---
title: "Basic Object-Oriented Concepts: Classes and Objects"
description: Class definition, object instantiation, constructors, and instance methods in Java.
---
## Classes and Objects

A **class** is a blueprint that defines attributes and methods. An **object** is a concrete instance of a class, created with the `new` keyword.

```java
ClassName obj = new ClassName();
```

## Constructors

Constructors are special methods that initialize an object when it is created.

- **Default constructor** — No parameters, assigns default values.
- **Parameterized constructor** — Accepts arguments to initialize fields.
- **Constructor overloading** — Multiple constructors with different parameter lists.

## The `this` Keyword

`this` refers to the current object instance. It is commonly used to:

- Distinguish instance variables from parameters with the same name.
- Call another constructor from within a constructor.

## Instance Methods

Instance methods operate on the data of a specific object and are called on an object reference.

## Example

```java
// StudentDemo.java
// Demonstrates class, constructors, and methods in Java.

class Student {
	// Instance variables
	String name;
	int age;

	// Default constructor
	Student() {
		name = "Unknown";
		age = 0;
	}

	// Parameterized constructor
	Student(String n, int a) {
		name = n;
		age = a;
	}

	// Method to display student info
	void display() {
		System.out.println("Name: " + name + ", Age: " + age);
	}
}

public class StudentDemo {
	public static void main(String[] args) {
		// Create students using both constructors
		Student s1 = new Student();
		Student s2 = new Student("Alice", 20);
		s1.display();
		s2.display();
	}
}
```

> Full source: [`examples/05-classes-objects/StudentDemo.java`](https://github.com/University-Of-Sri-Jayewardenepura/Java/blob/main/examples/05-classes-objects/StudentDemo.java)