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Activity 4.4 Static Variables and Static Methods

Introduction

Activity 4.4

Static Variables and Static Methods

Topics

Difference between instance and static members
Static variables (class-level data)
Static methods (class-level behavior)
Why main is static
Common AP mistakes involving static context

Instance vs. Static: The Big Idea

So far, all data you've worked with belonged to specific objects (instance variables).

Static means "belongs to the class itself," not to a specific object.

Instance Members (Review)

Each object gets its own copy
Used to store object-specific data
Accessed with objectName.variable

Static Members

Shared among ALL objects of the class
Stored at the class level, not inside objects
Called using ClassName.method()

Example: Counting Objects

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class Dog {
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    private String name;
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    private static int dogCount = 0; // static: shared across all Dog objects
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    public Dog(String name) {
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        this.name = name;
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        dogCount++; // increments whenever a Dog is created
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    }
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    public static int getDogCount() {
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        return dogCount; // static method can access static variables
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    }
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    public String getName() {
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        return name;
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    }
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}

Static Methods

Static methods do NOT require an object. They are often used for:

Utility functions (e.g., conversions)
Operations that don't need instance data
Working with static variables

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class MathTools {
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    public static int square(int x) {
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        return x * x;
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    }
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}

Common AP Mistake

Static methods cannot directly access instance variables.

Instance variables belong to objects, not classes.

main is static for this reason — it runs before any objects exist.

Key Terms

Static Variable: Shared by all objects of a class; belongs to the class itself.
Static Method: A method that can be called without creating an object.
Instance Variable: Stored per object; each object has its own copy.
Class Method / Class Variable: Another name for static methods/variables.

'F' → Fullscreen

Objectives

Distinguish between instance and static members
Write static variables and static methods
Count objects using static fields
Call static utility methods
Avoid common static-access errors

Activity Tasks

Create a new project named 4-4-Static-Members.
Complete each task individually.

Task 1: Counting Objects with Static Variables

Add a static counter to the Student class to track how many objects have been created.

Step 1 — Student.java

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class Student {
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    private String name;
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    private static int studentCount = 0;
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    public Student(String name) {
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        this.name = name;
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        studentCount++;
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    }
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    public String getName() {
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        return name;
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    }
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    public static int getStudentCount() {
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        return studentCount;
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    }
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}

Step 2 — Main.java

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public class Main {
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    public static void main(String[] args) {
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        Student s1 = new Student("Alex");
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        Student s2 = new Student("Jordan");
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        Student s3 = new Student("Riley");
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        System.out.println("Total students: " + Student.getStudentCount());
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    }
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}

Run the program. Notice how static variables track data shared across all objects.

Task 2: Static Utility Methods

Write a utility class with static methods that convert temperatures.

TemperatureConverter.java

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class TemperatureConverter {
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    public static double cToF(double c) {
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        return (c * 9.0 / 5.0) + 32;
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    }
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    public static double fToC(double f) {
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        return (f - 32) * (5.0 / 9.0);
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    }
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}

Main.java

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public class Main {
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    public static void main(String[] args) {
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        System.out.println("20C in F: " + TemperatureConverter.cToF(20));
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        System.out.println("72F in C: " + TemperatureConverter.fToC(72));
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    }
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}

Sample Output

Total students: 3

20C in F: 68.0
72F in C: 22.222...

Reflection Questions

Explain in your own words the difference between an instance variable and a static variable.
Why can static methods not access instance variables directly?
What is a real-world example of something that should be static?
Why is the main method static?
Describe a situation in which using static methods is preferable to instance methods.

Submission

Submit your completed project and reflection answers to the appropriate dropbox.