How Java runs, and Hello World

The big idea

Java is one of the most widely used programming languages in the world. It was released in 1995 and was designed around one promise: write once, run anywhere. You write your program once, and it runs on any computer that has Java installed, whatever the operating system.

If you already know some Python, a lot will feel familiar: variables, if, loops and functions all exist in Java. The big differences are that Java is compiled before it runs, every variable has a fixed type, and the code is organised with curly braces { } instead of indentation.

Explained simply

Think of a recipe written in English being translated into a universal sign language that every kitchen in the world understands. Java works in a similar way: you write the program once, it is translated once, and the translation runs anywhere.

1. You write source code: the instructions in Java, saved in a file ending in .java.
2. The compiler, a program called javac, reads your source code, checks it for mistakes and translates it into bytecode. Bytecode is a compact set of instructions that is not tied to any one kind of computer. It is saved in a .class file.
3. The Java Virtual Machine (JVM) is a program that runs bytecode. There is a JVM for Windows, one for macOS, one for Linux and more. Each one understands the same bytecode.