Over a million developers have joined DZone.

Size of an entry in a Map

DZone's Guide to

Size of an entry in a Map

· Java Zone
Free Resource

Learn how our document data model can map directly to how you program your app, and native database features like secondary indexes, geospatial and text search give you full access to your data. Brought to you in partnership with MongoDB.

There have been some very good articles on the size of a map. However as a map grows, it initial size become less important and the size per entry is what matters.

How are the sizes measured

In these tests an int key and long values are used. This adds a small but realistic size to each entry.

Size per entry of a medium sized Map

The following are the size per entry in bytes. The Map has 1024 entries.

Type of Map 32-bit 64-bit compressed 64-bit not compressed
TIntLongHashMap 26.9 26.9 27.0
FastMap (recycled) 32.0 39.9 47.9
IdentityHashMap 48.0 56.0 80.0
ConcurrentSkipListMap 68.3 76.1 108.3
TreeMap 64.0 80.0 112.0
HashMap 64.0 80.0 112.0
SynchronizedMap 64.0 80.0 112.0
ConcurrentHashMap 65.2 81.4 114.0
Properties 68.0 84.0 120.0
Hashtable 68.0 84.0 120.0
LinkedHashMap 72.0 88.0 128.1
WeakHashMap 80.0 88.0 136.1
The Javolution FastMap needed to be recycled. If it is not recycled, it was the largest per entry.


It may be worth considering The GNU Trove collections if you have primitive keys and/or values. However if you have non-trivial keys or values classes, the size of the collection is less likely to matter.

The Code



From http://vanillajava.blogspot.com/2011/07/size-of-entry-in-map.html

Discover when your data grows or your application performance demands increase, MongoDB Atlas allows you to scale out your deployment with an automated sharding process that ensures zero application downtime. Brought to you in partnership with MongoDB.


Opinions expressed by DZone contributors are their own.


Dev Resources & Solutions Straight to Your Inbox

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.


{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}