Implement a Minimal JVM That Runs Hello World

A Minimal JVM

Running \`Hello World\` on a toy JVM requires class parsing, constant pool resolution, and a bytecode interpreter for a handful of opcodes. Real HotSpot adds tiers, GC, and intrinsics; the core loop is the same fetch-decode-execute pattern as your bytecode VM.

Subset for Hello World

Minimum capabilities:

• Parse \`.class\` header and constant pool
• Resolve \`#main\` ([Ljava/lang/String;)V
• Execute \`ldc\`, \`invokevirtual\`, \`return\` for \`println\`

Native methods (\`System.out\`) can be stubbed: when bytecode hits \`getstatic\` on \`out\`, return a handle your runtime recognizes.

For example, \`getstatic java/lang/System.out\` followed by \`ldc "Hello"\` and \`invokevirtual println\` is the entire program path.

Frame setup mirrors the JVM spec: local array sized from \`max_locals\`, operand stack from \`max_stack\`.

Design Notes

Stub native methods with logging first so you know which host calls fire during Hello World. Real JVMs map hundreds of natives; a toy runtime can fake `println` with a single special case.

For This Exercise

This exercise asks you to implement a minimal JVM that loads one class and runs \`main\`. You will parse enough of the class file and interpret enough opcodes to print Hello World.

You will use the same mental model here when reading production interpreter source later in the track. Sketch one concrete input on paper, predict the outcome, then confirm with code. That discipline catches logic errors early and makes debugging far faster when you extend the implementation in follow-on tasks.