Java collection behavior

Showcases the behavior of various List, Set and Map implementations and their differences.

Some highlights are given in this readme. Refer to the unit tests for all the details.

Difference between Collectors#toList, Collectors#toUnmodifiableList and Stream#toList

Consider the following blocks:

List<Integer> list1 = Stream.of(1, 2, 3)
  .collect(Collectors.toList()); // Since Java 8
List<Integer> list2 = Stream.of(1, 2, 3)
  .collect(Collectors.toUnmodifiableList()); // Since Java 10
List<Integer> list3 = Stream.of(1, 2, 3)
  .toList(); // Since Java 16

Is there any difference?

• Collectors#toList currently returns an ArrayList, but the Javadoc actually does not guarantee what implementation is returned—not even if the list can be modified further! Prefer the more verbose Collectors.toCollection(ArrayList::new) if you need to make more changes to a List created by a Stream.
• Collectors#toUnmodifiableList does not support nulls. Also in read-only methods like list2.contains(null), an exception will be thrown by the list.
• Stream#toList supports nulls, and calling something like list3.contains(null) is fine.

Difference between Collections#unmodifiableSet, Set#copyOf and Guava's ImmutableSet#copyOf

Set<Integer> origin = new LinkedHashSet<>(Arrays.asList(1, 2, 3));

Set<Integer> set1 = Collections.unmodifiableSet(origin);
Set<Integer> set2 = Set.copyOf(origin);
Set<Integer> set3 = ImmutableSet.copyOf(origin);

• Set#copyOf and Guava's ImmutableSet#copyOf make a copy of the original collection, so they produce immutable collections: if origin is changed afterwards, set2 and set3 will not change. Collections#unmodifiableSet produces a set that delegates to the origin; set1 cannot be modified directly but any changes to origin are still reflected. It is unmodifiable but not immutable.
• Set#copyOf and ImmutableSet#copyOf throw an exception if any entry is null.
• The methods on Set#copyOf throw an exception if null is supplied to them (also, for instance, for set2.contains(null)).
• Collections#unmodifiableSet and ImmutableSet#copyOf keep the order of the original collection; Set#copyOf does not.