2.7 Class Variables

Learning Objectives

•Declare class (static) variables.
•Explain how class variables differ from instance variables in storage and sharing.
•Access a static variable through the class name.

Introduction

A class variable (declared with `static`) belongs to the class itself rather than to any single object — there is exactly one copy, shared by every instance. Class variables are useful for values that should be consistent across all objects, like a running count of how many objects have been created.

Key Concepts

`static` fields belong to the class, not to individual objects — there is only one copy total.

Class variables are conventionally accessed via the class name (`Student.totalCount`), not an instance.

A common use is a shared counter incremented in the constructor every time a new object is created.

Java Syntax

Code Examples

Every time a `Student` is constructed, the shared `totalCount` field increments. Because it's `static`, both objects update the same single counter.

Common Mistakes

⚠Expecting each object to have its own copy of a static field — they all share the exact same one.
⚠Accessing a static field through an instance (`ama.totalCount`) instead of the class name — it compiles but is misleading style.

Summary

Class (static) variables are shared across all instances of a class, unlike instance variables which are per-object.

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