Your first Java program

The parts of a Java program

Look at the example program below. It has a few important parts.

A Java statement such as System.out.println("Hello"); ends with a semicolon ;, like a full stop ends a sentence. Other parts, such as the class and method lines, follow different rules: they use curly brackets { } instead.

• public class Hello – a class is a block that holds your code. This class is named Hello.
• public static void main(String[] args) – the main method. In this course, we will initially use the traditional Java program structure, where execution begins here. Newer Java versions also support simpler forms, which we can explore later.
• System.out.println("..."); – prints the text inside the quotes and then moves to a new line.
• { } – curly brackets show where the class and the main method start and end.

Save the file with the right name

In your IDE, create a class named Hello. The IDE saves it as Hello.java for you. Without an IDE, type the program in any text editor, such as Notepad or TextEdit, and save it as Hello.java in a practice folder (next section).

A public top-level class named Hello must be declared in a file named Hello.java. Top-level means the class is not inside another class.

Java cares about capital letters. Hello.java and hello.java are different names.

Open a terminal in your practice folder

Skip this if you only use your IDE. To use the terminal, keep your files in one simple folder and open the terminal there. Do this once; later just open the folder again.

Run it: IDE or terminal

In your IDE (from the previous lesson), click Run. The IDE compiles and runs the program in one step, and shows the output at the bottom.

In the terminal, type these two commands in your practice folder. In this workflow, javac Hello.java compiles the file and java Hello launches the compiled class.

1. 1.javac Hello.java – the compiler checks your code and makes Hello.class (bytecode).
2. 2.java Hello – the JVM runs your program. Type the class name only: never java Hello.class.

Common beginner mistakes

Everyone makes these mistakes at first. The compiler shows the line number of the error. Read it, fix that line and compile again.

If something else goes wrong, use the Common errors → fixes list in the Summary tab. Check from the top: first the JDK, then the folder and file name, then the code.

• Missing semicolon – a statement like System.out.println("Hi"); needs ; at the end. If not, you see ';' expected.
• Wrong file name – a public class Hello must be saved in Hello.java.
• Wrong capital letters – write System, not system. Java sees them as different words.
• Missing quotes or brackets – text needs double quotes, and every { needs a }.