I have written some cheat sheets that summarise the main concepts of java.

They cover the following topics:

CLASS ACCESS
Class
Abstract Class
Final Class
Interfaces
Class Members Access Modifiers
Non-access member modifiers
Access and non-access modifiers (class/variable/methods)
Number types and values
Stack and Heap

FLOW CONTROL
Switch
For loops
Assertions
Exceptions

NUMBERS
Numbers
Literals
Primitive casting
Variables
Arrays

OBJECT ORIENTATION
Method selection
Overriding and Overloading
Constructors
Using super
Initialisation blocks
Initialisation blocks and Constructors
Method visibility
Casting
Polymorphism: Instance variables
Polymorphism: Static variables/Methods
Overloading method signatures

OPERATORS
Logical Operators
Short circuit
instanceof