The availability of Java projects on github is incredible.
There you can find Java projects for almost anything.
Java is a language that keeps firmly in its place among five most popular programming languages .
However, it is not possible to find online so many resources that make a classification of those repositories by content or popularity expressed in number of stars.
My post aims to bring a classification of some of the most popular Java repositories on github and classify them by :
1- Web development
2- Business
3- Game Development
4- Collections
5- Date and Time
6- Dependency Injection andAOP
7- Logging
8- Imagery and Video
9- API integration
10-Science
11- Databases
12- Functional Programming
13- Security andAuthentication
14- Serialization and I/O
15- Machine learning Natural language processing
16 - Web crawling and HTML parsing.
Lets start with :
1-Web development in Java :
playframework/playframework (https://github.com/playframework/playframework) Play Framework The Play Frameworkcombines productivity and performance making it easy to build scalable webapplications with Java and Scala. Number of stars 7777 at the moment of writting.
**2-Business **
Codecademy/EventHub (https://github.com/Codecademy/EventHub)Event analytics platform : Number of stars on github 1133
An alternative to that is also killbill/killbill (https://github.com/killbill/killbill)
3- Game development
libgdx/libgdx (https://github.com/libgdx/libgdx) with 8579 stars at the moment.
4- Collections
google/guava (https://github.com/google/guava) by 8811 stars is the first of its genre.
**5- Date and Time **
JodaOrg/joda-time (https://github.com/JodaOrg/joda-time)Joda is probably the best with 2061 stars
6- Dependency injection
The king is spring-projects/spring-framework (https://github.com/spring-projects/spring-framework) with 8631 stars
**7- Logging **
Here it is : Graylog2/graylog2-server (https://github.com/Graylog2/graylog2-server) with 1722 stars
**8- Imagery and Video **
facebook/rebound (https://github.com/facebook/rebound) with 3299 stars is the best. The alternative : processing/processing (https://github.com/processing/processing)with 2648 stars
**9- API integration **
openzipkin/zipkin (https://github.com/openzipkin/zipkin)Number of stars = 3427
**10- Science **
jgrapht/jgrapht (https://github.com/jgrapht/jgrapht)with 655 stars
**11- Database **
thinkaurelius/titan (https://github.com/thinkaurelius/titan) with 3672 stars.
**12- Functional programming **
orfjackal/retrolambda (https://github.com/orfjackal/retrolambda) Nr of stars 2174.
**13- security and authentication **
scribejava/scribejava (https://github.com/scribejava/scribejava) by 3297 stars
14-Serialization and IO
square/okio (https://github.com/square/okio) with 2280 stars
15- Machine learning Natural language processing :M
stanfordnlp/CoreNLP (https://github.com/stanfordnlp/CoreNLP)with 1757 stars.
**16- Web crawling and HTML parsing **
sparklemotion/nokogiri (https://github.com/sparklemotion/nokogiri) with 3805 stars
Finally, there are many other repositories and this post presented some of the most popular Java repositories.
The author will however continue to update this post with new repositories.

From my Quora blog "Algorithms and data fusion" you can find here : https://algo-data.quora.com/ while the link to this post is as follows https://algo-data.quora.com/Best-Java-repositories-on-github-by-category-and-sorted-by-number-of-stars?srid=n9bS