Use of Java's Collections.singletonList()?

Question

What is the use of Collections.singletonList() in Java? I understand that it returns a list with one element. Why would I want to have a separate method to do that? How does immutability play a role here?

Are there any special useful use-cases for this method rather than just being a convenient method?

Answer 1

The javadoc says this:

"Returns an immutable list containing only the specified object. The returned list is serializable."

You ask:

Why would I want to have a separate method to do that?

Primarily as a convenience ... to save you having to write a sequence of statements to:

• create an empty list object
• add an element to it, and
• wrap it with an immutable wrapper.

It may also be a bit faster and/or save a bit of memory, but it is unlikely that these small savings will be significant. (An application that creates vast numbers of singleton lists is unusual to say the least.)

How does immutability play a role here?

It is part of the specification of the method; see above.

Are there any special useful use-cases for this method, rather than just being a convenience method?

Clearly, there are use-cases where it is convenient to use the singletonList method. But I don't know how you would (objectively) distinguish between an ordinary use-case and a "specially useful" one ...