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Java Memory Management: Stack vs Heap

Understanding Java's memory model is crucial for writing efficient applications and debugging memory-related issues.

Stack

  • Purpose: Stores method call frames, local variables, and references.
  • Scope: Thread-local (each thread has its own stack).
  • Speed: Very fast (LIFO access).
  • Memory Management: Automatically freed after method execution.
  • Lifetime: Limited to the duration of the method.
  • Garbage Collected?: No.
  • Common Error: StackOverflowError for deep recursion.
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void method() {
    int x = 10;         // x is stored on the stack
    String s = "hi";    // reference is on the stack; "hi" is in the string pool
}

Heap

  • Purpose: Stores all Java objects and arrays.
  • Scope: Shared memory, accessible by all threads.
  • Speed: Slower than stack (due to GC and dynamic allocation).
  • Memory Management: Managed by the Garbage Collector.
  • Lifetime: Until no longer referenced.
  • Garbage Collected?: Yes.
  • Common Error: OutOfMemoryError when heap is exhausted.
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String s = new String("hi"); // a new object is created in the heap

Comparison Table

Feature Stack Heap
Stores Primitives, refs, call frames Objects, arrays
Scope Per-thread Shared across threads
Speed Fast Slower
GC-managed No Yes
Lifetime Until method exits Until dereferenced
Common Errors StackOverflowError OutOfMemoryError

Summary

  • Use stack for fast-access local data inside methods.
  • Use heap for objects and data that must persist beyond method calls.