Introduction to Classes & Objects

Prerequisites: Method overloading

A class is a blueprint describing a kind of thing; an object is a specific instance created from that blueprint.

Syntax

class Dog {
    String name;
    int age;

    void bark() {
        System.out.println(name + " says Woof!");
    }
}

Example

Dog myDog = new Dog();
myDog.name = "Rex";
myDog.age = 3;
myDog.bark();

Expected output

Rex says Woof!

Real-world analogy: a class is like a cookie cutter, and each object is a cookie made from it — every cookie has the same shape (fields and methods), but its own specific values.

Common mistakes

Forgetting new when creating an object — Dog myDog; only declares a variable that can hold a Dog; it doesn't create one.

Practice

Create a Cat class with a name field and a meow() method that prints "<name> says Meow!", then create and use one.

Show solution

class Cat {
    String name;

    void meow() {
        System.out.println(name + " says Meow!");
    }
}

Cat myCat = new Cat();
myCat.name = "Whiskers";
myCat.meow();