Methods Inside Classes

You know how to write methods with static: those belong to the class itself. But when you're building objects, you usually want methods that belong to each object. These are called instance methods, and they don't use the static keyword

Instance Methods

class Car {
    String model;

    Car(String model) {
        this.model = model;
    }

    void honk() {
        System.out.println(model + " goes: Beep!");
    }
}

public class Main {
    public static void main(String[] args) {
        Car c1 = new Car("Infernus");
        Car c2 = new Car("Cheetah");
        c1.honk();
        c2.honk();
    }
}

Output

Infernus goes: Beep!
Cheetah goes: Beep!

Notice: honk() has no static keyword. That's because it's an instance method: it operates on a specific car. When you call c1.honk(), Java knows model refers to c1's model

Methods That Return Values

Instance methods can return values just like static ones:

class Wallet {
    int cash;

    Wallet(int cash) {
        this.cash = cash;
    }

    int getCash() {
        return cash;
    }

    void addCash(int amount) {
        cash += amount;
    }
}

public class Main {
    public static void main(String[] args) {
        Wallet w = new Wallet(50);
        w.addCash(30);
        System.out.println("Cash: " + w.getCash());
    }
}

Output

Cash: 80

The pattern: void means the method does something but returns nothing. A return type like int means it gives back a value

Methods That Modify the Object

This is where it gets powerful. Methods can change the object's own fields:

class GangMember {
    String name;
    int respect;

    GangMember(String name) {
        this.name = name;
        this.respect = 0;
    }

    void completeMission() {
        respect += 10;
        System.out.println(name + " completed a mission! Respect: " + respect);
    }
}

public class Main {
    public static void main(String[] args) {
        GangMember tommy = new GangMember("Tommy Vercetti");
        tommy.completeMission();
        tommy.completeMission();
        tommy.completeMission();
    }
}

Output

Tommy Vercetti completed a mission! Respect: 10
Tommy Vercetti completed a mission! Respect: 20
Tommy Vercetti completed a mission! Respect: 30

Let's trace what happens to tommy:

• At creation, the constructor sets respect to 0
• First completeMission(): respect rises to 10, then the line with Respect: 10 is printed
• The second call starts from 10 and reaches 20
• The third starts from 20 and reaches 30

Each call to completeMission() modifies that specific object's respect field. If we had another GangMember, their respect would be separate

Static vs Instance: The Key Difference

Here's the rule:

• Static methods belong to the class. You call them with the class name: Math.max(5, 10)
• Instance methods belong to an object. You call them on a variable: tommy.completeMission()

Static methods can't access instance fields (because there's no object). Instance methods can access everything

class Example {
    int x = 10;          // instance field

    void show() {        // instance method, can use x
        System.out.println(x);
    }

    static void greet() { // static method, CANNOT use x
        System.out.println("Hello");
        // System.out.println(x);  // ERROR! No object, no x
    }
}

That's why main is static: it runs before any objects exist. It's the starting point, and from there you create objects and call their methods

Mission: Business Earnings

Tommy owns several businesses in Vice City. Each business has a name and a total of earnings that starts at 0. Every time the business brings in money, the total grows

Create a class that represents a business, with a field for the name and one for the total earnings. Write a method that adds an amount to the total and a method that returns the current total

In main, store the first business's name in name1 and its two takings in sale1 and sale2; store the second's name in name2 and its two takings in sale3 and sale4. Create two businesses from those variables, add each business's two takings, then print for each business the name, then : , then the total, for example Malibu Club: 500

Example: Malibu Club with takings 200, 300 and Print Works with 500, 250

Malibu Club: 500
Print Works: 750

Example: Cherry Popper with takings 100, 100 and Sunshine Autos with 700, 300

Cherry Popper: 200
Sunshine Autos: 1000