Question

I have this class :

public class Stack {

   private class Node {
       String item;
       Node next;
   }
   // some other methods here

}

In my book, the author says that the size per stack Node is 40 bytes including:

16 bytes (object overhead)
8 bytes (inner class extra overhead)
8 bytes (references to string)
8 bytes (references to node)
----------------------------------------------
40 bytes per stack node

I understand that the last two things refer to the size of the references to the String and Node. But I don’t know what the object overhead and inner class extra overhead correspond to. Can you please explain?

Answer 1

object overhead

Every object has a header which is typically 8-12 bytes long. Each object is also 8 byte aligned and a simple estimate is you say its about 16 bytes long.

inner class extra overhead

As your inner class is not static, it has a reference to the outer class.

If this were an anonymous inner class you might have copies of any number of final variables (any used in the anonymous class code)

8 bytes (inner class extra overhead)
8 bytes (references to string)
8 bytes (references to node)

Most JVMs use 32-bit references so the size would be 4 bytes. Even 64-bit JVM with a heap up to 32 GB can use 32-bit references.