How to construct immutable collections wiht only 1 item ?

Immutable collections can be constructed with a set of singleton* methods: sinleton for a Set, singletonMap for a Map and singletonList for a List:

@Test
public void should_correctly_create_singleton_list() {
  List<String> oneLetterList = Collections.singletonList("A");

  assertThat(oneLetterList.get(0)).isEqualTo("A");
  assertThat(oneLetterList).hasSize(1);
}

@Test(expected = UnsupportedOperationException.class)
public void should_correctly_create_singleton_list_and_assert_that_it_s_not_mutable() {
  Map<String, String> oneLetterMap = Collections.singletonMap("A", "Letter A");

  oneLetterMap.put("B", "Letter B");
}