With Java 9, new factory methods are added to List, Set and Map interfaces to create immutable instances. These factory methods are convenience factory methods to create a collection in less verbose and in concise way.
Old way to create collections
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
public class Tester {
public static void main(String []args) {
Set<String> set = new HashSet<>();
set.add("A");
set.add("B");
set.add("C");
set = Collections.unmodifiableSet(set);
System.out.println(set);
List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
list.add("C");
list = Collections.unmodifiableList(list);
System.out.println(list);
Map<String, String> map = new HashMap<>();
map.put("A","Apple");
map.put("B","Boy");
map.put("C","Cat");
map = Collections.unmodifiableMap(map);
System.out.println(map);
}
}
Output
It will print the following output.
[A, B, C]
[A, B, C]
{A=Apple, B=Boy, C=Cat}
New Methods
With java 9, following methods are added to List, Set and Map interfaces along with their overloaded counterparts.
static <E> List<E> of(E e1, E e2, E e3);
static <E> Set<E> of(E e1, E e2, E e3);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3);
static <K,V> Map<K,V> ofEntries(Map.Entry<? extends K,? extends V>... entries)
Points to Note
- For List and Set interfaces, of(…) method is overloaded to have 0 to 10 parameters and one with var args parameter.
- For Map interface, of(…) method is overloaded to have 0 to 10 parameters.
- In case of more than 10 paramters for Map interface, ofEntries(…) method can be used accepting var args parameter.
New way to create collections
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.AbstractMap;
import java.util.Map;
import java.util.Set;
public class Tester {
public static void main(String []args) {
Set<String> set = Set.of("A", "B", "C");
System.out.println(set);
List<String> list = List.of("A", "B", "C");
System.out.println(list);
Map<String, String> map = Map.of("A","Apple","B","Boy","C","Cat");
System.out.println(map);
Map<String, String> map1 = Map.ofEntries (
new AbstractMap.SimpleEntry<>("A","Apple"),
new AbstractMap.SimpleEntry<>("B","Boy"),
new AbstractMap.SimpleEntry<>("C","Cat"));
System.out.println(map1);
}
}
Output
It will print the following output.
[A, B, C]
[A, B, C]
{A=Apple, B=Boy, C=Cat}
{A=Apple, B=Boy, C=Cat}