Every program in java is a collection of objects communicating via invoking each others methods. Here are few java basic concepts.
A class is a template or blueprint which defines the properties, states and behavior that objects of the class support.
Object is an instance of a class which has behaviors and states. The properties and behaviors defined in a class are achieved through their objects.
Methods are behaviors which are defined inside a class. Logic is written is methods and many methods can be written in a class.
Instance variables are used to store the states of objects. They are declared in class outside any method declaration or constructor.
The keywords are reserved words with predefined meaning in java. They cannot be used by programmer as variable name, class name, method name or any other identifier name. Java has total 50 keywords.
comments are used to help the person reading code to better understand the functionality of the program. They are the integral part of any program.