I have a maven project build on java 7 say A. I have a another external maven project build on java 8 say B.
Is it possible to add dependency of project B in project A?
It's is not possible as the classes in project B (when compiled with Java 8) will have a different class format. The result will be some type of class format error like here.
If project B is compiled with target 1.7
and uses no API from Java 8 then it is possible that it will work.
The safest thing is to assume that you have to use Java 8 when using project B.
Usually not, but it depends on what APIs project B uses.