What is the Difference Between Class and Instance Variables?

The main difference between class and instance variables lies in their scope and visibility. Here are the key differences:

1. Scope: Class variables are defined within the class but outside of any class methods, while instance variables are defined within class methods, typically the constructor.
2. Accessibility: Class variables are shared between all instances of a class, meaning changes made to the class variable affect all instances. In contrast, instance variables are not shared between the objects of a class, and each instance has its own copy of the instance variables. Changes made to the instance variable through one object will not reflect in another object.
3. Memory Management: Instance variables are created when an object is created with the use of the keyword 'new' and are destroyed when the object is destroyed. Class variables, on the other hand, are created when the program starts and destroyed when the program stops.
4. Access Methods: Class variables can be accessed using either the class name or object reference. Instance variables can only be accessed through an object reference, not by the class name.
5. Typically Held: Class variables generally reserve memory for data that the class needs. Instance variables hold values that must be referenced by more than one method, constructor, or block, or essential parts of an object's state that must be present throughout the class.

In summary, class variables are shared among all instances of a class and can be accessed using the class name or an object reference, while instance variables are specific to each instance and can only be accessed through an object reference.

Comparative Table: Class vs Instance Variables

Here is a table highlighting the differences between class and instance variables:

In summary, class variables (also known as static variables) are shared across all instances of a class and can be accessed directly inside the class, while instance variables are specific to each object instance and can have different values for different instances of the same class.