Unit 1: How Java Actually Runs

Java is not a magic box where text goes in and answers come out. A Java program moves through a chain: source code -> compiler -> bytecode -> JVM -> observable behavior.

That chain matters. It tells you where an error can occur, why Java can run on different operating systems, and why a file that looks reasonable to a human can still be rejected before one line executes.

In this unit you will:

• run Java 25 code in the official Java Playground;
• distinguish source code, compilation, bytecode, JVM execution, and output;
• use Java 25 compact source files so the first program focuses on behavior rather than boilerplate;
• predict output before execution;
• change one variable at a time and preserve evidence of what changed.

Use the official browser runner for the core work: https://dev.java/playground/.

By the end, you should be able to explain what happened between the code you typed and the output you saw.