Sequenced collections

Definition

Sequenced collections (Java 21) add a common way to work with the first and last element and to reverse any collection that has a defined order. New methods include getFirst, getLast, addFirst, addLast and reversed.

Example code

First, last and reversed on a list and a LinkedHashMap

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;

class Demo {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(List.of("b", "c"));
        list.addFirst("a");                                    // Java 21
        list.addLast("d");
        System.out.println(list.getFirst() + " " + list.getLast());
        System.out.println(list.reversed());

        LinkedHashMap<String, Integer> map = new LinkedHashMap<>();
        map.put("one", 1);
        map.put("two", 2);
        map.put("three", 3);
        System.out.println(map.firstEntry() + " " + map.lastEntry());
    }
}

// Output:
// a d
// [d, c, b, a]
// one=1 three=3