9.4: The Hello World Program

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Traditionally, the first program you write when learning a new programming language is called the hello world program. All it does is display the words “Hello, World!” on the screen. In Java, it looks like this:

public class Hello {

    public static void main(String[] args) {
        // generate some simple output
        System.out.println("Hello, World!");
    }
}

When this program runs it displays:

Hello, World!

Notice that the output does not include the quotation marks.

Java programs are made up of class and method definitions, and methods are made up of statements. A statement is a line of code that performs a basic operation. In the hello world program, this line is a print statement that displays a message on the screen:

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

System.out.println displays results on the screen; the name println stands for “print line”. Confusingly, print can mean both “display on the screen” and “send to the printer”. In this book, we’ll try to say “display” when we mean output to the screen. Like most statements, the print statement ends with a semicolon (;).

Java is “case-sensitive”, which means that uppercase and lowercase are not the same. In this example, System has to begin with an uppercase letter; system and SYSTEM won’t work.

A method is a named sequence of statements. This program defines one method named main:

public static void main(String[] args)

The name and format of main is special: when the program runs, it starts at the first statement in main and ends when it finishes the last statement. Later, we will see programs that define more than one method.

A class is a collection of methods. This program defines a class named Hello. You can give a class any name you like, but it is conventional to start with a capital letter. The name of the class has to match the name of the file it is in, so this class has to be in a file named Hello.java.

Java uses squiggly braces ({ and }) to group things together. In Hello.java, the outermost braces contain the class definition, and the inner braces contain the method definition.

The line that begins with two slashes (//) is a comment, which is a bit of English text that explains the code. When the compiler sees //, it ignores everything from there until the end of the line. Comments have no effect on the execution of the program, but they make it easier for other programmers (and your future self) to understand what you meant to do.