When we consider a Java program it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods and instance variables mean. Object - Objects have states and behaviours.

Example: A dog has states - colour, name, breed as well as behaviours -wagging, barking, eating. An object is an instance of a class. Class - A class can be defined as a template/ blue print that describes the behaviours/states that object of its type support. Methods - A method is basically a behaviour. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed. Instance Variables - Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables.