Explain the difference between stack and heap memory in Java

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In Java, stack and heap memory serve distinct roles in managing data during program execution. Both are critical to memory management but differ in terms of structure, usage, and lifecycle.

Detailed Comparison:

1. Purpose:

  • Stack Memory: Stores method calls, local variables, and references to objects. Each thread has its own stack memory, ensuring isolation and thread safety.
  • Heap Memory: Stores all objects created using new. This memory is shared across threads, and objects remain until garbage collected.

2. Memory Allocation and Lifetime:

  • Stack Memory: Memory is automatically allocated when a method is called and deallocated when the method completes. Local variables are cleared once the method execution ends.
  • Heap Memory:

Memory is dynamically allocated when objects are created. Objects stay in heap memory until they are no longer referenced, at which point the garbage collector reclaims the memory.

3. Speed and Efficiency:

  • Stack Memory: Access to stack memory is faster because its allocation and deallocation follow a straightforward LIFO structure.
  • Heap Memory:

Access is slower because it involves dynamic memory allocation and garbage collection overhead.

Example:

public class Main {
    public static void main(String[] args) {
        int num = 5;  // Stored in stack memory
        String str = new String("Hello");  // Object in heap memory, reference in stack
    }
}

  • num is stored in the stack.
  • The String object is stored in the heap, and the reference to it (str) is stored in the stack.

Summary:

  • Stack Memory is used for method calls and local variables, with fast, automatic memory management and thread safety.
  • Heap Memory is used for objects and is managed by the garbage collector, allowing for dynamic memory allocation but at a slower speed.