Where Are Instance Variables Stored?

Direct Answer

An instance variable is a field that belongs to an object, so its value is stored as part of that object's logical heap state. A primitive field stores its value there; a reference field stores a reference there, while the referenced object is a separate heap object.

Deep Dive & Architecture

Instance fields form object state

When Java creates an object, its instance fields first receive default values: zero for numeric primitives, false for boolean, and null for references. Constructor code then assigns the intended values. Each instance has its own field values, so changing first.quantity does not change second.quantity. Local variables are different: the compiler requires definite assignment before a local is read.

Reference fields create a graph

Source syntax makes order.customer.address look like nested boxes, but the useful runtime model is a chain of reference values between separate objects. Two Order objects can point to the same Customer, and several Customer objects can point to one immutable Address. Each target's lifetime depends on every incoming reachable path.

Copying shows why the distinction matters

A shallow copy creates a new outer object but can copy its reference values unchanged. Both outer objects then reach the same mutable inner list. A deep copy deliberately creates selected inner objects as well. Neither behaviour follows merely from the word “copy”; the API must define it.

A final reference field prevents reassignment after Java's construction rules are satisfied, but it does not freeze the target object. Thread safety and immutability depend on the complete reachable state. Exact headers, padding, compressed references, and field offsets are JVM details; the stable rule is that an instance field belongs to its containing object and a reference field stores an arrow, not the target itself.

Code Example

class InstanceVariables {
    static final class Product {
        final String name;
        Product(String name) { this.name = name; }
    }

    static final class OrderLine {
        int quantity;
        Product product;
        OrderLine(int quantity, Product product) {
            this.quantity = quantity;
            this.product = product;
        }
    }

    public static void main(String[] args) {
        Product shared = new Product("Book");
        OrderLine first = new OrderLine(2, shared);
        OrderLine second = new OrderLine(1, shared);
        System.out.println(first.quantity + second.quantity);
        System.out.println(first.product == second.product);
    }
}