Code Structure

In a source file, put a class. In a class, put methods. In a method, put statements.

What goes in a source file

A source file (a file with the .java extension) typically holds one class definition. The class is a piece of your program, and the class must go within a pair of curly braces.

public class Dog {

}

What goes in a class

A class has one or more methods. In the Dog class, the bark method will hold instructions for how the dog will bark. Your methods must be declared inside a class (within the curly braces of the class).

public class Dog {
    void bark() {

    }
}

What goes in a method

Inside your method, write your instructions for how that method should be performed. Method code is basically a set of statements, and you can think of a method like a function or procedure.

public class Dog {
    void bark() {
        statement1;
        statement2;
    }
}

Anatomy of a class

When the JVM starts running, it looks for the class you give it at the command line, then it starts looking for a specially written method:

public static void main(String[] args) {
    // code goes here
}

The JVM runs everything within the curly braces of the main method. Every Java app has to have at least one class, and at least one main method (not one main per class, just one main per application):

public class MyFirstApp {
    public static void main(String[] args) {
        System.out.print("I Rule!");
    }
}

1. public — so everyone can access it
2. class — definition of a class
3. MyFirstApp — the name of the class
4. static — we’ll get there later
5. void — the return type of the method; void means it has no return value
6. main — the name of the method
7. String[] args — arguments to the method; this method must be given an array of Strings, and the array will be called args
8. System.out.print — print to the standard output the string “I rule”

Notes

A Java source file ends with the .java extension, and a compiled bytecode file ends with the .class extension. In every Java program, you must have only one top-level public class, and that class has to match the name of the source file.

Let’s say you have a source file with multiple classes in it. When you compile the code, the JVM creates bytecode .class files for each class in your source file. To avoid any confusion here, the JVM is designed to treat the class that has the same name as your source file as the entry point to your application.