From source code to running program¶

☕ Java 17 · How Java Works Under the Hood · 1/3

Three steps, every time¶

Every Java program follows the same journey:

Step	Tool	Input → output
1. Write	your editor	`Movie.java` — source code, plain text
2. Compile	`javac`	`Movie.java` → `Movie.class` — bytecode
3. Run	`java` (the JVM)	executes `Movie.class`

$ javac Movie.java     # compile: creates Movie.class
$ java Movie           # run: the JVM executes the bytecode

Bytecode is a middle language. It is no longer human-readable source code, but it is not machine code either. The JVM (Java Virtual Machine) turns it into instructions for the machine it runs on.

Write once, run anywhere¶

Bytecode doesn't depend on the operating system. The same Movie.class runs unchanged on Windows, macOS or Linux, as long as a JVM is installed. That's the promise Java made in 1995, and it still holds: you compile once, and the JVM of each machine does the rest.

The tools in the JDK¶

The JDK (Java Development Kit) is the toolbox you install to write Java. Four of its tools come up early:

Tool	Role	Example
`javac`	compiler: `.java` → `.class`	`javac Movie.java`
`java`	launcher: starts the JVM and runs a class	`java Movie`
`jar`	packager: bundles `.class` files into one `.jar` archive	`jar cf app.jar *.class`
`javadoc`	generates HTML documentation from code comments	`javadoc Movie.java`

To start, you only need two: javac to compile, java to run. The other tools come in later, when a project grows.

Two details that matter from day one¶

javac takes a file name, with its extension: javac Movie.java.
java takes a class name, without any extension: java Movie, never java Movie.class.

Java is also case-sensitive everywhere: Movie, movie and MOVIE are three different names.

Now that the journey is clear, let's look at the JDK itself and which version to install.

💻 Code: Java17_Part_1_Fundamentals_Core_Syntax on GitHub

Next: your JDK →