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Essentials are a collection of general-purpose classes we found useful in many occasions.

  • Write less code in your projects
  • Specialized implementations for better performance compared to standard APIs
  • Super light weight: < 100k in size
  • Compatible with Android and Java

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.


JavaDoc (3.0 release candidate)



  • IO utilities help with streams (byte and character based) and digests (e.g. MD5 and SHA-1).
  • File utilities simplify reading and writing strings/bytes/objects from or to files. Also includes getting hashes from files and copying files.
  • String utilities allow efficient splitting and joining of strings, hex and MD5 creation, and other useful string helpers.
  • Date utilities.
  • Better hash functions: our Murmur3 implementation provides superior hash quality and outperforms standard Java hash functions
  • Specialized Streams: for example an optimized PipedOutputStream replacement (based on a circular byte buffer)
  • Hash set and map for primitive long keys outperform the generic versions of the Java Collection APIs
  • Multimaps provide a map of lists or sets to simplify storing multiple values for a single key
  • Object cache with powerful configuration options: soft/weak/strong references, maximum size, and time-based expiration
  • Base64 implementation (bundled from for lower versions of Java & Android (includes input/output streams)

Read more on our website.

Add the dependency to your project

You may also try the 3.0 release candidate with additional features and a cleaned up API:

compile 'org.greenrobot:essentials:3.0.0-RC1'

Or, grab the older version:

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

And for Maven:


Code samples (V2.3.1)

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, so this code might look familiar to you:

Murmur3A murmur = new Murmur3A();
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


// 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();

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.

Build setup

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:



Copyright (C) 2012-2016 Markus Junginger, greenrobot (

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

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