What are static variables?

Static variables: belong to the class rather than a particular instance.

  • These types of variables are variables shared across all instances of a class.
import java.util.ArrayList;
import java.util.List;

public class Gadget {
    public static int totalGadgets = 0;  // Static variable to track total gadgets made
    private String gadgetName;  // Instance variable to store the name of the gadget
    //public static List<Gadget> gadgetsList = new ArrayList<>();  // Static list to track all gadgets

    // Constructor to set the gadget name and increment totalGadgets
    public Gadget(String gadgetName) {
        this.gadgetName = gadgetName;
        totalGadgets++;  // Increment the total gadgets count
        // gadgetsList.add(this);  // Add this gadget to the static list
    }

}
// In the Main class:
public class Main {
    public static void main(String[] args) {
        // Create three gadgets
        Gadget g1 = new Gadget("Freeze Ray");
        Gadget g2 = new Gadget("Banana Blaster");
        Gadget g3 = new Gadget("Lipstick Taser");

        // Print the total number of gadgets
        System.out.println("Total gadgets made: " + Gadget.totalGadgets);
    }
}

Main.main(null);


Total gadgets made: 3

Cool, but why did I have to use a static variable?

  • The totalGadgets was made as a static variable because it is tracking data that is shared across all instances of the Gadget class.
  • If totalGadgets was an instance variable, then it would always be 1 since it would only reflect how many gadgets are in the specific instance.

Popcorn hacks:

  • Look at some of the code I’ve commented out and try experimenting with gadgetsList if you want. Otherwise, just make a static variable that serves a purpose in the program.
public class Minion {
    // Static variable to count the number of minions
    public static int minionCount = 0;

    // Instance variables
    private String name;

    // Constructor
    public Minion(String name) {
        this.name = name;
        minionCount++; // Increase count whenever a new minion is created
    }

    // Method to get the total number of minions
    public static int getMinionCount() {
        return minionCount;
    }

    public static void main(String[] args) {
        // Create some minions
        Minion kevin = new Minion("Kevin");
        Minion stuart = new Minion("Stuart");

        // Print the total number of minions
        System.out.println("Total minions: " + Minion.getMinionCount());
    }
}

Minion.main(null);

Total minions: 2

What are static methods?

Static methods are associated with the class and not any object of the class

  • Static methods can only directly access other static methods and static variables of the class.
  • They cannot use the “this” keyword because they don’t belong to any instance.

Here’s an example:

import java.util.ArrayList;
import java.util.List;

public class Gadget {
    public static int totalGadgets = 0;  // Static variable to track total gadgets made
    private String gadgetName;  // Instance variable to store the name of the gadget
    public static List<Gadget> gadgetsList = new ArrayList<>();  // Static list to track all gadgets

    // Constructor to set the gadget name and increment totalGadgets
    public Gadget(String gadgetName) {
        this.gadgetName = gadgetName;
        totalGadgets++;  // Increment the total gadgets count
        gadgetsList.add(this);  // Add this gadget to the static list
    }

    // Getter for the gadget name
    public String getGadgetName() {
        return gadgetName;
    }

    // Static method to print all gadgets in the list
    public static void printAllGadgets() {
        System.out.println("Gadgets created:");
        for (int i = 0; i < gadgetsList.size(); i++) {
            System.out.println("- " + gadgetsList.get(i).getGadgetName());
        }
    }
}

// In the Main class:
public class Main {
    public static void main(String[] args) {
        // Create three gadgets
        Gadget g1 = new Gadget("Freeze Ray");
        Gadget g2 = new Gadget("Banana Blaster");
        Gadget g3 = new Gadget("Lipstick Taser");

        // Print the total number of gadgets
        System.out.println("Total gadgets made: " + Gadget.totalGadgets);

        // Print all gadgets stored in the static list
        Gadget.printAllGadgets();
    }
}

Main.main(null);


Total gadgets made: 3
Gadgets created:
- Freeze Ray
- Banana Blaster
- Lipstick Taser

Why did I use a static method?

  • Static methods can only directly access other static variables and methods, such as gadgetsList
  • I needed to print inforamtion about all instances of the Gadget class, which applied to the entire class as a whole.

Popcorn hack:

Dr. Nefario and Gru need to calculate the cost of their equipment to remain under the budget for this year! Add a second parameter to the Gadget constructor to include cost for Gadget instances, and make a static method to calculate the price of all gadgets that have been made so far.

public class Gadget {
    // Instance variables
    private String name;
    private double cost;

    // Static variable to keep track of total cost
    public static double totalCost = 0;

    // Constructor to initialize name and cost of each gadget
    public Gadget(String name, double cost) {
        this.name = name;
        this.cost = cost;
        totalCost += cost; // Add the cost of this gadget to the total cost
    }

    // Static method to get the total cost of all gadgets
    public static double getTotalCost() {
        return totalCost;
    }

    public static void main(String[] args) {
        // Create some gadgets with name and cost
        Gadget laserGun = new Gadget("Laser Gun", 500.50);
        Gadget jetpack = new Gadget("Jetpack", 1200.75);
        Gadget bananaLauncher = new Gadget("Banana Launcher", 299.99);

        // Print the total cost of all gadgets
        System.out.println("Total cost of all gadgets: $" + Gadget.getTotalCost());
    }
}
Gadget.main(null);


Total cost of all gadgets: $2001.24