Why always have single implementaion interfaces in service and dao layers?

There are more advantages to interfaces - As in proxying . If your class implements an interface , JDK dynamic proxies will be used by default for AOP . If you use the implementations directly, you'll be forced to use CGLIB proxies by making proxy-target-class=true .

These require byte code manipulation unlike JDK proxies .

It's a very controversial subject. In brief, there's none—at least for you, the developer.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.