SmoothieMap is a Map implementation for Java with the lowest memory usage and absence of rehash latency spikes. Under the hood, it is a version of extendible hashing with 48-slot mini segments.

This project also includes a proof-of-concept implementation of SwissTable algorithm in Java: see SwissTable.java.

See this post for a more detailed introduction, performance and memory comparisons, etc.

Maven:

<dependency>
  <groupId>io.timeandspace</groupId>
  <artifactId>smoothie-map</artifactId>
  <version>2.0.2</version>
</dependency>

Then, in Java:

Map<String, String> myMap = SmoothieMap.<String, String>newBuilder().build();

See Javadocs.