I am not sure how first case is more explicit. Try Ctrl+click (say we are in Eclipse world) on getCommentById(...) in first case and you will see interface signature - bummer. And while you will be pressing Ctrl+Shift+G and trying to guess (just guess because only Spring knows for sure) which implementation is more likely to be used on this particular line, I will jump straight to getCommentCache(...) implementation (2nd example, non-interface) and move miles ahead.
EDIT: Not to mention, that passing references to cache object along each method call bloats the code. Now you have references to cache not in 50 cases where cache is used (example #2), but in 5000 cases because each method signature needs to include reference to cache (each method along the call hierarchy until you reach the point where you use cache object).
That's not what I mean. When you see an instance of explicit `isCommentSpam` used, you know immediately that it is using the CommentCache. Compare:
if (isCommentSpam(423)) {
doSomething();
}
if(isCommentSpam(423, commentCache)) {
doSomething();
}
See, the fact that the first kind uses the CommentCache is completely opaque to you when you're reading the code. You don't know it does until you actually look inside `isCommentSpam`. The second way, you can trace the flow of information through the program very easily.
EDIT: Not to mention, that passing references to cache object along each method call bloats the code. Now you have references to cache not in 50 cases where cache is used (example #2), but in 5000 cases because each method signature needs to include reference to cache (each method along the call hierarchy until you reach the point where you use cache object).