Object-Oriented Programming: Classes and Objects

A class is a blueprint; an object is a specific thing built from that blueprint. This is the foundation everything else in Java builds on.

Defining a class

public class Student {
    // fields (state)
    String name;
    int age;

    // constructor - runs when you create a new Student
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // method (behavior)
    public void introduce() {
        System.out.println("Hi, I am " + name + ", age " + age);
    }
}

Creating objects

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student("Riya", 21);
        Student s2 = new Student("Aman", 22);
        s1.introduce(); // Hi, I am Riya, age 21
        s2.introduce(); // Hi, I am Aman, age 22
    }
}

Why "this"?

this.name refers to the field on the object itself, distinguishing it from the constructor parameter name - without this, name = name would just assign the parameter to itself and do nothing useful.

Encapsulation: private fields, public methods

In real code, fields are usually private and accessed through public "getter" and "setter" methods - this lets the class control how its data is read and changed:

public class Student {
    private int age;

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        if (age > 0) {
            this.age = age;
        }
    }
}