Agreed - I'm an old hand at C and C++, but it'd take me ... 3 mo. to half a year or so to really get a good grip on serious Java.
Don't get me wrong - I've used it ( but not extensively ) and it's not scary or anything, but there would eb a few things what would take some woodshedding.
Even then, there's many, many more than one way to use Java.
Nobody is becoming a "top ranked" Java developer in 3 months.
I say this as somebody who spent many years developing in Java, and I was not even close to being the best on my team.