Use of Java's Collections.singletonList()?

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 ...

Here's one view on the singleton methods:

I have found these various "singleton" methods to be useful for passing a single value to an API that requires a collection of that value. Of course, this works best when the code processing the passed-in value does not need to add to the collection.

From the javadoc

@param  the sole object to be stored in the returned list.
@return an immutable list containing only the specified object.

example

import java.util.*;

public class HelloWorld {
    public static void main(String args[]) {
        // create an array of string objs
        String initList[] = { "One", "Two", "Four", "One",};

        // create one list
        List list = new ArrayList(Arrays.asList(initList));

        System.out.println("List value before: "+list);

        // create singleton list
        list = Collections.singletonList("OnlyOneElement");
        list.add("five"); //throws UnsupportedOperationException
        System.out.println("List value after: "+list);
    }
}

Use it when code expects a read-only list, but you only want to pass one element in it. singletonList is (thread-)safe and fast.