Heap and stack

Learn heap and stack memory regions.

Heap and stack

The JVM splits memory into two main regions. The stack is allocated per method call; local primitives and object references live here. The heap holds all objects created with new and is shared across the application.

Example: int x = 5 is on the stack; in Box b = new Box() the reference b is on the stack while the object itself is on the heap. The stack is freed when the method returns; the heap is left to the garbage collector.

Reference versus object

A variable holds a reference to an object, not the object itself. The reference is on the stack; the object is on the heap. Two variables can point to the same object.

Example: Box a = new Box(); Box b = a;. Here a and b reference the same heap object; a change a.v = 9 is also visible through b.v. Primitives instead hold their value directly on the stack; assignment copies. This distinction drives side effects and copying behavior.

Exercise Use stack and heap together

Define a Box class with an int v field and a Box(int v) constructor. A static int square(int n) method returns n * n. In main, declare int x = 5, create an object with new Box(x), and print the result of square(b.v) with System.out.println.

Show example solution

class Box {
    int v;
    Box(int v) { this.v = v; }
}

public class MemoryDemo {
    static int square(int n) { return n * n; }
    public static void main(String[] args) {
        int x = 5;
        Box b = new Box(x);
        System.out.println(square(b.v));
    }
}

Test yourself

In which memory region does an object created with new live?
- On the stack
- In Metaspace
- In a register file
- On the heap
Show answer

On the heap — Objects created with new live on the heap.
Complete the object creation operator
```
Box b = ____ Box(5);
```
Show answer
```
Box b = new Box(5);
```
Match the memory regions

Heap

new objects
Stack

Local variables
Metaspace

Class metadata

Next lesson: happens-before

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