Head First Design Patterns

Chapter 4. The Factory Pattern: Baking with OO Goodness

Get ready to bake some loosely coupled OO designs. There is more to making objects than just using the new operator. You’ll learn that instantiation is an activity that shouldn’t always be done in public and can often lead to coupling problems. And you don’t want that, do you? Find out how Factory Patterns can help save you from embarrassing dependencies.

When you see “new,” think “concrete.”

Yes, when you use new you are certainly instantiating a concrete class, so that’s definitely an implementation, not an interface. And it’s a good question; you’ve learned that tying your code to a concrete class can make it more fragile and less flexible.

When you have a whole set of related concrete classes, often you’re forced to write code like this:

Here we’ve got several concrete classes being instantiated, and the decision of which to instantiate is made at runtime depending on some set of conditions.

When you see code like this, you know that when it comes time for changes or extensions, you’ll have to reopen this code and examine what needs ...