Heap and stack
- Java course
- Expert
- Lesson 7/46
- 5 min
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.
Practice 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.
Starter code
// Define the Box class
// Add the square method
// Print the result in mainSample 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));
}
}Expected output
25
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In which memory region does an object created with new live?
- On the stack
- In Metaspace
- In a register file
- On the heap Correct answer
Explanation: Objects created with new live on the heap.
2 Complete the object creation operator
Box b = ____ Box(5);Box b = new Box(5);Explanation: The new operator creates an object on the heap.
3 Match the memory regions
| Term | Meaning |
|---|---|
Heap |
new objects |
Stack |
Local variables |
Metaspace |
Class metadata |