How much memory takes TreeMap<Long, Long> collection granularly?

Question

How much memory takes TreeMap<Long, Long> collection filled with 1000 (all unique) key-value pairs?

Yes I can just watch memory dump, but need to know granularly why exactly:

• Long = n bytes
• Entry<Long, Long > = 2n bytes, so 1000 entries = 2000n bytes
• reference in tree node: k bytes, 1 node have 2 links to children, so 2k bytes,
• etc.

Total: X ?

Answer 1

JOL (Java Object Layout)

To answer such questions you can use the tool, JOL.

In your case it will get such results:

java.util.TreeMap@17046283d footprint:
     COUNT       AVG       SUM   DESCRIPTION
      2000        24     48000   java.lang.Long
         1        48        48   java.util.TreeMap
      1000        40     40000   java.util.TreeMap$Entry
      3001               88048   (total)

Total 88048 bytes used

Granularly

TreeMap itself:

OFF  SZ                    TYPE DESCRIPTION               VALUE
  0   8                         (object header: mark)     N/A
  8   4                         (object header: class)    N/A
 12   4                  Set<K> AbstractMap.keySet        N/A
 16   4           Collection<V> AbstractMap.values        N/A
 20   4                     int TreeMap.size              N/A
 24   4                     int TreeMap.modCount          N/A
 28   4   Comparator<? super K> TreeMap.comparator        N/A
 32   4             Entry<K, V> TreeMap.root              N/A
 36   4                EntrySet TreeMap.entrySet          N/A
 40   4               KeySet<K> TreeMap.navigableKeySet   N/A
 44   4      NavigableMap<K, V> TreeMap.descendingMap     N/A
Instance size: 48 bytes

TreeMap.Entry

OFF  SZ          TYPE DESCRIPTION               VALUE
  0   8               (object header: mark)     N/A
  8   4               (object header: class)    N/A
 12   1       boolean Entry.color               N/A
 13   3               (alignment/padding gap)   
 16   4             K Entry.key                 N/A
 20   4             V Entry.value               N/A
 24   4   Entry<K, V> Entry.left                N/A
 28   4   Entry<K, V> Entry.right               N/A
 32   4   Entry<K, V> Entry.parent              N/A
 36   4               (object alignment gap)    
Instance size: 40 bytes

And Long:

OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     N/A
  8   4        (object header: class)    N/A
 12   4        (alignment/padding gap)   
 16   8   long Long.value                N/A
Instance size: 24 bytes

Also you may need do read additional questions about alignment and object header.