Module 4: Memory

JVM Memory Architecture

The Java Virtual Machine (JVM) manages memory in several different areas:

• Stack: Stores method frames, local variables, and partial results
• Heap: Stores all objects and their instance variables
• Method Area: Stores class structures, static variables, and method code

Stack vs. Heap

Understanding the difference between stack and heap memory is crucial:

• The stack is organized as frames, with each frame representing a method call
• Each stack frame contains local variables and references to objects in the heap
• The heap stores all objects, regardless of where they were created
• References to heap objects can be stored in local variables (on the stack) or as fields in other objects (on the heap)

Example: Memory Allocation

public class MemoryExample {
    public static void main(String[] args) {
        // Local primitive variable - stored on stack
        int count = 5;
        
        // Object reference stored on stack, object stored on heap
        Person person = new Person("Alice", 30);
        
        // Method call creates new stack frame
        doSomething(person);
    }
    
    public static void doSomething(Person p) {
        // Local primitive - stored on stack in the doSomething frame
        int age = p.getAge();
        
        // New object created on heap, reference on stack
        String message = "Hello, " + p.getName();
        
        System.out.println(message);
        
        // When method exits, stack frame is removed
        // If no more references to message exist, it becomes eligible for garbage collection
    }
}

class Person {
    // Instance variables stored on heap with the object
    private String name;
    private int age;
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() { return name; }
    public int getAge() { return age; }
}