Helper to remove null references in a Java List?

Given the following List:

List<String> list = new ArrayList<String>();

list.add("s1");
list.add("s2");
list.add(null);
list.add("s3");
list.add(null);
list.add("s4");

I need a helper class that removes null references. Something like:

SomeHelper.removeNullReference(list);

such that the list only contains "s1", "s2", "s4", "s4" (non-null references).

What should I use to fulfill this requirement?