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. 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

Colt distribution consists of several free Java libraries bundled under one single uniform umbrella. Namely the Colt library, the Jet library, the CoreJava library, and the Concurrent library. 

 The Colt library provides fundamental general-purpose data structures optimized for numerical data, such as resizable arrays, dense and sparse matrices (multi-dimensional arrays), linear algebra, associative containers and buffer management. The Jet library contains mathematical and statistical tools for data analysis, powerful histogramming functionality, Random Number Generators and Distributions useful for (event) simulations, and more. The CoreJava library contains C-like print formatting. The Concurrent library contains standardized, efficient utility classes commonly encountered in parallel & concurrent programming.

Colt distribution consists of several free Java libraries bundled under one single uniform umbrella. Namely the Colt library, the Jet library, the CoreJava library, and the Concurrent library. It provides support for resizable arrays, dense, sparse matrices, histogramming functionality, Random Number Generators etc.

Colt - Scientific and Technical Computing in 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

Mapsforge project uses a compact file format for fast ad-hoc rendering of OpenStreetMap data. It provides tools to compile your own maps with detailed instructions and also precompiled maps, Simple boilerplate code to build applications for Android that display OpenStreetMap-based maps. Mapsforge maps can be flexibly styled with XML style files (render themes).

Mapsforge - Vector map library written in Java - running on Android and Desktop.

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

Trove provides fast, lightweight implementations of the java.util Collections API. These implementations are designed to be pluggable replacements for their JDK equivalents. This gap in the JDK is often addressed by using the "wrapper" classes (java.lang.Integer, java.lang.Float, etc.) with Object-based collections. For most applications, however, collections which store primitives directly will require less space and yield significant performance gains. Trove eliminates the need to create Map.Entry wrappper objects for every item in a table and so reduces the O (big-oh) in the performance of the hashtable algorithm. The size of the tables used in Trove's maps/sets is always a prime number, improving the probability of an optimal distribution of entries across the table, and so reducing the the likelihood of performance-degrading collisions. Trove sets are not backed by maps, and so using a THashSet does not result in the allocation of an unused "values" array.

Trove provides fast, lightweight implementations of the java.util Collections API. These implementations are designed to be pluggable replacements for their JDK equivalents. This gap in the JDK is often addressed by using the "wrapper" classes (java.lang.Integer, java.lang.Float, etc.) with Object-based collections. For most applications, however, collections which store primitives directly will require less space and yield significant performance gains.

Trove - High performance collections for Java

Mango is a Java library consisting of a number of iterators, algorithms and functions, loosely inspired by the C++ Standard Template Library. It has support of 12 iterators, 16 generic algorithms and variety of function types. It supports Array Iterator, Bounded Iterator, Chain Iterator, Null Iterator, Reverse Iterator, Selecting Iterator, Singleton Iterator, Skipping Iterator, StringIterator, Tee Iterator, Transform Iterator, Zip Iterator. It supports function types such as Generators, Functions, Binary Functions, Predicates, Binary Predicates.

Mango is a Java library consisting of a number of iterators, algorithms and functions, loosely inspired by the C++ Standard Template Library. It has support of 12 iterators, 16 generic algorithms and variety of function types.

Mango - specialized iterators and functions

Capsule aims to become a full-fledged (immutable) collections library for Java 8+ that is solely built around persistent tries. The library is designed for standalone use and for being embedded in domain-specific languages. Capsule still has to undergo some incubation before it can ship as a well-rounded collection library. Nevertheless, the code is stable and performance is solid.
 
The standard libraries of recent Java Virtual Machine languages, such as Clojure or Scala, contain scalable and well-performing immutable collection data structures that are implemented as Hash-Array Mapped Tries (HAMTs). HAMTs already feature efficient lookup, insert, and delete operations, however due to their tree-based nature their memory footprints and the runtime performance of iteration and equality checking lag behind array-based counterparts.
 
We introduce CHAMP (Compressed Hash-Array Mapped Prefix-tree), an evolutionary improvement over HAMTs. The new design increases the overall performance of immutable sets and maps. Furthermore, its resulting general purpose design increases cache locality and features a canonical representation.

Capsule aims to become a full-fledged (immutable) collections library for Java 8+ that is solely built around persistent tries. The library is designed for standalone use and for being embedded in domain-specific languages. Capsule still has to undergo some incubation before it can ship as a well-rounded collection library. Nevertheless, the code is stable and performance is solid.

Capsule - The Capsule Hash Trie Collections Library

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

This library provides high-quality Java implementations of mutable and immutable data structures, each sharing a common API. Rather than using the existing collection interfaces in java.util such as List or Map, it provides its own interfaces (IList, IMap, ISet) that provide functional semantics - each update to a collection returns a reference to a new collection. 

bifurcan - impure functional data structures

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