🖥️ This lecture covers how the Java Virtual Machine (JVM) works internally — its memory model, the difference between static and instance variables, how method calls use the Java Stack (frames), and how bytecode is executed on the operand stack.

At-a-glance Checklist

• [ ] Understand why Java uses a JVM instead of compiling directly to machine code
• [ ] Know the Java compilation pipeline: .java → javac → .class (bytecode) → JVM
• [ ] Learn what JIT (Just-In-Time) compilation is and why modern JVMs use it
• [ ] Memorise the 3 main JVM memory areas: Method Area, Heap, Java Stack
• [ ] Understand what is stored in the Method Area (class info, static variables, methods)
• [ ] Understand what is stored on the Heap (object instances, instance variables)
• [ ] Understand what is stored on the Java Stack (frames for each method call)
• [ ] Know the difference between static and instance variables and where each is stored
• [ ] Trace through a method call chain using stack frames (push/pop)
• [ ] Walk through a bytecode execution example using the operand stack
• [ ] Understand how recursion creates new frames on the stack
• [ ] Know the Thread.dumpStack() debugging command

Why Does Java Need a Virtual Machine?

The Problem with Direct Compilation

Languages like C and C++ compile source code directly into machine binaries that are specific to a particular hardware architecture (Windows, Mac, Linux, etc.).

Java's goal was to be a universal language — write once, run anywhere. This means Java code cannot be compiled all the way down to architecture-specific binaries.

The Solution: An Intermediate Layer

Java introduces the JVM as a middle layer: