. lang。UnsupportedClassVersionError: .class文件中的错误版本号?

I am getting this error when I include an opensource library that I had to compile from source. Now, all the suggestions on the web indicate that the code was compiled in one version and executed in another version (new on old). However, I only have one version of JRE on my system. If I run the commands:I am getting this error when I include an opens