Factory Pattern

What

Defines an interface for creating an object, but lets subclasses alter the type of object that will be created. Subclasses decide which concrete classes to create.

When

You need to delegate instantiation logic without specifying concrete classes. You want to delegate object creation to avoid new scattered everywhere.

class App {
    public static void main(String[] args) {
        Shape s;
        String type = "circle";
        if (type.equals("circle")) {
            s = new Circle(); // tight coupling to concrete class
        } else {
            s = new Square();
        }
        s.draw();
    }
}
 

How

• Define a factory method in a creator class.
• Subclasses override the method to instantiate different products.

class ShapeFactory {
    public static Shape getShape(String type) {
        return type.equals("circle") ? new Circle() : new Square();
    }
}

abstract class Dialog {
    public void render() {
        Button b = createButton(); b.render();
    }
    protected abstract Button createButton();
}

Examples

• UI toolkits that create different widgets for Mac/Windows.
• Document editors creating different document types.

Why

Advantages

• Promotes loose coupling.
• Supports Open/Closed Principle.
• Allows code to work with interfaces, not implementations.

Disadvantages

• Can introduce complexity with many subclasses.
• Harder to trace which implementation is used at runtime.