What is the use of singletonList?

Question

I was looking around for some elegant solution to removing null values from a List. I came across the following post, which says I can use list.removeAll(Collections.

Answer 1

It works like a charm:

List<String> list = new ArrayList<String>();
list.add("abc");
list.add(null);
list.add("def");
list.removeAll(Collections.singletonList(null));
System.out.println(list);  //[abc, def]

Indeed Collections.singletonList(null) is immutable (which is unfortunately hidden in Java^[1]), but the exception is thrown from your list variable. Apparently it is immutable as well, like in example below:

List<String> list = Arrays.asList("abc", null, "def");
list.removeAll(Collections.singletonList(null));

This code will throw an UnsupportedOperationException. So as you can see singletonList() is useful in this case. Use it when client code expects a read-only list (it won't modify it) but you only want to pass one element in it. singletonList() is (thread-)safe (due to immutability), fast and compact.

^[1] E.g. in scala there is a separete hierarchy for mutable and immutable collections and API can choose whether it accept this or the other (or both, as they have common base interfaces)

Answer 2

To answer your actual question :

what would be a typical use of this singletonList? To represent a collection of size 1 when you're sure you don't want to actually do anything with the collection?

The typical use is if you have one element and want to pass it to a method that accepts a List, ie

public void registerUsers(List<User> users) {...}

User currentUser = Login Manager.getCurrentUser();
registerUsers(Collections.singletonList(currentUser));

The removeAll() is a special case for this.

Answer 3

Has your list been protected with

Collections.unmodifiableList(list)

Because if you have protected it and try to modify it later you get that error.