Sheets about stuff I tend to forget.
Software objects are conceptually similar to real-world objects. They too consist of state and related behavior. The goals behind their use are mainly reusability, debugging ease, modularity (the source code for an object can be written and maintained independently of the source code for other objects) and information-hiding (By interacting only with an object's methods, the details of its internal implementation remain hidden from the outside world).
It models the behaviour and state of real-world objects. It is the prototype or blurptint of that object.
Inheritance in OOP allows classes to inherit commonly used state and behavior from other classes.
It is a namespace for organizing classes and interfaces in a logical manner. Placing your code into packages makes large software projects easier to manage.
There are four types of them: Instance variables, class variables, local variables and parameters. They can start with a unicode letter, $ or underscore _. You should avoid the reserved words in Java that you can find in this list. Camel Case for normal variables and UPPER_CASE for constant values.
For a list of the operators used.