Builder Design Pattern – Is StringBuilder an Application?

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Is the "Builder" pattern restricted to addressing the "telescoping constructor" anti-pattern, or can it be said to also address the more general problem of complicated creation of immutable objects?

The StringBuilder class has the word "builder" in its name, but it has nothing to do with telescoping constructors, it simply helps us collect all the data that we need to pass to the constructor of an immutable object.

To me it appears like the answer is a very clear "yes", but there appears to be some disagreement on the topic, so I was hoping that someone could perhaps clarify it.

I was answering this question: Programmers SE: Legitimate “real work” in a constructor? where the OP wants to create a (presumably immutable) object containing a complex tree, and the idea of the "builder" pattern popped up, and while researching it I found this Q&A which seems to say that "StringBuilder" style of object creation is not an application of the "Builder" pattern, for reasons which are not clear to me: Stackoverflow – StringBuilder and Builder Pattern. (Those answering that question failed to make a convincing point as far as I can tell.)

Best Answer

A StringBuilder is similar to a the Builder Pattern, but does not share much with the GoF description of this design pattern. The original point of the design pattern was

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

— from Design Patterns, by Gamma, Helm, Johnson, Vlissides.

(note: “complex” primarily means “composed of multiple parts”, not necessarily “complicated” or “difficult”)

The “different representations” is key here. E.g. assuming this construction process:

interface ArticleBuilder {
  void addTitle(String title);
  void addParagraph(String paragraph);
}

void createArticle(ArticeBuilder articleBuilder) {
  articleBuilder.addTitle("Is String Builder an application of ...");
  articleBuilder.addParagraph("Is the Builder Pattern restricted...");
  articleBuilder.addParagraph("The StringBuilder class ...");
}

we might end up with a HtmlDocument or a TexDocument or a MarkdownDocument depending on what concrete implementation is provided:

class HtmlDocumentBuilder implements ArticleBuilder {
  ...
  HtmlDocument getResult();
}

HtmlDocumentBuilder b = new HtmlDocumentBuilder();
createArticle(b);
HtmlDocument dom = b.getResult();

So one central point of the Builder pattern is polymorphism. The Design Patterns book compares this pattern to the Abstract Factory:

Abstract Factory is similar to the Builder in that it too may construct complex objects. The primary difference is that the Builder pattern focuses on constructing a complex object step by step. […] Builder returns the product as a final step, but as far as the Abstract Factory is concerned, the product gets returned immediately.

— from Design Patterns, by Gamma, Helm, Johnson, Vlissides.

This step-by-step aspect has then become the more popular aspect of the Builder pattern, so that in common parlance the Builder pattern is understood like this:

Split construction of an object into multiple steps. This allows us to use named arguments or optional parameters even in languages that do not support these features.

Wikipedia defines the pattern like this:

The builder pattern is an object creation software design pattern. Unlike the abstract factory pattern and the factory method pattern whose intention is to enable polymorphism, the intention of the builder pattern is to find a solution to the telescoping constructor anti-pattern^{[citation needed]}. […]

The builder pattern has another benefit. It can be used for objects that contain flat data (html code, SQL query, X.509 certificate...), that is to say, data that can't be easily edited. This type of data cannot be edited step by step and must be edited at once. The best way to construct such an object is to use a builder class.^{[citation needed]}

— from Builder Pattern on Wikipedia, by various contributors.

So as we can see, there is no truly common understanding of which pattern this name refers to, and in some points different definitions even contradict one another (e.g. regarding the relevance of polymorphism for Builders).

The only common property of the StringBuilder with various interpretations of the pattern is that the product is created step by step rather than in one go. It does not meet a strict reading of the GoF definition of the design pattern, but please note that design patterns are malleable concepts meant to facilitate communication. I would continue to call StringBuilder an example of the Builder Pattern, albeit an atypical one – the main reason for that structure in Java is performant concatenation in the presence of immutable strings, but not some interesting object-oriented design.

Related Solutions

Design Patterns – Do Named Arguments Replace the Builder Pattern?

Builders are most useful when your object needs a lot of arguments/dependencies to be useful, or you want to allow many different ways of constructing the object.

Off the top of my head, I can imagine someone might want to "build" objects in a 3D game like this:

// Just ignore the fact that this hypothetical god class is coupled to everything ever
new ObjectBuilder(x, y, z).importBlenderMesh("./meshes/foo")
                          .syncWithOtherPlayers(serverIP)
                          .compileShaders("./shaders/foo.vert", "./shaders/foo.frag")
                          .makeDestructibleRigidBody(health, weight)
                          ...

I would argue this example is more readable with the builder methods I made up just now than it would be with optional parameters:

new Object(x, y, z, meshType: MESH.BLENDER,
                    meshPath: "./meshes/foo",
                    serverToSyncWith: serverIP,
                    vertexShader: "./shaders/foo.vert",
                    physicsType: PHYSICS_ENGINE.RIGID_DESTRUCTIBLE,
                    health: health,
                    weight: weight)
                    ...

In particular, the information implied by the builder method names has to get replaced by yet more parameters, and it's much easier to forget about one parameter in a group of closely related parameters. In fact, the fragment shader is missing, but you wouldn't notice that unless you knew to look for it.

Of course, if your object only takes one to five arguments to construct, there's no need to get the builder pattern involved, whether or not you have named/optional parameters.

Java Design Patterns – Legitimate Real Work in a Constructor?

And, besides what Ross Patterson suggested, consider this position which is the exact opposite:

Take maxims such as "Thou Shalt Not Do Any Real Work In Thy Constructors" with a grain of salt.
A constructor is, really, nothing but a static method. So, structurally, there is really not much difference between:

a) a simple constructor and a bunch of complex static factory methods, and

b) a simple constructor and a bunch of more complex constructors.

A considerable part of the negative sentiment towards doing any real work in constructors comes from a certain period of the history of C++ when there was debate as to precisely what state the object will be left in if an exception is thrown within the constructor, and whether the destructor should be invoked in such an event. That part of the history of C++ is over, and the issue has been settled, while in languages like Java there never was any issue of this kind to begin with.

My opinion is that if you simply avoid using new in the constructor, (as your intention to employ Dependency Injection indicates,) you should be fine. I laugh at statements like "conditional or looping logic in a constructor is a warning sign of a flaw".

Besides all that, personally, I would take the XML parsing logic out of the constructor, not because it is evil to have complex logic in a constructor, but because it is good to follow the "separation of concerns" principle. So, I would move the XML parsing logic into some separate class altogether, not into some static methods that belong to your ModelDef class.

Amendment

I suppose that if you have a method outside of ModelDef which creates a ModelDef from XML, you will need to instantiate some dynamic temporary tree data structure, populate it by parsing your XML, and then create your new ModelDef passing that structure as a constructor parameter. So, that could perhaps be thought of as an application of the "Builder" pattern. There is a very close analogy between what you want to do and the String & StringBuilder pair. However, I have found this Q&A which seems to disagree, for reasons which are not clear to me: Stackoverflow - StringBuilder and Builder Pattern. So, to avoid a lengthy debate over here as to whether the StringBuilder does or does not implement the "builder" pattern, I would say feel free to be inspired by how StrungBuilder works in coming up with a solution that suits your needs, and postpone calling it an application of the "Builder" pattern until that little detail has been settled.

See this brand new question: Programmers SE: Is “StringBuilder” an application of the Builder Design Pattern?

Best Answer

Related Solutions

Design Patterns – Do Named Arguments Replace the Builder Pattern?

Java Design Patterns – Legitimate Real Work in a Constructor?

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