Is java.util.List mutable?

Yes. It's mutable. If you want to have an immutable one, you can wrap it using java utility Collections class: java.util.Collections#unmodifiableList

like this:

  List<Object> mutableList = new ArrayList<>();
  List<Object> immutableList = Collections.unmodifiableList(mutableList);

List mutability

Since List is an interface, the only promise it makes is: "these are the methods you will get".

The List interface describes a mutable List. Notice that it has functions such as add(), set() and remove(). But notice also that these mutators are designated as "optional" operations.

There exist implementations of the List interface which are, in practice, immutable.

List<Integer> mutable = new ArrayList<>();
mutable.add(1);

List<Integer> immutable = Collections.unmodifiableList(mutable);
// try to modify the list
immutable.add(2);
// Exception in thread "main" java.lang.UnsupportedOperationException

Collections.unmodifiableList() returns an instance of the List implementation Collections.UnmodifiableList.

Collections.UnmodifiableList forwards all calls of immutable List functions, correctly to the underlying List. Whereas it implements all mutable List functions by throwing java.lang.UnsupportedOperationException.

List element mutability

If we do:

List<Date> mutable = new ArrayList<>();
mutable.add(new Date());
List<Date> immutable = Collections.unmodifiableList(mutable);
Date myCoolDate = immutable.get(0);

Can we mutate myCoolDate? Absolutely.

myCoolDate.setTime(99999); // works! mutates the original Date object in the `List`.

The List — whether immutable or otherwise — stores copies of references to objects. Once we have a reference to the object (Date myCoolDate = immutable.get(0);), we can mutate the object freely.

The items in an immutable list have no immutability guarantee. They are exactly as mutable as they have always been.

java.util.List is an interface. Implementation could be mutable or immutable depending on which you use. If you're wondering about java.util.ArrayList - it is mutable and it is not creating another List instance on add() or remove().

If you are looking for immutable list - check Guava implementation of ImmutableList or Collections.unmodifiableList which throws java.lang.UnsupportedOperationException on modifications.