In this article, we will see another JDK 9 feature — creating immutable collections. Until Java 8, if we wanted to create immutable collections, we used to call unmodifiableXXX() methods on java.util.Collections. For example,  To create an unmodifiable list, we should write:

jshell> List<String> list = new ArrayList<String>();
list ==> []

jshell> list.add("Smart")
$2 ==> true

jshell> list.add("Techie")
$3 ==> true

jshell> System.out.println("The values are: "+ list);
The list values are: [Smart, Techie]

jshell> // make the list unmodifiable

jshell>    List<String> immutablelist = Collections.unmodifiableList(list);
immutablelist ==> [Smart, Techie]

jshell> // try to modify the list

jshell>    immutablelist.add("Smart_1")
|  java.lang.UnsupportedOperationException thrown:
|        at Collections$UnmodifiableCollection.add (Collections.java:1056)
|        at (#6:1)

jshell>

The above code is too verbose to create a simple unmodifiable List. As Java is adopting functional programming style, Java 9 came up with convenient, more compacted factory methods to create unmodifiable collections, as seen in JEP 269. Let us see how that works.

Create Empty List

jshell> List immutableList = List.of();
immutableList ==> []

//Add an item to the list
jshell> immutableList.add("Smart")
|  Warning:
|  unchecked call to add(E) as a member of the raw type java.util.List
|  immutableList.add("Smart")
|  ^------------------------^
|  java.lang.UnsupportedOperationException thrown:
|        at ImmutableCollections.uoe (ImmutableCollections.java:70)
|        at ImmutableCollections$AbstractImmutableList.add (ImmutableCollections.java:76)
|        at (#2:1)

Create Non-Empty List

jshell> List immutableList = List.of("Smart","Techie");
immutableList ==> [Smart, Techie]

jshell> //add an item to the list

jshell> immutableList.add("Smart_1")
|  Warning:
|  unchecked call to add(E) as a member of the raw type java.util.List
|  immutableList.add("Smart_1")
|  ^--------------------------^
|  java.lang.UnsupportedOperationException thrown:
|        at ImmutableCollections.uoe (ImmutableCollections.java:70)
|        at ImmutableCollections$AbstractImmutableList.add (ImmutableCollections.java:76)
|        at (#2:1)

jshell>

Create Non-Empty Map

jshell> Map immutableMap = Map.of(1,"Smart",2,"Techie");
immutableMap ==> {1=Smart, 2=Techie}

//Get item from Map
jshell> immutableMap.get(1);
$2 ==> "Smart"

//Add item to map
jshell> immutableMap.put(3,"Smart_1");
|  Warning:
|  unchecked call to put(K,V) as a member of the raw type java.util.Map
|  immutableMap.put(3,"Smart_1
|  ^---------------------------^
|  java.lang.UnsupportedOperationException thrown:
|        at ImmutableCollections.uoe (ImmutableCollections.java:70)
|        at ImmutableCollections$AbstractImmutableMap.put (ImmutableCollections.java:557)
|        at (#3:1)

jshell>

If you look at the above Java 9 factory method, the code is a simple one liner to create immutable collections. In an upcoming article, we will see another Java 9 feature. Stay tuned!