I have been programming in Java for about six years now.
During this period I have learned a lot in terms of experience in using the different Java libraries for various task. The libraries I have listed here are only the ones that became useful to me as a data science professional.
I would list the following libraries which in my opinion are the ones worthy of mention :
1-JFreeChart for data visualizations as bar charts, grouped bar charts, line charts, boxplot diagrams etc.. : JFreeChart (http://www.jfree.org/jfreechart/)
2-for language detection : language-detection - Language Detection Library for Java - Google Project Hosting (https://code.google.com/p/language-detection/)
3-for FlickrData harvesting : flickrapi flickrapi-1.2.jar - socialjeju - Flickr API - flickrj - jeju univ sns study - Google Project Hosting (https://code.google.com/p/socialjeju/downloads/detail?name=flickrapi-1.2.jar&can=2&q=)
4-for json parsing : simple-json json-simple - JSON.simple - A simple Java toolkit for JSON - Google Project Hosting (https://code.google.com/p/json-simple/)
5- for feed parsing : Feed4j Parser Reading RSS Feed (http://www.tutorialsavvy.com/2013/02/feed4j-parser-reading-rss-feed.html)
6-for basic linear algebra calculations: JAMA Java Matrix Package (http://math.nist.gov/javanumerics/jama/)
7- For statistical calculations and linear algebra . It also has some algorithms for clustering such as BDSCAN and linear regression : Commons Math: The Apache Commons Mathematics Library (https://commons.apache.org/proper/commons-math/)
8- Mallet MALLET homepage (http://mallet.cs.umass.edu) and deeplearning4j : Open-source, distributed deep learning for the JVM (http://deeplearning4j.org) for respectively Machine Learning and Deep Learning algorithms.
9- joda-time : Joda-Time - Home (http://www.joda.org/joda-time/) which is a Java library for kind of by-passing the Java DateTime API
10- and obviously last but not least JUnit for testing Java code in a repeated way : JUnit - About (http://junit.org)
This was my list of ten libraries. However, it is difficult to list just ten and exclude other libraries.
So some more libraries that are also useful are :
a - iText : iText is an API that helps developers to generate or update PDF documents on the fly programmatically without manual process. iText (http://itextpdf.com)
b- JMeter : Apache JMeter is a Java-based desktop application, which can be used for load testing to measure the performance of a system or used for stress testing to see if a system is crashed gracefully. It can generate reports to help eliminate bottlenecks of the system or to see how it performs under heavy loads.
Apache JMeter - Apache JMeter™ (http://jmeter.apache.org)
c- JavaCC : JavaCC (Java Compiler Compiler) is an open source lexical analyzer generator and a parser generator for use with the Java applications, which takes a grammar specification (e.g., EBNF) and generates the Java source code of a lexical analyzer and a parser. JavaCC Home (https://javacc.java.net)
d- Apache POI : (Poor Obfuscation Implementations) is the Java API for Microsoft Documents, which allows you to read and write MS Office documents such as Excel, Word and PowerPoint using Java. Through POI, you can generate MS Office documents dynamically based on requests or to generate personalized reports on the fly. POI supports OLE2 files such as XLS, DOC and PPT and new XML based standard Office OpenXML (OOXML) files such as XLSX, DOCX and PPTX. Apache POI - the Java API for Microsoft Documents (https://poi.apache.org)