Java Immutable Collections

Question

Unmodifiable collections are usually read-only views (wrappers) of other collections. You can't add, remove or clear them, but the underlying collection can change.

Immutable collections can't be changed at all - they don't wrap another collection - they have their own elements.

Here's a quote from guava's ImmutableList

Unlike Collections.unmodifiableList(java.util.List<? extends T>), which is a view of a separate collection that can still change, an instance of ImmutableList contains its own private data and will never change.

So, basically, in order to get an immutable collection out of a mutable one, you have to copy its elements to the new collection, and disallow all operations.

The difference is that you can't have a reference to an immutable collection which allows changes. Unmodifiable collections are unmodifiable through that reference, but some other object may point to the same data through which it can be changed.

e.g.

List<String> strings = new ArrayList<String>();
List<String> unmodifiable = Collections.unmodifiableList(strings);
unmodifiable.add("New string"); // will fail at runtime
strings.add("Aha!"); // will succeed
System.out.println(unmodifiable);

Collection<String> c1 = new ArrayList<String>();
c1.add("foo");
Collection<String> c2 = Collections.unmodifiableList(c1);

c1 is mutable (i.e. neither unmodifiable nor immutable).
c2 is unmodifiable: it can't be changed itself, but if later on I change c1 then that change will be visible in c2.

This is because c2 is simply a wrapper around c1 and not really an independent copy. Guava provides the ImmutableList interface and some implementations. Those work by actually creating a copy of the input (unless the input is an immutable collection on its own).

Regarding your second question:

The mutability/immutability of a collection does not depend on the mutability/immutability of the objects contained therein. Modifying an object contained in a collection does not count as a "modification of the collection" for this description. Of course if you need a immutable collection, you usually also want it to contain immutable objects.

Now java 9 has factory Methods for Immutable List, Set, Map and Map.Entry .

In Java SE 8 and earlier versions, We can use Collections class utility methods like unmodifiableXXX to create Immutable Collection objects.

However these Collections.unmodifiableXXX methods are very tedious and verbose approach. To overcome those shortcomings, Oracle corp has added couple of utility methods to List, Set and Map interfaces.

Now in java 9 :- List and Set interfaces have “of()” methods to create an empty or no-empty Immutable List or Set objects as shown below:

Empty List Example

List immutableList = List.of();

Non-Empty List Example

List immutableList = List.of("one","two","three");

I believe the point here is that even if a collection is Unmodifiable, that does not ensure that it cannot change. Take for example a collection that evicts elements if they are too old. Unmodifiable just means that the object holding the reference cannot change it, not that it cannot change. A true example of this is Collections.unmodifiableList method. It returns an unmodifiable view of a List. The the List reference that was passed into this method is still modifiable and so the list can be modified by any holder of the reference that was passed. This can result in ConcurrentModificationExceptions and other bad things.

Immutable, mean that in no way can the collection be changed.

Second question: An Immutable collection does not mean that the objects contained in the collection will not change, just that collection will not change in the number and composition of objects that it holds. In other words, the collection's list of references will not change. That does not mean that the internals of the object being referenced cannot change.

Pure4J supports what you are after, in two ways.

First, it provides an @ImmutableValue annotation, so that you can annotate a class to say that it is immutable. There is a maven plugin to allow you to check that your code actually is immutable (use of final etc.).

Second, it provides the persistent collections from Clojure, (with added generics) and ensures that elements added to the collections are immutable. Performance of these is apparently pretty good. Collections are all immutable, but implement java collections interfaces (and generics) for inspection. Mutation returns new collections.

Disclaimer: I'm the developer of this