Java object memory footprint - what to believe ? jhat ? ClassLayout?

Question

I have a simple java class "Tile" that declares 4 fields :

• two integers
• two objects (pointers)

I'm trying to know what is the exact memory footprint of an instance of this class.

Using the org.openjdk.jol.info.ClassLayout.parseInstance(obj) method, I'm getting this :

 OFFSET  SIZE               TYPE DESCRIPTION                               VALUE
      0     4               (object header)                           09 00 00 00 (00001001 00000000 00000000 00000000) (9)
      4     4               (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4               (object header)                           00 40 0b 01 (00000000 01000000 00001011 00000001) (17514496)
     12     4                int   Tile.x                                    0
     16     4                int   Tile.y                                    0
     20     4                Board Tile.board                                (object)
     24     4  java.util.ArrayList Tile.elements                             (object)
     28     4                (loss due to the next object alignment)
Instance size: 32 bytes

So far so good, that seems to make sense.

Now... I'm also dumping the heap with jmap jmap -dump:all,file=dump.dump 56311 and exploring it with jhat jhat dump4.dump.

I'm going under the Tile class, and there I read the following :

Tile@0x7fbdf0da0 (40 bytes)

So here it looks like that/jmap calculates 40 bytes instead of 32.

And now I'm going under the "Heap Histogram" feature, and I'm getting the number of instances of each class, along with the "Total Size" :

class Tile  9   216

That means 24 bytes...

So... what should I believe ? 40 ? 32 ? 24 ? How to make sense of all this ?