Though there are tools like Pig and Hive which automatically converts your MapReduce jobs in Java, Java becomes important when you need to go deeper into code to solve tiny problems.
Learning "just enough" Java is learning Java. Either you learn all the core principles and language design decisions, or you suffer along making easily avoidable mistakes. Considering that you already know how to program, a lot of the information can be skimmed. As you will start learning Java you will notice how it differs from C++ and soon you will be able to cope with the different syntax of java, the language will become familiar.
The basic things you need to learn:
You just need to take it easy, Since you have good command in C++, it will be easy for you to grasp Java language. I would personally suggest you go through some online java tutorials and you can also read some Java books.
If you want more information regarding Hadoop, then do check out this video tutorial which will help you in learning Hadoop from scratch: