Unfortunately saying "I need to learn Java" is a pretty broad statement these days. The standard library isn't too hard to pick up on once you figure out how to google stuff correctly but I assume coming from a C/C++ background you're not used to or comfortable quickly picking up third parties and using them in your software stack. You'd also need to learn a few things that are in pretty much every Java solution such as the build framework (most likely will be Maven, could be Gradle), Spring (at least the IoC framework but most places use Web MVC and AoP), possibly some persistence layer/ORM (Hibernate, MyBatis) and if you're not comfortable writing unit tests you need to learn JUnit like the back of your hand.
But I may be overstepping for how to teach a novice the basics. But honestly its programming so you'll have for loops and classes. That shit is easy to pick up on if you're already a programmer. Its the mountain of 3rd parties for Java (and realizing that someone PROBABLY has done whatever you need and exposed it as a third party), proper OOP/D, clean code and well tested code that is the hard part of the job. Chances are if you're being required to write Java code the odds of you exposing yourself as a service and talking down to a database is fairly high. So, again, that is Spring and some ORM. Unless you really love raw SQL.
