OOP Basics in Java Classes, Objects, Constructors: Tutorial, Examples, FAQs & Interview Tips

Classes and Objects

A class is a blueprint that defines the state (fields) and behavior (methods) of objects. An object is a concrete instance of a class created with the new keyword.

Class, Object & Constructors

public class Car {
    // Instance fields (state)
    String brand;
    String model;
    int year;
    double price;

    // Default constructor (no parameters)
    public Car() {
        brand = "Unknown";
        model = "Unknown";
        year  = 2000;
        price = 0.0;
    }

    // Parameterized constructor
    public Car(String brand, String model, int year, double price) {
        this.brand = brand;   // 'this' refers to the current object
        this.model = model;
        this.year  = year;
        this.price = price;
    }

    // Copy constructor
    public Car(Car other) {
        this.brand = other.brand;
        this.model = other.model;
        this.year  = other.year;
        this.price = other.price;
    }

    // Instance method (behavior)
    public void displayInfo() {
        System.out.printf("%s %s (%d) - $%.2f%n", brand, model, year, price);
    }

    public static void main(String[] args) {
        Car c1 = new Car();                              // default constructor
        Car c2 = new Car("Toyota", "Camry", 2023, 26000.0);  // parameterized
        Car c3 = new Car(c2);                            // copy constructor

        c1.displayInfo();  // Unknown Unknown (2000) - $0.00
        c2.displayInfo();  // Toyota Camry (2023) - $26000.00
        c3.displayInfo();  // Toyota Camry (2023) - $26000.00
    }
}

The `this` Keyword

this is a reference to the current object. It has three main uses:

Disambiguate fields from parameters - this.name = name;
Call another constructor - this(arg1, arg2); (must be first statement)
Pass the current object - someMethod(this);

this() Constructor Chaining

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

    // Full constructor
    public Person(String name, int age, String email) {
        this.name  = name;
        this.age   = age;
        this.email = email;
    }

    // Delegates to full constructor using this()
    public Person(String name, int age) {
        this(name, age, "not provided");  // must be first line
    }

    // Delegates further
    public Person(String name) {
        this(name, 0);
    }

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

    public static void main(String[] args) {
        System.out.println(new Person("Alice", 30, "alice@example.com"));
        System.out.println(new Person("Bob", 25));
        System.out.println(new Person("Charlie"));
    }
}

Key Takeaways

OOP organizes code around objects that combine data (fields) and behavior (methods).
The four pillars of OOP are: Encapsulation, Inheritance, Polymorphism, and Abstraction.
A class is a blueprint; an object is an instance of a class created with the new keyword.
Constructors initialize objects - Java provides a default no-arg constructor if none is defined.
this refers to the current object instance; super refers to the parent class.
Java supports single inheritance for classes but multiple inheritance through interfaces.

OOP Basics in Java Classes, Objects, Constructors: Tutorial, Examples, FAQs & Interview Tips

Classes and Objects

The this Keyword

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The `this` Keyword