Java 9 provides new static factory methods for creating instances of collections and maps conveniently with a small number of elements. Here, we are going to discuss how to create List, Set, Map with Java 9 Factory Method for Collections.
1. List
Create an Immutable List in Java 8
In Java 8, we can create the Immutable list using the Collections utility class method. Check the below example of the same:
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class ImmutableListJ8 {
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("Java 8");
list.add("java 9");
System.out.println("Mutable list "+list);
List<String> imList = Collections.unmodifiableList(list);
System.out.println("Immutable list "+imList);
/** There will be java.lang.UnsupportedOperationException
* while modifying the Immutable list. */
imList.add("java");
System.out.println(imList);
}
}
Create an Immutable List in Java 9
In Java 9, we can create an immutable list using interface static methods as described in the below example:
import java.util.List;
public class ImmutableListJ9 {
public static void main(String[] args) {
List<String> immList = List.of("Java 8","Java 9");
System.out.println(immList);
immList.add("java");
}
}
There are few static methods to create an immutable List
// for empty list
static <E> List<E> of()
// for list containing one element
static <E> List<E> of(E e1)
// for list containing two element
static <E> List<E> of(E e1, E e2)
// ...
// for list containing an arbitrary number of elements
static <E> List<E> of(E... elements)
It returns an immutable list containing an arbitrary number of elements. See Immutable List Static Factory Methods for details.
Type Parameters: <E> the List's element type
Parameters: elements the elements to be contained in the list
Returns: a List containing the specified elements
Throws: NullPointerException - if an element is null or if the array is null
Since: 9
Important points
1. There will be NullPointerException while creating the immutable list with null.
import java.util.List;
public class ImmutableListJ9 {
public static void main(String[] args) {
List<String> immList = List.of(null,"Java 9");
System.out.println(immList);
}
}
Output:
Exception in thread "main" java.lang.NullPointerException
at java.base/java.util.Objects.requireNonNull(Objects.java:221)
at java.base/java.util.ImmutableCollections$List2.<init>(ImmutableCollections.java:185)
at java.base/java.util.List.of(List.java:822)
at ImmutableListJ9.main(ImmutableListJ9.java:6)
2. As the property of Immutable list, the program will throw an UnsupportedOperationException while adding the element to the Immutable list.
import java.util.List;
public class ImmutableListJ9 {
public static void main(String[] args) {
List<String> immList = List.of("Java 9");
System.out.println(immList);
immList.add("Java 8");
}
}
Output:
[Java 9]
Exception in thread "main" java.lang.UnsupportedOperationException
at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:71)
at java.base/java.util.ImmutableCollections$AbstractImmutableList.add(ImmutableCollections.java:77)
at ImmutableListJ9.main(ImmutableListJ9.java:8)
Solution for above problem
import java.util.ArrayList;
import java.util.List;
public class ImmutableListJ9 {
public static void main(String[] args) {
List<String> immList = new ArrayList<>(List.of("Java 9"));
System.out.println(immList);
immList.add("Java 8");
System.out.println(immList);
}
}
Output:
[Java 9]
[Java 9, Java 8]
2. Set
Create an Immutable Set in Java 8
In Java 8, we can create the Immutable Set using the Collections utility class method. Check the below example of the same:
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
public class ImmutableSetJ8 {
public static void main(String[] args) {
Set<String> set = new HashSet<>();
set.add("Java 8");
set.add("java 9");
System.out.println("Mutable set "+set);
Set<String> imSet = Collections.unmodifiableSet(set);
System.out.println("Immutable set "+imSet);
/** There will be java.lang.UnsupportedOperationException
* while modifying the Immutable set. */
imSet.add("java");
System.out.println(imSet);
}
}
Create an Immutable Set in Java 9
In Java 9, we can create an immutable set using interface static methods as described in the below example:
import java.util.Set;
public class ImmutableListJ9 {
public static void main(String[] args) {
Set<String> imSet = Set.of("Java 8","Java 9");
System.out.println(imSet);
imSet.add("java");
}
}
There are few static methods to create an immutable Set
// for empty set
static <E> Set<E> of()
// for set containing one element
static <E> Set<E> of(E e1)
// for set containing two element
static <E> Set<E> of(E e1, E e2)
// ...
// for set containing an arbitrary number of elements
static <E> Set<E> of(E... elements)
It returns an immutable set containing an arbitrary number of elements. See Immutable Set Static Factory Methods for details.
