Build Status

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

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)



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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