design patterns - Static factory methods vs Instance (normal) constructors?

Question

In a language where both are available, would you prefer to see an instance constructor or a static method that returns an instance?

For example, if you're creating a String from a char[]:

1. String.FromCharacters(chars);

2. new String(chars);

Answer 1

In Effective Java, 2nd edition, Joshua Bloch certainly recommends the former. There are a few reasons I can remember, and doubtless some I can't:

• You can give the method a meaningful name. If you've got two ways of constructing an instance both of which take an int, but have different meanings for that int, using a normal method makes the calling code much more readable.
• A corollary of the first - you can have different factory methods with the same parameter list
• You can return null for "potentially expected failure" cases whereas a constructor will always either return a value or throw an exception
• You can return a type other than the declared (e.g. return a derived class)
• You can use it as a factory, to potentially return a reference to the same object several times

The downsides:

• It's not as idiomatic, currently - developers are more used to seeing "new"
• If you see "new" you know you're getting a new instance (modulo the oddity I mentioned recently)
• You need to make appropriate constructors available for subclasses
• In C# 3, constructor calls are able to set fields/properties in a compact manner with object initializer expressions; the feature doesn't apply to static method calls