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Your First Program

Scope: one file, compiled and run two ways, with every line explained. All samples below were compiled and run with OpenJDK 21.

The file

Create Hello.java:

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

Three rules hiding in five lines:

  • The file name must match the class name. public class Hello lives in Hello.java — capital H, both. Get this wrong and the compiler refuses (see Troubleshooting).
  • main is where the program starts. The exact incantation public static void main(String[] args) is non-negotiable; you'll memorize it by the third program. String[] args holds command-line arguments — ignore it for now.
  • System.out.println prints a line. System.out is "the screen," println is "print line." Java names are long but literal.

Compile, then run

javac Hello.java
java Hello

Check: the first command silently creates Hello.class (bytecode — see What Is Java?); the second prints:

Hello, world!

Note: you run java Hello, not java Hello.class and not java Hello.java. The runner wants the class name.

The shortcut: run the source directly

Since Java 11 you can skip the separate compile step for single-file programs:

java Hello.java

Check: prints Hello, world! with no .class file created. Handy for experiments; real projects still compile explicitly.

Reading error messages

Break it on purpose — delete the semicolon and compile:

javac Hello.java

Hello.java:3: error: ';' expected
        System.out.println("Hello, world!")
                                             ^
1 error

Java errors give you file, line, and a caret pointing at the problem. Read the first error only; later ones are often echoes of the first. Fix, recompile, move on.

Try it in jshell

jshell

jshell> System.out.println("Hello, jshell!")
Hello, jshell!

No class, no main, no file — jshell wraps snippets for you. Use it to test one-liners from the coming chapters.

What's next

A program that just prints is a parlor trick. Next: Variables and Types — storing values and doing arithmetic with them.