In object-oriented Java, what are instance variables and how do they differ from class (static) variables?

Introduction / Context:
Understanding the difference between instance variables and other types of variables is fundamental to object-oriented programming in Java. Instance variables represent the state of an object and are one of the key reasons to use classes and objects in the first place. Interviewers ask this question to confirm that you understand how data is stored in each object and how it differs from shared static data or temporary local variables.

Given Data / Assumptions:

• We are working with Java classes that can define fields and methods.
• Objects are created from classes using the new keyword.
• Variables can be declared as static or non-static (instance).
• Local variables can also be declared inside methods or blocks.

Concept / Approach:
Instance variables are fields declared in a class without the static keyword. For each object created from the class, a separate copy of each instance variable is allocated. These variables hold the unique state of that particular object, such as name, age, or balance. In contrast, static variables (class variables) belong to the class itself and are shared across all instances. Local variables, on the other hand, live only inside methods and are not part of the object state. Correctly identifying instance variables is important for modeling real world entities and understanding memory layout in Java programs.

Step-by-Step Solution:
Step 1: Look at where the variable is declared: instance variables are declared directly in the class body, but outside any method or constructor. Step 2: Confirm that there is no static keyword in the declaration; for example, int age; in a class is an instance variable if it is non-static. Step 3: Recognize that when you create two different objects of the same class, each object has its own copy of the instance variables. Step 4: Understand that changes to an instance variable of one object do not affect the instance variable value of another object. Step 5: Distinguish this from static variables, which are declared with static and are shared by all objects of that class.

Verification / Alternative check:
Consider a class Person with an instance variable String name. If you create Person p1 and Person p2 and assign different names to each, printing p1.name and p2.name shows independent values. This confirms that each object has its own instance variable state. By contrast, if you declare a static int counter in the same class and increment it each time a Person is created, the value of counter will be shared across all objects, proving that instance variables and static variables behave differently.

Why Other Options Are Wrong:
Option B incorrectly describes local variables, which are declared inside methods and are not stored as part of each object. Option C describes static variables, which are class variables shared among all instances. Option D is wrong because instance variables are not restricted to interfaces; in fact, interface fields are static and final by definition, not instance variables.

Common Pitfalls:
A common pitfall is accidentally declaring fields as static when they should be instance variables, leading to shared state where separate object state was intended. Another issue is shadowing instance variables with local variables of the same name, which can confuse code readers and cause bugs. Using clear naming conventions and understanding where variables are declared helps avoid these mistakes.

Final Answer:
In Java, instance variables are non-static fields declared in a class for which each object gets its own copy, representing the unique state of that specific instance.