A multimap implementation which trades some CPU time in favour of compact memory representation.
Works well for collecting large amount of "append-only" data in-memory.

It basically represents a Map[K, List[V]] as a Map[K, Array[Byte]]. So when a new item is put into the map it gets serialized and appended to a byte array. When a user defined byte size is reached (8 kB by default) the array gets zipped and stored separately. This separate compation allows better memory footprint while keeping the original array at a constant size therefore the time of insertion won't degrade by the number of elements in the list. This in-memory storage is optimized for minimal memory footprint and fast instertion. For retrieval operations like get, remove and contains it is necessary to unzip and deserialize the byte array in order to access the actual objects so those operations are expected to be slower than standard multimap implementations.
So choose wisely according to your constraints/usage patterns.

• Low memory usage due to byte array value list representation.
• Values are compressed automatically above a certain size.
• Supports ordered list of values, duplicates are possible.
• Specialized implementation for primitive values.
• Support for custom inner map for efficient primitive key storage (fastutil, Koloboke).
• Efficient put operations.
• Single threaded.
• Scala wrapper.

• Persistence support.
• Set support in values.

Get and remove operations are computationally more expensive compared to standard implementations, especially for large amount of values for a single key.

import com.github.bezsias.multimap.*;
import java.util.*;

MultiMap<String, String> map = new MultiMapBuilder<String>().objectMap();
map.put("a", "1");
map.put("a", "2");
map.put("b", "1");
map.put("a", "3");

List<String> values = map.get("a");

import com.github.bezsias.multimap.scala._

val map = CompactMultiMap.objectMap[String, String]()
map.put("a", "1");
map.put("a", "2");
map.put("b", "1");
map.put("a", "3");

map.get("a");
res4: List[String] = List(1, 2, 3)

map.get("b");
res5: List[String] = List(1)

You can get compact-multimap from maven central.

<dependency>
    <groupId>com.github.bezsias</groupId>
    <artifactId>compact-multimap_2.11</artifactId>
    <version>1.0.0</version>
</dependency>

libraryDependencies += "com.github.bezsias" %% "compact-multimap" % "1.0.0"

Distributed under MIT license.