Java and C# are relatively different in comparison.
Just wanted to point out that java is very similar to c# more so then c++.
If you know c# you practically already have learned java.
I was using java before c# came out then jumped ship c# provides more flexability pinvokes unsafe ect.
Some basic things are different like the way base and super work in java vs base in c# and a few other minor things. However code java is extremely similar to c#.
You could easily get a program running in no time at all in java and feel like your in very familiar territory, if you already know c#.