Why Static Factory Methods?

Example: Rectangle

The constructors just use the object type name (e.g., Rectangle).

It's certainly not clear what the passed parameters are. 😕

let rec1 = new Rectangle(500, 250);
let rec2 = new Rectangle(300);

Example: Circle

This won't work 😢

We would have to add unnecessary complexity and add more parameters to the constructors to allow this to work.

//constructing using radius
let circ1 = new Circle(10);
//constructing using diameter
let circ2 = new Circle(20);

Example: Rectangle

Now it is clear that we are passing width and height in the first case, and a square side in the second case. 😊

let rec = Rectangle.fromWidthHeight(500, 2500);
let square = Rectangle.fromSide(700);

Example: Circle

Circle will be left as a fun drill problem in this chapter.

Example: Date

Objects often provide multiple alternatives for construction.

Static factory methods (shown) provide clear protocol.

Standard constructors would not support these alternatives (same method signature). They would need to use a clumsy workaround such as a date-type parameter.

let date1 = Date.fromMMDDYY('6/1/99');
let date2 = Date.fromMMDDYY('6/1/1999');
let date3 = Date.fromYYYYMMDD('1999-6-1');

Example: FullName

Another example of an object providing multiple alternatives for construction.

With standard constructors, e.g., "new FullName('Asha', 'Gupta')" there is no indication of what the parameters are.

let name1 = FullName.fromFirstLast('Asha', 'Gupta')
let name2 = FullName.fromFirstMiddleLast('Asha', 'Kanvar', 'Gupta')