Java.Collections.What is Object header ?

Every object in Java has an object header — a block of metadata that the JVM attaches to every object on the heap.

It’s not part of your fields or values — it’s internal data used by the JVM for:

• 🔁 Object identity (hashCode())
• 🧹 Garbage collection
• 🔐 Synchronization (e.g., when using synchronized)
• 📏 Type information

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📐 How big is the object header?

Depends on the JVM and system architecture:

JVM Architecture	Object Header Size
32-bit JVM	8 bytes
64-bit JVM	12 bytes (with compressed oops) ✅
64-bit JVM (no compression)	16 bytes ❌

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🧠 Breakdown (on 64-bit JVM with compressed oops):

• 8 bytes for the Mark Word
  • Stores hash code, GC flags, locking info, etc.
• 4 bytes for the Class Pointer
  • Points to the object’s class metadata (type info)

So total = 12 bytes

Then, the JVM adds padding to align the object size to 8-byte boundaries.

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🧪 Example:

class A {
    int x;
}

This tiny object:

• 12 bytes (header)
• 4 bytes (int field)
• +4 bytes padding
  → total = 20 bytes (rounded to 24 due to alignment)

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🧠 Why is this important?

• Every object has this overhead — even empty ones!
• So thousands/millions of small objects (like Node in LinkedList) → significant memory use

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📌 Summary:

Term	Description
Object Header	JVM metadata stored with every object
Typical Size	12–16 bytes on 64-bit JVM
What’s Inside?	Hash info, class pointer, lock state
You Can’t See It	But it’s always there 😄