For example, walls of a cabin may be built of wood, but the castle walls must be built with stone. Some of the construction steps might require different implementation when you need to build various representations of the product.
You can call only those steps that are necessary for producing a particular configuration of an object. The important part is that you don’t need to call all of the steps. To create an object, you execute a series of these steps on a builder object. The pattern organizes object construction into a set of steps ( buildWalls, buildDoor, etc.). The Builder doesn’t allow other objects to access the product while it’s being built. The Builder pattern lets you construct complex objects step by step. The Builder pattern suggests that you extract the object construction code out of its own class and move it to separate objects called builders. For instance, only a fraction of houses have swimming pools, so the parameters related to swimming pools will be useless nine times out of ten. In most cases most of the parameters will be unused, making the constructor calls pretty ugly. The constructor with lots of parameters has its downside: not all the parameters are needed at all times. While this approach indeed eliminates the need for subclasses, it creates another problem. You can create a giant constructor right in the base House class with all possible parameters that control the house object. There’s another approach that doesn’t involve breeding subclasses. Any new parameter, such as the porch style, will require growing this hierarchy even more. But eventually you’ll end up with a considerable number of subclasses. The simplest solution is to extend the base House class and create a set of subclasses to cover all combinations of the parameters. But what if you want a bigger, brighter house, with a backyard and other goodies (like a heating system, plumbing, and electrical wiring)?
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To build a simple house, you need to construct four walls and a floor, install a door, fit a pair of windows, and build a roof.
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You might make the program too complex by creating a subclass for every possible configuration of an object.įor example, let’s think about how to create a House object. Or even worse: scattered all over the client code. Such initialization code is usually buried inside a monstrous constructor with lots of parameters. Imagine a complex object that requires laborious, step-by-step initialization of many fields and nested objects.
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