can someone please tell me how much time will take to learn java , i have already learnt c++ and looking forward to learning java

According to Sams, it will only take 24 hours, see Sams Teach Yourself Java in 24 Hours.

commented: lol +14
commented: Good thinking! +14

Learning the language itself from c++ ... you'll have 95% of it with a few days
Learning the API - a lifetime

In my words, Java == C++ with training wheels. The biggest issue is which components to "include", much like C and C++... :-)