into this definition. Why then are all these files .class?

.class files are the output from the javac compiler when it compiles a .java source program.
They do define classes. Their contents are in byte code vs text and are not meant to be read with an editor.

I suppose what I meant is that the primary function of the HelloWorld.class file is not to define a class within it, it does use them, but it itself just prints the words out... From what I know a 'class' should have constructors, fields and methods for objects within that class. In this case, HelloWorld has none of these things.

the primary function of the HelloWorld.class file is to define a class.
A minimal class, but still a class.

So what exactly is the definition of a class then? How do you justify calling HelloWorld a class? I was looking at this link:

What Is a Class? (The Java™ Tutorials > Learning the Java Language > Object-Oriented Programming Concepts)

and it seems that Bicycle.class and BicycleDemo.class are two quite different bits of code and yet they are still both defined to be classes... Bicycle.class contains the information/blueprints for the Bicycle object and BicycleDemo actually uses this information to create some "useful" application.

It is a class with one static method.

Let me try to clear up some of the confusion here. First: the .class you see at the end of your files has nothing to do with the class as in classes and objects. Classes and objects are one thing, but .class is a file type.

When you write a program, it is saved as a .java file. For example, if your program were called Runner, it would be saved first as Runner.java.

However, your computer can't understand the stuff written in the .java file. Therefore, you have to convert the .java file into a language that the JVM can understand. (The JVM is the Java Virtual Machine, which is what runs your java programs.) You do this by compiling. The language that it is converted to is called byte code. The compiler makes new copies of your files in this language, and saves them as .class files. So, if you compile Runner.java, it would create a new file named Runner.class.

The .class files are what the JVM interprets when it runs your file.


Classes and Objects are different things entirely. All Java files are classes, for the most part (there are interfaces and enums and stuff too but you will learn about those eventually.) Even your "hello world" is written in a class, which is why you have to start it with "public class Something". (Just because you didn't use it as a class doesn't mean it isn't a class!) Your "hello world" class does have one method, which is the main method: public static void main(String args[])

When you get to making classes that represent things, like "public class Person" or something, you will make instances of these classes. An Object is an instance of a class.

All of that is very important to Java programming, but has little to do with .class files! Remember, .class is just a file type that represents byte code, and it's made by the compiler.

Hope this helps...

Some slightly incorrect statements:

So, if you compile Runner.java, it would create a new file named Runner.class.

There can be more than one class defined in a .java file and none of the names of the classes have to match the name of the .java file. The standard is that you name the source file with the name of the class it contains. However the compiler doesn't care.

First: the .class you see at the end of your files has nothing to do with the class

The compiler generates a .class file for each class you define in your source. The .class file will have the same name as the class.

Classes and Objects are different things entirely

Class definitions are used to build objects. You can not get an instance of an object without a class definition.

Actually, when I said "classes and objects are different things entirely", I meant they are different from the .class file type, not from each other. I have been misconstrued, it seems.