Learn Java From C#
This is the source code for the book Learn Java From C#, by Nick Fisher.
The book's sole mission in life is to teach someone Java when said person already has a good grasp on C#. Because the languages are largely similar, it would seem silly to re-learn all of C# if all you want to know is some Java.
E-book
The kindle version (DRM-free) is available on Amazon at https://www.amazon.com/Learn-Java-C-Introduction-Spring-ebook/dp/B07NKR2LYY.
Free Online
You can also read the entire book for free online at Learn Java From C#, with an Introduction to Spring.