Is "StringBuilder" an application of the Builder Design Pattern?

Question

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.)

Answer 1

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.