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Java 9 Collection factory methods

Java 9 New Features

Java 9 New static factory methods can create immutable instances of these collections in List, Set, and Map interfaces.

These factory methods can create collections in a more concise way.

Old method to create a collection

Example

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);
　　　}
}

The execution output is as follows:

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

New method to create a collection

Java 9 In which, the following methods are added to the List, Set and Map interfaces and their overloaded objects.

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)

The of(...) method of the List and Set interfaces overloads 0 ~ 10 parameters.
The of(...) method of the Map interface overloads 0 ~ 10 parameters.
Different methods of the Map interface if more than 10 parameter, you can use the ofEntries(...) method.