greenrobot's Essentials library provides general purpose utilities for Android and Java projects. Having its root in the early days of Android development, the library is minimalistic, tiny in size (jar < 100k), and focuses on efficiency and performance.

See Website.

The Utility classes cover stream-based IO, files, strings, and date/time. There are also efficient hash map and hash set implementation for primitive long keys.

Another important part of Essentials are hash functions. Our Murmur3A&F implementations are the fastest Java implementations known to us. Murmur3 hash functions are one of today's best hash functions available with excellent properties. At 3,6 GByte/s, we measured our Murmur3F implementation to outperform Guava's by factor 10. Find more information on the hash functions page or jump directly to our Java hash function benchmark PDF.

Speaking of Guava, this project is bare bones compared to a rich menu offered by Guava or Apache Commons. Essentials is not a framework, it's rather a small set of utilities to make Java standard approaches more convenient or more efficient.

Just grab it from Maven Central (or jCenter). For Gradle, add the following dependency:

compile 'de.greenrobot:java-common:2.3.1'

And for Maven:

<dependency>
    <groupId>de.greenrobot</groupId>
    <artifactId>java-common</artifactId>
    <version>2.3.1</version>
</dependency>

Example code on how to use some of the utility classes:

// Get all bytes from stream and close the stream safely
byte[] bytes = IoUtils.readAllBytesAndClose(inputStream);

// Read the contents of an file as a string (use readBytes to get byte[])
String contents = FileUtils.readUtf8(file);

// How many days until new year's eve?
long time2 = DateUtils.getTimeForDay(2015, 12, 31);
int daysToNewYear = DateUtils.getDayDifference(time, time2);

Multimaps (added in V2.2):

ListMap<String,String> multimap = new ListMap<>();
multimap.putElement("a", "1");
multimap.putElement("a", "2");
multimap.putElement("a", "3");
List<String> strings = multimap.get("a"); // Contains "1", "2", and "3"

Our hash functions implement java.util.zip.Checksum, so this code might look familiar to you:

Murmur3A murmur = new Murmur3A();
murmur.update(bytes);
long hash = murmur.getValue();

All hashes can be calculated progressively by calling update(...) multiple times. Our Murmur3A implementation goes a step further by offering updates with primitive data in a very efficient way:

// reuse the previous instance and start over to calculate a new hash
murmur.reset();

murmur.updateLong(42L);

// Varargs and arrays are supported natively, too  
murmur.updateInt(2014, 2015, 2016);

murmur.updateUtf8("And strings, of course");

// Hash for the previous update calls. No conversion to byte[] necessary.
hash = murmur.getValue();

We may not write a lot of documentation for this project. The utility classes are straight forward and don't have dependencies, so you should be fine to grasp them by having a look at their source code. Most of the method names should be self-explaining, and often you'll find JavaDocs where needed. Code is the best documentation, right?

Currently, Maven is used to build greenrobot-common. Inside of build-common, there are two parent POMs defined that might be useful: parent-pom and parent-pom-with-checks. The latter integrates FindBugs and Checkstyle in your build. Use it like this:

<parent>
    <groupId>de.greenrobot</groupId>
    <artifactId>parent-pom-with-checks</artifactId>
    <version>2.0.0</version>
    <relativePath></relativePath>
</parent>

For more details on greenrobot Essentials please check the Essentials' website. Here are some direct links you may find useful:

Features

Changelog

Copyright (C) 2012-2016 Markus Junginger, greenrobot (http://greenrobot.org)

EventBus binaries and source code can be used according to the Apache License, Version 2.0.

EventBus is a central publish/subscribe bus for Android with optional delivery threads, priorities, and sticky events. A great tool to decouple components (e.g. Activities, Fragments, logic components) from each other.

greenDAO is an ORM optimized for Android: it maps database tables to Java objects and uses code generation for optimal speed.

Follow us on Google+ to stay up to date.