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

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

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