Using the builder pattern with subclasses

 Josh Bloch's Effective Java popularized the Builder Pattern as a more palatable way of constructing objects than constructors or factory methods when there are potentially many constructor parameters. The formulation in Effective Java makes for a particularly readable construction, like this: new Rectangle.Builder().height(250).width(300).color(PINK).build(); The advantage over a constructor invocation like new Rectangle(250, 300, PINK); here is that you don't have to guess whether 250 is the height or the width. More generally, if the constructor allowed you to specify many other parameters — such as position, opacity, transforms, effects, and so on — it would quickly get very messy. The builder pattern stays clean. But one question that arises is: how does it work in the presence of inheritance? For example, suppose you have an abstract Shape class that represents an arbitrary graphical shape, with a set of properties that are common to all shapes, such as opacity and transforms. And suppose you have a number of concrete subclasses such as Rectangle, Circle, Path and so on, each with its own properties, like Rectangle's height and width. As a reminder, here's what the builder pattern looks like in the absence of subclassing: public class Rectangle { private final double opacity; private final double height; ... publi...

Date: October, 25 2010
Url: http://www.java.net/blog/emcmanus/archive/2010/10/25/using-builder-pattern-subclasses

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