Static Variables

This notebook will explain what static variables are, how they differ from instance variables, and provide examples, including a mini-project with broken code to fix.

1. Introduction to Static Variables

Static variables belong to the class rather than any particular instance. They are shared across all instances of the class.
Example:

public class BankAccount {
    // Static variable
    static double interestRate = 0.03;

    // Instance variable
    double balance;

    // Constructor
    public BankAccount(double balance) {
        this.balance = balance;
    }

    // Static method
    static void setInterestRate(double newRate) {
        interestRate = newRate;
    }
}

In this BankAccount class, interestRate is a static variable shared by all instances, and setInterestRate() is a static method that updates this variable.

2. Defining Static Variables

Static variables are defined using the static keyword and accessed using the class name.
Example:

public class Student {
    // Static variable
    static String schoolName = "XYZ School";

    // Instance variable
    String name;

    // Constructor
    public Student(String name) {
        this.name = name;
    }
}

// Accessing static variable
System.out.println(Student.schoolName);

Here, schoolName is a static variable shared by all Student instances.

3. Static Methods

Static methods can access static variables but cannot access instance variables directly.
Example:

public class BankAccount {
    static double interestRate = 0.03;

    static void updateInterestRate(double newRate) {
        interestRate = newRate;
    }
}

// Using static method
BankAccount.updateInterestRate(0.04);

The updateInterestRate method updates the static variable interestRate.

4. Mini Project: Fix the Code

Below is a class with broken code. The goal is to fix the class so it correctly implements and accesses static variables and methods.
Broken code:

public class Vehicle {
    // Static variable to keep count of the number of vehicles
    static int count = 0;

    // Constructor
    public Vehicle() {
        count++;  
    }

    // Static method to display the total number of vehicles
    public static void displayCount() {
        System.out.println("Total vehicles: " + count);
    }
}
Vehicle car1 = new Vehicle();
Vehicle car2 = new Vehicle();
Vehicle car3 = new Vehicle();
Vehicle.displayCount();

Total vehicles: 3

Task: Debug the Vehicle class to ensure count is properly incremented and displayed. Consider how displayCount() should be modified if count is a static variable.

Unit 5 Classes - Static Variables