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Collection Factory Method


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A new factory method is added to the list with java 9. The new methods like Set and Map interfaces create immutable instances. To create a collection in a less verbose and concise way, then these are the best convenience methods.

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);
   }
}

Result

Your output must look like this:

[A, B, C]
[A, B, C]
{A=Apple, B=Boy, C=Cat}

New Methods

Set and Map interfaces along with their overloaded counterparts are the added factory methods that are added to the list.

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)

Note the points

  • For List and Set interfaces, of(...) way is overloaded to have 0 to 10 parameters and one with var args parameters.
  • For the Map interface, of(...) way is overloaded to have 0 to 10 parameters.
  • In case of more than 10 parameters for Map interface, ofEntries(...) method can be used accepting var args parameters.

A 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);
   }
}

Result

You will get the below-given output.

[A, B, C]
[A, B, C]
{A=Apple, B=Boy, C=Cat}
{A=Apple, B=Boy, C=Cat}