Koloboke aims to replace the standard Java collections and streams  with more efficient implementations.  The current version of Koloboke focuses on replacing java.util.HashSet and java.util.HashMap. It provides a complete set of primitive type implementations for each collection. Its able to avoid the expensive boxing/unboxing of primitives and saves memory for boxed primitive objects. It is the fastest and the most memory efficient library implementing hash maps and sets.

Koloboke -   Java Collections till the last breadcrumb of memory and performance

Fastutil extends the Java Collections Framework by providing type-specific maps, sets, lists and queues with a small memory footprint and fast access and insertion. It also provides also big (64-bit) arrays, sets and lists, and fast, practical I/O classes for binary and text files.<BR><BR>It has support of bidirectional iterators. It has support of Big arrays and Big Lists with 64 Bit indices.

Fastutil extends the Java Collections Framework by providing type-specific maps, sets, lists and queues with a small memory footprint and fast access and insertion. It also provides also big (64-bit) arrays, sets and lists, and fast, practical I/O classes for binary and text files.

Fastutil - Fast & compact type-specific collections for Java

Hazelcast is a clustering and highly scalable data distribution platform for Java. It supports Distributed implementations of java.util.{Queue, Set, List, Map}, java.util.concurrency.locks.Lock, java.util.concurrent.ExecutorService, Distributed Indexing and Query support, Dynamic scaling, partitioning with backups, fail-over, Web-based cluster monitoring tool and lot more. 

Hazelcast - In-Memory Data Grid for Java

GS Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings. The iteration protocol was inspired by the Smalltalk collection framework. It provides replacements for ArrayList, HashSet, and HashMap optimized for performance and memory usage.

gs-collections - A supplement or replacement for the Java Collections Framework

Commons-Collections seek to build upon the JDK classes by providing new interfaces, implementations and utilities.
    <ul><li>Bag interface for collections that have a number of copies of each object</li>
	<li>Buffer interface for collections that have a well defined removal order, like FIFOs</li>
	<li>BidiMap interface for maps that can be looked up from value to key as well and key to value</li>
	<li>MapIterator interface to provide simple and quick iteration over maps</li>
	<li>Type checking decorators to ensure that only instances of a certain type can be added</li>
	<li>Transforming decorators that alter each object as it is added to the collection</li>
	<li>Composite collections that make multiple collections look like one</li>
	<li>Ordered maps and sets that retain the order elements are added in, including an LRU based map</li>
	<li>Identity map that compares objects based on their identity (==) instead of the equals method</li>
	<li>Reference map that allows keys and/or values to be garbage collected under close control</li>
	<li>Many comparator implementations</li>
	<li>Many iterator implementations</li>
	<li>Adapter classes from array and enumerations to collections</li>
	<li>Utilities to test or create typical set-theory properties of collections such as union, intersection, and closure</li>
	</ul>

Commons-Collections seek to build upon the JDK classes by providing new interfaces, implementations and utilities.  It provides collection class like Identity map, Ordered map, Reference map, Composite collections, Bag interface etc.

Apache Commons - Alternative to Java collection

GraphHopper is a fast and Open Source road routing engine. It works with OpenStreetMap data. It has  additional implementations for A*, Dijkstra and the bidirected versions, Takes care of the road type, the surface, barriers, access restrictions, ferries, Supports Car, Bike, Pedestrian and you can easily create your own or customize existing vehicles and lot more. It is tuned towards road networks with OpenStreetMap data but can be useful for public transport problems as well.

GraphHopper -  Fast and Open Source Road Routing Engine in Java

HPPC provides template-generated implementations of typical collections, such as lists, sets and maps, for all Java primitive types. The primary driving force behind HPPC is optimization for highest performance and memory efficiency. 

HPPC - High Performance Primitive Collections for Java

A small library that provide a disk backed map implementation for storing large number of key value pairs. The map implementations (HashMap, HashTable) max out around 3-4Million keys/GB of memory for very simple key/value pairs and in most cases the limit is much lower. DiskBacked map on the other hand can store betweeen 16Million (64bit JVM) to 20Million(32bit JVM) keys/GB, regardless the size of the key/value pairs.

Disk-backed-map - A simple java disk backed map 

Immutable Clojure collections and a Transformation abstraction for Java 8+, immutably, type-safely, and with good performance. It provide support for RRB Tree, which is an immutable List (like Clojure's PersistentVector) that also supports random inserts, deletes, and can be split and joined back together in logarithmic time.

Paguro - Immutable Collections and Functional Transformations for the JVM

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Koloboke - Java Collections till the last breadcrumb of memory and performance