CompactHashMap
Usage
Add Maven dependency:
<dependency>
<groupId>com.github.vlsi.compactmap</groupId>
<artifactId>compactmap</artifactId>
<version>1.3.0</version>
</dependency>
Gradle:
compile("com.github.vlsi.compactmap:compactmap:1.3.0")
About
This is a memory efficient alternative to HashMap. Main design goal is taken from "Fast Property Access" http://code.google.com/apis/v8/design.html#prop_access.
This implementation however can store specific key-value pairs out of the map, so they do not consume memory when repeated in different maps.
The expected memory consumption (8u40, 64 bit, compressed references) is as follows:
# of elements CompactHashMap HashMap (with 1.0 fillFactor)
0 32 48
1 32 104
2 32 136
3 32 176
4 64 208
5 64 256
6 64 288
7 72 320
8 72 352
In other words, the first three non default values consume the same 32 bytes, then map grows as 32 + 16 + 4 * (n-2) == 40 + 4 * n. Regular HashMap grows as 64 + 36 * n.
The runtime of put and get is constant. The expected runtime is as follows (measured in hashmap and array accesses):
best case worst case
get 1 hashmap + 1 array 2 hashmap
put 1 hashmap + 1 array 6 hashmap
Sample
// Mark height->auto a default mapping entry, so it would not consume memory in CompactHashMaps
CompactHashMapDefaultValues.add("height", "auto");
// Mark all values of width as default, so they would not consume memory in real maps
CompactHashMapDefaultValues.add("width");
CompactHashMap<String, String> map = new CompactHashMap<String, String>();
map.put("height", "auto"); // does not consume memory in map
map.put("width", "100%"); // does not consume memory in map either
map.put("id", "myFirstButton"); // consumes some memory
map.get("height"); // => "auto"
map.get("width"); // => "100%"
map.get("id"); // => "myFirstButton"
map.put("height", "50px"); // consumes some memory (switches from default to custom)
map.get("height"); // => "50px"
License
This library is distibuted under terms of GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
Change log
v2.0:
- Change license: LGPL 2.0 -> Apache-2.0
v1.3.0
- Improvement: implement null keys
- Improvement:
Map#toString
- Improvement:
Map#hashCode
+equals
- Improvement:
Map.Entry#hashCode
+equals
- Improvement:
Map.Entry#toString
- Improvement:
Map#containsValue
(it is slow but it works) - Test: use
guava-testlib
forMap
implementation testing
v1.2.1
- Improvement: release to Maven Central
- Improvement: fix EntrySet.remove method
v1.2.0
- Improvement: reduce size of hidden classes by using persistent dexx-collections.
- Improvement: mavenize build
- Switch to semantic versioning
v1.1
- Fix: #1 containKey returns true on non existing key
- Fix: #2 size should account removed keys
- Improvement: #3 Default values should be serialized as map
Author
Vladimir Sitnikov sitnikov.vladimir@gmail.com