تشريح Hello World

Written Explanation

The smallest useful program

Here is the program from the module's first demo. Create a file named Hello.java with this content:

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
    }
}

Compile and run it as you did in BEG-007:

javac Hello.java
java Hello

You should see Hello, Java! on its own line. Now let us name the parts.

Part 1: the class

public class Hello {
    ...
}

Java code does not float free in a file. In Java, code that does work lives inside a class. For now, think of a class as a named container for code. The line public class Hello has three pieces:

• public means this class can be used from anywhere. We need it for the file-name rule below.
• class is the keyword that says "a class starts here".
• Hello is the name you chose. By convention, class names start with a capital letter.

The opening brace { begins the class and the final } ends it. Everything between them belongs to the class.

Part 2: the main line

public static void main(String[] args) {

This line is the entry point. When you type java Hello, the Java launcher loads the class Hello and then calls its main method. The Java Language Specification requires that this method be public, static, void, and take one parameter whose type is an array of String. The java documentation describes the same form.

You do not need to understand every word yet. Here is what each one signals, with the full explanation arriving in later modules:

The name args is only a convention. String[] words works equally well, because it is just a name for the parameter. The words public, static, void, main, and String[] are the part you must not change.

Treat this line as a fixed phrase, like the title of a form. Learn it exactly, and type it the same way every time.

A note on newer Java. Some recent JDK releases add shorter ways to write a program's entry point. This course uses the classic full form above, because it works in every JDK you are likely to meet, and because it makes the structure visible. Check your own JDK's documentation if you want to read about the newer forms.

Part 3: braces and blocks

Curly braces mark where something begins and ends. In our program there are two pairs:

• The outer pair belongs to the class Hello.
• The inner pair belongs to the method main.

The inner pair sits inside the outer pair, like a small box inside a larger box. Every { must be closed by a matching }, and the compiler stops with an error if it cannot find the partner. We study this in detail in the next lesson.

Part 4: statements

Inside main's braces we have one statement:

System.out.println("Hello, Java!");

A statement is one instruction, and it ends with a semicolon. When the program runs, the JVM starts at the first statement inside main and carries them out one after another, top to bottom. When it runs out of statements, main ends, and the program ends with it.

This particular statement sends the text between the quotes to the screen and then moves to the next line. We look at it closely in Lesson 3.

Two rules that trip up beginners

Rule 1: Java is case sensitive. Main and main are different words. So are System and system, and String and string. If you write public static void Main(String[] args), the program compiles, because it is a valid method with a legal name. But the launcher looks for exactly main, so running it fails with a message saying it cannot find the main method. (You will try this yourself in Lesson 5.)

Rule 2: a public class lives in a file with the same name. The Java Language Specification says that when a class is declared public, it must be stored in a file whose name is the class name plus .java. So public class Hello must be in Hello.java, including capital letters. If it is in hello.java, javac refuses, as you saw in the BEG-007 demo. You then run the program with the class name, not the file name: java Hello.