Java Immutable Collections

Question

From Java 1 6 Collection Framework documentation      Collections that do not support any modification operations  such as add  remove and clear  are referred to as unmodifiable        Collections that additionally guarantee that no change in the Collection object will ever be visible are referred to as immutable    The second criteria confuses me a bit  Given the first collection is unmodifiable  and assuming that the original collection reference has been disposed away  what are the changes that are referred to in the second line  Is it referring to the changes in the elements held in the collection ie the state of the elements    Second question  For a collection to be immutable  how does one go about providing the additional guarentees specified  If the state of an element in the collection is updated by a thread  is it sufficient for immutability that those updates in the state are not visible on the thread holding the immutable collection   For a collection to be immutable  how does one go about providing the additional guarantees specified

User · Accepted Answer

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.

User · Answer

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

User · Answer

Collection lt String gt  c1   new ArrayList lt String gt     c1 add  foo    Collection lt String gt  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

User · Answer

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

User · Answer

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

User · Answer

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 lt String gt  strings   new ArrayList lt String gt     List lt String gt  unmodifiable   Collections unmodifiableList strings   unmodifiable add  New string       will fail at runtime strings add  Aha        will succeed System out println unmodifiable

[java] Java Immutable Collections

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