Creating & Initializing Objects in Java With new Keyword

1. Creating an Object

The new keyword creates an object (instance) of a class.

Point origin = new Point(23, 94);

Here:

new Point(23, 94)
      ↓
creates a Point object
      ↓
constructor is called
      ↓
reference is returned
      ↓
stored in origin

Instantiating a class = creating an object (instance) of that class.

2. Constructor

A constructor:

• Has the same name as the class

• Has no return type

• Is called automatically when using new

• Initializes the object's data

class Point {

    int x;
    int y;

    Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

Point p = new Point(10, 20);

3. Multiple Constructors

A class can have multiple constructors as long as their parameter lists are different.

class Rectangle {

    Rectangle() {
    }

    Rectangle(int width, int height) {
    }

    Rectangle(Point origin, int width, int height) {
    }
}

This is called constructor overloading.

Java chooses the constructor based on the number and types of arguments.

new Rectangle();                  // Rectangle()
new Rectangle(50, 100);           // Rectangle(int, int)
new Rectangle(p, 50, 100);        // Rectangle(Point, int, int)

4. No-Argument Constructor

A constructor with no parameters is called a no-argument constructor.

Rectangle rect = new Rectangle();

If you don't define any constructor, Java automatically provides a default no-argument constructor.

class Rectangle {
    // Java provides Rectangle() automatically
}

Important: If you define your own constructor, Java does not automatically provide the no-argument constructor.

5. Object References

A variable of an object type stores a reference to the object, not the object itself.

Point p = new Point(10, 20);

Multiple variables can refer to the same object:

Point p1 = new Point(10, 20);
Point p2 = p1;

Now:

p1 ──┐
     ├──→ Point object
p2 ──┘

Changing the object through p2 also affects what p1 sees.

Quick Summary

new
 ↓
Creates object
 ↓
Calls constructor
 ↓
Returns object reference

new creates the object; the constructor initializes it.