/*
Project website:
https://github.com/FasterXML/jackson-datatype-hppc

Maven: 
<dependency>
  <groupId>com.fasterxml.jackson.datatype</groupId>
  <artifactId>jackson-datatype-hppc</artifactId>
  <version>2.0.0-RC1</version>
</dependency>
*/

import com.carrotsearch.hppc.ObjectObjectOpenHashMap;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.datatype.hppc.HppcModule;

// Serialize:
ObjectObjectOpenHashMap<String, ObjectDoubleOpenHashMap<String>> mapStringMapStringDouble = new ObjectObjectOpenHashMap<String, ObjectDoubleOpenHashMap<String>>();

// fill map
               
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new HppcModule());
mapper.writeValue(new FileOutputStream("filename-goes-here"), mapStringMapStringDouble);

// De-serialize:
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new HppcModule());
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

ObjectObjectOpenHashMap<String, ObjectDoubleOpenHashMap<String>> mapStringMapStringDouble = mapper.readValue(inputStream, new TypeReference<ObjectObjectOpenHashMap<String, ObjectDoubleOpenHashMap<String>>>() {});

(De-)serialising an HPPC map using the jackson HPPC module.

Jackson HPPC serialization example

Java