Collection factory methods

Definition

List.of, Set.of and Map.of (Java 9) create small immutable collections in one line. They do not allow null, and Set.of and Map.of reject duplicates.

Example code

Immutable collections in one line

import java.util.List;
import java.util.Map;
import java.util.Set;

class Demo {
    public static void main(String[] args) {
        List<String> languages = List.of("Java", "Kotlin", "Scala");
        Set<Integer> primes = Set.of(2, 3, 5);
        Map<String, Integer> ages = Map.of("Asha", 30, "Ravi", 28);
        Map<String, Integer> many = Map.ofEntries(Map.entry("a", 1), Map.entry("b", 2));

        System.out.println(languages);
        System.out.println(primes.contains(3));
        System.out.println(ages.get("Asha") + " " + many.get("b"));

        try {
            languages.add("Go");
        } catch (UnsupportedOperationException e) {
            System.out.println("immutable");
        }
        try {
            List.of("a", null);
        } catch (NullPointerException e) {
            System.out.println("null is not allowed");
        }
    }
}

// Output:
// [Java, Kotlin, Scala]
// true
// 30 2
// immutable
// null is not allowed