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

• Holds local variables (not instance or class fields)
• Organized into call frames (one per method call)
• Follows a Last-In-First-Out (LIFO) order
• Each call frame contains:
  • Local variables
  • Method parameters
  • Reference to this (for non-static methods)
• A new frame is created when a method is called and removed when the method finishes

Heap

• Stores objects created with new
• Objects can be shared between methods and variables
• Each object contains:
  • Class name
  • Instance fields and their values
  • Captured variables (for nested classes)
• No LIFO restriction; objects can outlive the methods that created them

Constructor Example

Point p;                  // Stack: p (uninitialized)
p = new Point(1, 2);      // Heap: new Point object
                          // Stack: p now references the Point
                          // Constructor call creates a new call frame

Declare Point variable on stack (null placeholder)Allocate Point object on heapInvoke Constructor (new Point(1, 2))Constructor completes (pop frame)Assign object reference to p

• The symbol ∅ indicates an uninitialized variable
• Each constructor call creates a new stack frame containing:
  • The this reference
  • Constructor parameters
  • Local variables

Method Call Example

class Point {
  private double x;
  private double y;

  public Point(double x, double y) {
    this.x = x;
    this.y = y;
  }

  public void move(double x, double y) {
    this.x = x;
    this.y = y;
  }
}

Point p1 = new Point(0, 0);
Point p2 = new Point(1, 1);
double x = 5;
double y = 5;
p1.move(x, y);

After Lines 1-2: p1 and p2 objects createdAfter Lines 3-4: x and y primitives assignedLine 5: move method invokedMethod completes, frame poped, p1 updated

• Each method call creates a new stack frame
• The frame is destroyed when the method returns
• Method parameters are copied into the new frame

Best Practices

• Understand the lifetime of objects and variables
• Be aware of when objects are shared or referenced
• Know the difference between stack and heap allocation
• Clean up references when no longer needed
• Be mindful of memory usage patterns