Heap and Stack Summary

JVM Memory Regions

• Method area: Stores method code
• Metaspace: Stores class metadata
• Heap: Stores dynamically allocated objects
• Stack: Stores local variables and call frames

Stack

• Contains variables (not instance/class fields)
• Organized in call frames
• LIFO (Last-In-First-Out) structure
• Call frames:
  • Created when method invoked
  • Removed when method completes
  • Contains local variables
  • Contains method parameters
  • Contains reference to this (for non-static methods)

Heap

• Stores objects created with new
• Objects persist across method calls
• Can be shared between methods
• Contains:
  • Class name
  • Instance fields and values
  • Captured variables
• No LIFO restriction

Example Stack and Heap Interactions

Point p;                    // Stack: p (uninitialized)
p = new Point(1, 2);       // Heap: Point object
                          // Stack: p (reference to Point)

Circle c = new Circle(p, 3); // Heap: Circle object
                           // Stack: c (reference to Circle)

Constructor Example

Point p1 = new Point(0, 0);  // Creates call frame for Point constructor
Point p2 = new Point(1, 1);  // Creates new call frame

- Each constructor call creates new frame - Frame contains: - this reference - Constructor parameters - Local variables

Method Call Example

void foo(Point p) {
  int x = p.getX();  // Creates call frame for getX
}

- Method call creates new frame - Frame destroyed after method returns - Parameters copied to new frame

Call Stack Behavior

• Methods can be nested
• Each nested call adds new frame
• Frames removed in reverse order
• Example:

  a() {
    b();      // b's frame stacked on a's
    return;   // b's frame removed, back to a's
  }
  

Best Practices

• Understand object lifetime
• Be aware of object sharing
• Consider stack vs heap allocation
• Clean up references when no longer needed
• Be mindful of memory usage patterns