Classes and objects are the foundation of object-oriented programming. They help to structure real-world entities in our code base.

• Classes: Blueprint that creates objects. Shares common properties of the object.
• Objects: Specific entity that is created from a class.

Real Life Analogy: Building Blueprint

✔ Class = Blueprint, Object = Real Building

Java Class (Blueprint)

The Building class is like a blueprint:

class Building {
    int floors;
    String color;
    
    Building(int floors, String color) {
        this.floors = floors;
        this.color = color;
    }
}

Object = Real Building

Objects are created from the class using the new keyword.

Building building1 = new Building(5, "Blue");
Building building2 = new Building(10, "White");

System.out.println("This building has " + building1.floors + " floors and is " + building1.color + ".");
System.out.println("This building has " + building2.floors + " floors and is " + building2.color + ".");

Ways to Create Objects

There are four main ways to create objects in Java:

1. Using new Keyword

Most common way to create an object.

Building building1 = new Building(5, "Blue");

2. Using Reflection

Create an object without using the new keyword, by asking Java to create it at runtime using the class name.

Class<?> clazz = Class.forName("Building");
Building building = (Building) clazz.getDeclaredConstructor().newInstance();

3. Using clone() Method

The clone() method creates a copy of an existing object instead of creating a new one manually.

class Building implements Cloneable {
    int floors;
    String color;
    
    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

Building original = new Building(5, "Blue");
Building cloned = (Building) original.clone();

4. Using Deserialization

Deserialization is a technique of reading an object from the saved state in a file. The object is recreated from a stored byte stream.

ObjectInputStream in = new ObjectInputStream(new FileInputStream("building.ser"));
Building building = (Building) in.readObject();
in.close();