Creating immutable Collections before JDK9:
In Java 9 added some factory methods to create immutable collections. See JEP 269: Convenience Factory Methods for Collections. But this guide will explain the techniques to create immutable collections before JDK 9 and flaws in using these techniques. Java 9 how solved these flaws will be explained in next guide.
Case 1:
Creating immutable collections using unmodifiableList(), unmodifiableSet(), unmodifiableMap().
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public class ImmutableCollectionBeforeJDKDemo { public static void main(String[] args) { //Creating Unmodifiable List List<String> list = new ArrayList<>(); list.add("Peter"); list.add("Gerhard"); list = Collections.unmodifiableList(list); System.out.println(list); //list.add("Philip"); /*if above line uncomment will get java.lang.UnsupportedOperationException*/ //Creating Unmodifiable Set Set<String> set = new HashSet<>(); set.add("Peter"); set.add("Gerhard"); set = Collections.unmodifiableSet(set); //set.add("Philip"); /*if above line uncomment will get java.lang.UnsupportedOperationException*/ //Creating Unmodifiable Map Map<Integer, String> map = new HashMap<>(); map.put(1, "Peter"); map.put(2, "Gerhard"); map = Collections.unmodifiableMap(map); System.out.println(map); map.put(3, "Philip"); /*if above line uncomment will get java.lang.UnsupportedOperationException*/ } } |
Flaw in Case 1:
In above example you see how to create immutable collections, you can still modify the collection, to see how let’s take set from example, you cannot modify the set using the set2 variable. However, you can still use the set variable to modify the set and the modifications will be reflected when you read the set using the set2 variable. See below example.
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public class ImutableModifiableListDemo { public static void main(String[] args) { //Creating Unmodifiable Set Set<String> set = new HashSet<>(); set.add("Peter"); set.add("Gerhard"); Set<String> set2 = Collections.unmodifiableSet(set); System.out.println(set); System.out.println(set2); set.add("Philip");// set2 also will effect System.out.println(set); System.out.println(set2); } } |
Output:
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1 2 3 4 |
[Gerhard, Peter] [Gerhard, Peter] [Gerhard, Peter, Philip] [Gerhard, Peter, Philip] |
Case 2:
Converting an array to List or Set using Arrays class.
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public class ArraysAsListDemo { public static void main(String[] args) { // Using Arrays.asList() creating List List<String> list = Collections.unmodifiableList( Arrays.asList("Peter", "Gerhard")); System.out.println(list); // Using Arrays.asList() creating Set Set<String> set = Collections.unmodifiableSet( new HashSet<>(Arrays.asList("Peter", "Gerhard"))); System.out.println(set); } } |
Output:
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1 2 |
[Peter, Gerhard] [Gerhard, Peter] |
Case 3:
Using double-brace.
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public class AnonymousClassDoubleBraceDemo { public static void main(String[] args) { // Using double-brace technique List<String> list = Collections.unmodifiableList( new ArrayList<>(){{add("Peter"); add("Gerhard");}}); System.out.println(list); Set<String> set = Collections.unmodifiableSet( new HashSet<>(){{add("Peter"); add("Gerhard");}}); System.out.println(set); } } |
Output:
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1 2 |
[Peter, Gerhard] [Gerhard, Peter] |
Case 4:
Using Java 8 Streams.
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public class Java8StreamsImmutableCollectionDemo { public static void main(String[] args) { // Using a Java 8 stream Creating immutable List List<String> list = Stream.of("Peter", "Gerhard").collect( Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)); System.out.println(list); // Using a Java 8 stream Creating immutable Set Set<String> set = Stream.of("Peter", "Gerhard").collect( Collectors.collectingAndThen(Collectors.toSet(), Collections::unmodifiableSet)); System.out.println(set); } } |
Output:
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1 2 |
[Peter, Gerhard] [Gerhard, Peter] |
Flaws in Case 2, 3, 4:
These techniques case2, case3 and case 4 are also inefficient. For example, just to hold two values in a collection, these techniques create multiple objects backing to hold the values and little verbose also.
