Chapter 5 · Lesson 1

Classes and Objects

Classes and Objects in Java

Java is an object-oriented language — almost everything is an object.

Class definition A class is a blueprint. It contains fields (state) and methods (behaviour).

Constructors A constructor initialises a new object. Its name matches the class name and it has no return type. If you don't define one, Java provides a no-argument constructor automatically.

this keyword Inside instance methods and constructors, this refers to the current object. It is used to:

Distinguish field names from parameter names.
Call another constructor in the same class: this(arg).

new keyword Allocates memory for a new object and calls its constructor:

java

Dog d = new Dog("Rex");

Key points:

Fields should generally be private to enforce encapsulation.
Provide public getters and setters to control access.
Multiple constructors with different parameters are allowed (constructor overloading).

Code Examples

Class with fields, constructor, and methodsjava

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() { return name; }
    public int getAge() { return age; }

    public void birthday() {
        age++;
    }

    @Override
    public String toString() {
        return name + " (age " + age + ")";
    }

    public static void main(String[] args) {
        Person p = new Person("Alice", 30);
        System.out.println(p);
        p.birthday();
        System.out.println(p.getName() + " is now " + p.getAge());
    }
}

Private fields with public getters/setters enforce encapsulation. toString() provides a readable string representation.

Constructor overloading with this()java

public class Rectangle {
    private double width;
    private double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    // Delegates to the two-argument constructor
    public Rectangle(double side) {
        this(side, side);
    }

    public double area() { return width * height; }
    public double perimeter() { return 2 * (width + height); }

    public static void main(String[] args) {
        Rectangle r = new Rectangle(4, 6);
        System.out.println("Area: " + r.area());
        Rectangle sq = new Rectangle(5);
        System.out.println("Square area: " + sq.area());
    }
}

this() calls another constructor in the same class, avoiding code duplication. It must be the first statement.

Quick Quiz

1. What does the `new` keyword do?

2. What is the purpose of `this` in a constructor?

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