Stack vs Heap
0. Introduction
Understanding the difference between Stack and Heap memory is one of the most fundamental concepts in Java. In this article, we will explore how Stack and Heap work, and the difference between them.
1. What is Stack Memory?
Stack memory is used for method execution. Each thread has its own stack, which makes it thread-safe by design.
1.1. What is stored in the Stack?
Local variables
Method parameters
Reference variables (the reference itself, not the object)
Stack frames (one per method call)
Every time a method is called, a new stack frame is created. When the method finishes, the frame is automatically removed.
1.2. Key Characteristics
Thread-specific
LIFO structure
Very fast allocation and deallocation
Automatically managed
Limited size
If too many stack frames accumulate, a StackOverflowError occurs.
2. What is Heap Memory?
Heap memory is used to store objects created using the new keyword. Unlike Stack, Heap memory is shared among all threads.
2.1. What is stored in the Heap?
Objects
Instance variables
Arrays
Static variables (in the method area, part of heap in modern JVM implementations)
Heap memory is managed by the Garbage Collector.
2.2. Key Characteristics
Shared among threads
Larger than stack
Slower allocation compared to stack
Managed by Garbage Collector
If too many objects are created and not properly reclaimed, an OutOfMemoryError may occur.
3. Stack vs Heap Comparison
| Feature | Stack | Heap |
|---|---|---|
| Scope | Per thread | Shared across threads |
| Stores | Local variables, references | Objects, instance data |
| Speed | Faster | Slower |
| Memory size | Smaller | Larger |
| Management | Automatic (method-based) | Garbage Collector |
