Unfortunately it's kind of hard to gauge how much is 'enough'.
Ideally, you'd have an understanding of:
Classes
Methods
Primitive types (boolean, int, short, long, float, char, as well as String)
Inheritance (extending classes and also using interfaces)
Basic data structures (Arrays, Lists, etc)
Using Public/Private/Final/etc modifiers for methods and variables when appropriate
If, For, Else, While, Do, Switch statements - How they work and when to use them
Recursive methods (which can save you a lot of coding)
Basic logic (!= not equal, && and, || or, == equa