Java Sequenced Collections
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SequencedCollection<Type> collection = new ArrayList<>();
collection.getFirst();
collection.getLast();
collection.reversed();What are Sequenced Collections?
Sequenced Collections, added in Java 21, introduce a uniform way to access the first and last elements of an ordered collection -- methods like getFirst, getLast, addFirst, and addLast now work consistently across List, Deque, and LinkedHashSet.
Example: What are Sequenced Collections?
import java.util.*;
public class Main {
public static void main(String[] args) {
List<Integer> list = new ArrayList<>(List.of(1, 2, 3));
System.out.println(list.getFirst() + " " + list.getLast()); // uniform across List, Deque, LinkedHashSet
}
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SequencedCollection Interface
SequencedCollection is a new interface that List and Deque now extend, defining a consistent set of methods -- getFirst, getLast, addFirst, addLast, removeFirst, removeLast, and reversed -- for any collection with a well-defined encounter order.
Example: SequencedCollection Interface
// Import java.util.* so it can be used by its short name
import java.util.*;
// Define the class Main
public class Main {
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Create a new ArrayDeque object and store it in deque
Deque<String> deque = new ArrayDeque<>(List.of("a", "b", "c"));
deque.addFirst("start");
deque.addLast("end");
// Print a line to the console
System.out.println(deque);
}
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SequencedSet and SequencedMap
SequencedSet extends both Set and SequencedCollection, implemented by LinkedHashSet and TreeSet, while SequencedMap adds firstEntry, lastEntry, and reversed methods to ordered maps like LinkedHashMap and TreeMap.
Example: SequencedSet and SequencedMap
// Import java.util.* so it can be used by its short name
import java.util.*;
// Define the class Main
public class Main {
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Create a new LinkedHashMap object and store it in map
LinkedHashMap<String, Integer> map = new LinkedHashMap<>();
map.put("a", 1); map.put("b", 2);
// Print a line to the console
System.out.println(map.firstEntry() + " " + map.lastEntry());
}
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Reversed Views
The reversed() method returns a reverse-ordered view of the original collection rather than a separate copy, meaning changes made to the original collection are immediately visible when iterating the reversed view.
Example: Reversed Views
import java.util.*;
public class Main {
public static void main(String[] args) {
List<Integer> list = new ArrayList<>(List.of(1, 2, 3));
List<Integer> reversed = list.reversed(); // a view, not a copy
list.add(4);
System.out.println(reversed); // sees the update
}
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Retrofitted Existing Collections
Existing collection classes like ArrayList, ArrayDeque, LinkedHashSet, TreeSet, LinkedHashMap, and TreeMap were all retrofitted to implement the new sequenced interfaces, so this consistent first/last API became available without requiring any new collection types.
Example: Retrofitted Existing Collections
import java.util.*;
public class Main {
public static void main(String[] args) {
TreeSet<Integer> set = new TreeSet<>(List.of(3, 1, 2)); // retrofitted, no new collection type needed
System.out.println(set.getFirst() + " " + set.getLast());
}
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- Calling
getFirst()orgetLast()on an empty collection, which throws aNoSuchElementException. - Thinking
reversed()gives an independent copy, when it is a view, so changes in one show up in the other. - Expecting
HashSetorHashMapto havegetFirst(), when only ordered collections such asLinkedHashSetandTreeSetare sequenced.
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