Introduction to Java

First program — Hello World

• The public class name must match the file name: save this as HelloWorld.java.
• Compile with javac HelloWorld.java (creates HelloWorld.class), then run java HelloWorld (no .class suffix).
• main must be public static void main(String[] args) so the launcher can start the program without an instance.

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

Compilation, bytecode, and the launcher

• javac is a compiler, not an interpreter. The JVM then interprets and/or JIT-compiles hot methods to native code.
• java HelloWorld uses the application class loader; a public class in the default package is found on the current directory classpath (.).
• If you use a package (package com.demo;), the file must live in a matching folder and you run java com.demo.HelloWorld from the package root.
• One public top-level class per .java file. Additional package-private classes in the same file are allowed but uncommon in tutorials.
• main can also be public static void main(String... args) (varargs). A non-static main will not start the program.

Hello World

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

Hello with command-line args

public class HelloArgs {
    public static void main(String[] args) {
        String name = args.length > 0 ? args[0] : "learner";
        System.out.println("Welcome to Java, " + name);
    }
}