I am using a LGPL (GNU Lesser General Public License version 2.1) licensed library in my Java project I'm soon about to release. In order to solve a bug in this library I have copied a particular class from the source code, modified it, and made it a part of my project.
Library A is available in binary form at central nexus repo. This library A is also available as source code and licensed with LGPL 2.1. Library A contains class B that is buggy.
Project C built in Java and Maven depends on library A. In order to solve the bug in class B, B is copied to a new class D and stored as a part of project C source code. The bug is then fixed in D.
Project C's source code is about to be released and available for others.
Question: What must project C contain, license-wise?
I am looking for another answer than answers like: "In order to follow LGPL 2.1, you must implement the things listed in license LGPL 2.1".
The main thing to note is that the conditions for distributing modified copies are a hassle and basically amount to having to maintain your personal fork with carefully marked modification and a distribution channel for it. You want to get rid of that hassle as soon as possible, so you should try getting your bug fixes accepted upstream.
Apart from that advice, I don't think the gist of your request "what do I have to do to heed the conditions of the LPGL? Please don't ask me to read the conditions of the GPL" makes a lot of sense. You have to read through and understand a set of conditions anyway, so why not read the original rather than something regurgitated by someone whose reading is legally irrelevant?