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

Guava is a set of core libraries that includes new collection types (such as multimap and multiset), immutable collections, a graph library, functional types, an in-memory cache, and APIs/utilities for concurrency, I/O, hashing, primitives, reflection, string processing, and much more. 
It is memory efficient. Its most interesting class is <A HREF="http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/MapMaker.html" target="_blank">MapMaker</A>, which is a ConcurrentMap builder, providing any combination of these features: soft or weak keys, soft or weak values, timed expiration, and on-demand computation of values. Check out their video in <A HREF="http://www.youtube.com/watch?v=ZeO%5FJ2OcHYM" target="_blank">Youtube</A>.

Guava is a set of core libraries that includes new collection types (such as multimap and multiset), immutable collections, a graph library, functional types, an in-memory cache, and APIs/utilities for concurrency, I/O, hashing, primitives, reflection, string processing, and much more.

Guava - Google Core Libraries for Java

Java Concurrency Tools for the JVM. This project aims to offer some concurrent data structures currently missing from the JDK.
 
<ul>
<li>
SPSC/MPSC/SPMC/MPMC variations for concurrent queues:
<ul>
<li>SPSC - Single Producer Single Consumer (Wait Free, bounded and unbounded)</li>
<li>MPSC - Multi Producer Single Consumer (Lock less, bounded and unbounded)</li>
<li>SPMC - Single Producer Multi Consumer (Lock less, bounded)</li>
<li>MPMC - Multi Producer Multi Consumer (Lock less, bounded)</li>
</ul>
</li>
<li>
SPSC/MPSC linked array queues offer a balance between performance, allocation and footprint
</li>
<li>
An expanded queue interface (MessagePassingQueue):
<ul>
<li>relaxedOffer/Peek/Poll: trade off conflated guarantee on full/empty queue state with improved performance.</li>
<li>drain/fill: batch read and write methods for increased throughput and reduced contention</li>
</ul>
</li>
</ul>

Java Concurrency Tools for the JVM. This project aims to offer some concurrent data structures currently missing from the JDK.

JCTools - Java Concurrency Tools for the JVM

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

A collection of Concurrent and Highly Scalable Utilities. These are intended as direct replacements for the java.util.* or java.util.concurrent.* collections but with better performance when many CPUs are using the collection concurrently. Single-threaded performance may be slightly lower. 
 The other utilities do not have direct JDK replacements includes 
	<UL>
		<LI>NonBlockingHashMap - Fast, concurrent, lock-free HashMap. Linear scaling to 768 CPUs</LI>
		<LI>NonBlockingHashMapLong - Same as above, but using primitive 'long' keys</LI>
		<LI>NonBlockingHashSet - A Set version of NBHM</LI>
		<LI>NonBlockingSetInt - A fast fully concurrent BitVector</LI>
		<LI>Counter - A simple counter that scales linearly even when extremely hot</LI>
	</UL>

A collection of Concurrent and Highly Scalable Utilities. These are intended as direct replacements for the java.util.* or java.util.concurrent.* collections but with better performance when many CPUs are using the collection concurrently. 


high-scale-lib

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

Eclipse Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API, additional types not found in the JDK like Bags, Multimaps, 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 maximizes the power of Java 8 Lambda expressions and method references with rich APIs directly available on your collections. 

Eclipse Collections - Best Java Collection Framework 

Agrona provides a library of data structures and utility methods that are a common need when building high-performance applications in Java and C++. It supports Buffers, Map, Sets, Cache, Queues and lot more.
<ul>
<li>Buffers - Thread safe direct and atomic for working with on and off heap memory with memory ordering semantics.</li>
<li>Lists - Array backed lists of int/long primitives to avoid boxing.</li>
<li>Maps - Open addressing and linear probing with int/long primitive keys to object reference values.</li>
<li>Maps - Open addressing and linear probing with int/long primitive keys to int/long values.</li>
<li>Sets - Open addressing and linear probing for int/long primitives and object references.</li>
<li>Cache - Set Associative with int/long primitive keys to object reference values.</li>
<li>Clocks - Clock implementations to abstract system clocks, allow caching, and enable testing.</li>
<li>Queues - Lock-less implementations for low-latency applications.</li>
<li>Ring/Broadcast Buffers - implemented off-heap for IPC communication.</li>
<li>Basic Agent framework.</li>
<li>Signal handling to support "Ctrl + c" in a server application.</li>
<li>Scalable Timer Wheel.</li>
<li>Basic code generation from annotated implementations that can vary on primitive types.</li>
<li>Off-heap counters implementation for application telemetry and position tracking.</li>
<li>Implementations of InputStream and OutputStream that can wrap direct buffers.</li>
<li>DistinctErrorLog: A log of distinct errors to avoid filling disks with existing logging approaches.</li>
</ul>

Agrona provides a library of data structures and utility methods that are a common need when building high-performance applications in Java and C++. It supports Buffers, Map, Sets, Cache, Queues and lot more.

Agrona - Library to build high-performance applications in Java and C++

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

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

Cactoos is a collection of object-oriented Java primitives as an alternative to Google Guava and Apache Commons.

Cactoos - Object-Oriented Java primitives, as an alternative to Google Guava and Apache Commons

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