Your first Java program

A program is a list of instructions for a computer. Picture sheet music for an orchestra, the score. It lists what to play in order, and the orchestra plays it bar by bar, from the first to the last. A computer runs a program the same way: instruction after instruction, top to bottom.

Music is written in special signs that every musician understands. A program is also written in a special language, and in this course that language is Java. Here is a program that prints one line:

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

Let's walk through it line by line:

1. public class Main { opens a class named Main, a named block that holds code. In Java, every instruction lives inside some class.
2. public static void main(String[] args) { opens the method main, a named set of instructions inside a class. Every program in this course writes this line the same way, word for word.
3. System.out.println("Hello, Java"); is the only instruction: print Hello, Java.
4. The first } closes the method main.
5. The second } closes the class Main.

Text from // to the end of the line is a comment, and Java skips it. In the course examples, a comment at the end of a line shows what that line prints.

Where execution starts

When the program starts, Java finds the main method in the class and runs its body: the instructions between the curly braces after main(String[] args). That body is the score the computer plays:

• instructions run one after another, top to bottom;
• after the last instruction, the program ends.

Braces and semicolons

Java text has a strict shape. Three rules hold it together:

• curly braces { and } come in pairs: the opening one starts a block, and the closing one ends it;
• the pair of braces of the method main sits inside the pair of braces of the class Main, like a box inside a box;
• every instruction ends with a semicolon, the way a sentence ends with a period.

Java does not read indentation, but it shows a person what sits inside what. In this course, each level of nesting is indented by four spaces.

Printing: println and print

System.out is the program's output: the place where printed text appears. Read System.out.println(…) as: print to the output whatever is in the parentheses. Text in double quotes is printed as is, without the quotes. A number can be written without quotes.

Printing comes in two variants. The difference between them is the line break.

println: with a line break

System.out.println("Hello"); // Hello
System.out.println(42);      // 42

println prints a value and ends the line, so the next print starts on a new line. The ending ln helps you remember the difference: it is short for "line".

print: no line break

System.out.print("Ja");
System.out.print("va");
System.out.println("!"); // Java!

print prints a value and stays on the same line. Let's see what each instruction does:

1. print("Ja") starts the line: it holds Ja.
2. print("va") adds va right after it. print itself adds no spaces.
3. println("!") adds ! and ends the line. The result is Java!.

In the widget, these lines sit inside a whole program. Press "Next step" and watch which step finishes each line of output. Next to the code, the widget shows the method running now: Main.main, that is, the method main of the class Main. A line appears in the "output" box once println finishes it.

The order of lines in the output always matches the order of the instructions: Java neither reorders nor skips them.

Running the program

To run the program, take three steps:

1. Save the text to a file named Main.java. The file name matches the class name.
2. Open a terminal, a window for text commands, in the folder with this file.
3. Type the command java Main.java.

Before the program runs, the compiler checks the text: a program that enforces Java's rules and translates the text into a form that can run. It is a strict proofreader: if even one rule is broken, it shows an error, and the program does not start.

This is where the orchestra picture falls short. A musician can guess which note hides under an ink blot and play the piece to the end. The compiler does not guess: because of one error, no line runs, not even a correct one.

Common mistake

The closing brace of the class is missing:

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

error: reached end of file while parsing

The compiler read the file to the end and found no partner for the { on the first line: the class Main was left open. Add } as the last line. To see the pairs, put each closing brace right under the start of the line with its opening brace.

Key points

• A Java program is a Main class with a main method; execution starts at main and runs top to bottom.
• Curly braces come in pairs, and every instruction ends with a semicolon.
• println prints a value and ends the line, while print stays on the same line and adds no spaces.
• java Main.java checks the program and runs it; if the compiler finds an error, not a single line runs.