Heap vs Stack

In the JVM, memory is divided into runtime data areas. The two most critical areas for executing method calls and allocating objects are the Stack and the Heap.

Memory Allocation Rules

• Stack Memory: Each thread gets its own private Stack. Methods are allocated Stack Frames containing primitive local variables and object reference handles. Access is LIFO (Last-In-First-Out) and extremely fast.
• Heap Memory: All objects created via the new keyword reside on the Heap. The Heap is globally shared among all threads. It is managed by automatic Garbage Collection (GC).
• Variables: An instance variable of primitive type resides on the heap inside the object. A local reference variable resides on the stack, but points to an object on the heap.

Code Walkthrough

The following example maps how references on the stack point to object allocations on the heap.

public class MemoryLayoutDemo {
    public static void main(String[] args) {
        int localPrimitive = 10; // Stored in main() Stack frame
Point point = new Point(5, 7); // Reference "point" on Stack; "new Point" on Heap
processPoint(point);
    }
private static void processPoint(Point p) {
        // "p" is a copy of reference on processPoint() Stack frame, pointing to same Heap object
        int val = p.x;
    }
}
class Point {
    int x; // Stored on Heap as part of Point object
    int y;
Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

Interview-Relevant Information