diff --git a/core-java-modules/core-java-21/README.md b/core-java-modules/core-java-21/README.md
new file mode 100644
index 0000000000..c63f3b360b
--- /dev/null
+++ b/core-java-modules/core-java-21/README.md
@@ -0,0 +1 @@
+## Relevant Articles
\ No newline at end of file
diff --git a/core-java-modules/core-java-21/pom.xml b/core-java-modules/core-java-21/pom.xml
new file mode 100644
index 0000000000..83b2b1c858
--- /dev/null
+++ b/core-java-modules/core-java-21/pom.xml
@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-21</artifactId>
+    <name>core-java-21</name>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+<!--    <build>-->
+<!--        <plugins>-->
+<!--            <plugin>-->
+<!--                <groupId>org.apache.maven.plugins</groupId>-->
+<!--                <artifactId>maven-compiler-plugin</artifactId>-->
+<!--                <configuration>-->
+<!--                    <source>{maven.compiler.source.version}</source>-->
+<!--                    <target>{maven.compiler.target.version}</target>-->
+<!--                </configuration>-->
+<!--            </plugin>-->
+<!--        </plugins>-->
+<!--    </build>-->
+
+<!--    <properties>-->
+<!--        <maven.compiler.source.version>21</maven.compiler.source.version>-->
+<!--        <maven.compiler.target.version>21</maven.compiler.target.version>-->
+<!--        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>-->
+<!--    </properties>-->
+
+</project>
\ No newline at end of file
diff --git a/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedCollection.java b/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedCollection.java
new file mode 100644
index 0000000000..58f5dbd014
--- /dev/null
+++ b/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedCollection.java
@@ -0,0 +1,15 @@
+package com.baeldung.sequenced.collections;
+
+/*
+interface SequencedCollection<E> extends Collection<E> {
+    // new method
+    SequencedCollection<E> reversed();
+    // methods promoted from Deque
+    void addFirst(E);
+    void addLast(E);
+    E getFirst();
+    E getLast();
+    E removeFirst();
+    E removeLast();
+}
+ */
diff --git a/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedMap.java b/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedMap.java
new file mode 100644
index 0000000000..925449a090
--- /dev/null
+++ b/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedMap.java
@@ -0,0 +1,18 @@
+package com.baeldung.sequenced.collections;
+
+/*
+interface SequencedMap<K,V> extends Map<K,V> {
+    // new methods
+    SequencedMap<K,V> reversed();
+    SequencedSet<K> sequencedKeySet();
+    SequencedCollection<V> sequencedValues();
+    SequencedSet<Entry<K,V>> sequencedEntrySet();
+    V putFirst(K, V);
+    V putLast(K, V);
+    // methods promoted from NavigableMap
+    Entry<K, V> firstEntry();
+    Entry<K, V> lastEntry();
+    Entry<K, V> pollFirstEntry();
+    Entry<K, V> pollLastEntry();
+}
+ */
\ No newline at end of file
diff --git a/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedSet.java b/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedSet.java
new file mode 100644
index 0000000000..a8fb4caa1f
--- /dev/null
+++ b/core-java-modules/core-java-21/src/main/java/com/baeldung/sequenced/collections/SequencedSet.java
@@ -0,0 +1,7 @@
+package com.baeldung.sequenced.collections;
+
+/*
+interface SequencedSet<E> extends Set<E>, SequencedCollection<E> {
+    SequencedSet<E> reversed();    // covariant override
+}
+ */