You are forcing Java 8 and other Java programs like jdownloader are forcing Java 7. You have to use a bugged jdowloader 2 beta that runs under Java 8. This is so great. If as you said, a well coded java 7 app runs perfectly under Java 8, keep coding the way you want but compile it to run under Java ...

Since v4.5 you require Java 8. You can't install Java 8 in Windows XP, only 7.

Multi platform... except for Windows XP, such a great idea!