Type Parameters: <E> the Set's element type
Parameters: elements the elements to be contained in the set
Returns: a Set containing the specified elements
Throws: NullPointerException - if an element is null or if the array is null
Since: 9
Important points
1. There will be NullPointerException while creating the immutable set with null.
import java.util.Set;
public class ImmutableSetJ9 {
public static void main(String[] args) {
Set<String> immSet = Set.of(null, "Java 9");
System.out.println(immSet);
}
}
Output:
Exception in thread "main" java.lang.NullPointerException
at java.base/java.util.ImmutableCollections$Set2.<init>(ImmutableCollections.java:379)
at java.base/java.util.Set.of(Set.java:483)
at ImmutableSetJ9.main(ImmutableSetJ9.java:6)
2. As the property of an Immutable set, the program will throw an UnsupportedOperationException while adding the element to Immutable set.
import java.util.Set;
public class ImmutableSetJ9 {
public static void main(String[] args) {
Set<String> set = Set.of("Java 9");
System.out.println(set);
set.add("Java 8");
System.out.println(set);
}
}
Output:
[Java 9]
Exception in thread "main" java.lang.UnsupportedOperationException
at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:71)
at java.base/java.util.ImmutableCollections$AbstractImmutableSet.add(ImmutableCollections.java:281)
at ImmutableSetJ9.main(ImmutableSetJ9.java:9)
Solution for above problem
import java.util.HashSet;
import java.util.Set;
public class ImmutableSetJ9 {
public static void main(String[] args) {
Set<String> set = new HashSet<>(Set.of("Java 9"));
System.out.println(set);
set.add("Java 8");
System.out.println(set);
}
}
Output:
[Java 9]
[Java 8, Java 9]
3. Map
1. Map.of()
To create a Map, we use those static methods:
import java.util.Map;
public class ImmutableMapJ9 {
public static void main(String[] args) {
Map<String, Integer> map = Map.of("Java 8", 8, "Java 9", 9);
System.out.println(map);
}
}
Output: {Java 9=9, Java 8=8}
Factory methods to create the Map
// for empty Map
static <K, V> Map<K, V> of()
//for Map containing a single mapping
static <K, V> Map<K, V> of(K k1, V v1)
//for Map containing two mappings
static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2)
//...
//for Map containing up to ten mappings
static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10)
2. Map.ofEntries()
import java.util.Map;
public class ImmutableMapJ9 {
public static void main(String[] args) {
Map<Integer, String> newImmutableMap = Map.ofEntries(Map.entry(1, "one"), Map.entry(2, "two"),
Map.entry(3, "three"));
System.out.println(newImmutableMap);
}
}
Output: {2=two, 3=three, 1=one}
Important points
1. There will be NullPointerException while creating the immutable map with null values.
import java.util.Map;
public class ImmutableMapJ9 {
public static void main(String[] args) {
Map<String, String> map = Map.of("Java 8", "8" , "Java 9", null);
System.out.println(map);
}
}
Output:
Exception in thread "main" java.lang.NullPointerException
at java.base/java.util.Objects.requireNonNull(Objects.java:221)
at java.base/java.util.ImmutableCollections$MapN.<init>(ImmutableCollections.java:678)
at java.base/java.util.Map.of(Map.java:1326)
at ImmutableMapJ9.main(ImmutableMapJ9.java:5)
2. As the property of an Immutable set, the program will throw an UnsupportedOperationException while adding the element to the Immutable map.
import java.util.Map;
public class ImmutableMapJ9 {
public static void main(String[] args) {
Map<String, String> map = Map.of("Java 8", "8" , "Java 9", "9");
System.out.println(map);
map.put("Java", "2");
}
}
Output:
{Java 9=9, Java 8=8}
Exception in thread "main" java.lang.UnsupportedOperationException
at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:71)
at java.base/java.util.ImmutableCollections$AbstractImmutableMap.put(ImmutableCollections.java:558)
at ImmutableMapJ9.main(ImmutableMapJ9.java:8)
Solution for above problem
import java.util.HashMap;
import java.util.Map;
public class ImmutableMapJ9 {
public static void main(String[] args) {
Map<String, String> map = new HashMap<>(Map.of("Java 8", "8" , "Java 9", "9"));
System.out.println("Map: "+map);
map.put("Java", "2");
System.out.println("Modified Map: "+map);
}
}
Output:
Map: {Java 8=8, Java 9=9}
Modified Map: {Java=2, Java 8=8, Java 9=9}
