diff --git a/src/main/java/org/warp/commonutils/type/HashAssociation.java b/src/main/java/org/warp/commonutils/type/HashAssociation.java
index 2cbc68d..1fd6073 100644
--- a/src/main/java/org/warp/commonutils/type/HashAssociation.java
+++ b/src/main/java/org/warp/commonutils/type/HashAssociation.java
@@ -136,7 +136,9 @@ public class HashAssociation<T, U> implements Association<T, U> {
 	@Override
 	public Set<U> getLinks(T src) {
 		Objects.requireNonNull(src);
-		return associations.get(src).clone();
+		var dests = associations.get(src);
+		if (dests == null) return Set.of();
+		return dests.clone();
 	}
 
 	@Override
diff --git a/src/main/java/org/warp/commonutils/type/HashMultiAssociation.java b/src/main/java/org/warp/commonutils/type/HashMultiAssociation.java
new file mode 100644
index 0000000..f8adc20
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/HashMultiAssociation.java
@@ -0,0 +1,185 @@
+package org.warp.commonutils.type;
+
+import it.unimi.dsi.fastutil.objects.Object2ObjectOpenHashMap;
+import it.unimi.dsi.fastutil.objects.ObjectOpenHashSet;
+import it.unimi.dsi.fastutil.objects.ObjectSet;
+import java.util.Objects;
+import java.util.Set;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public class HashMultiAssociation<T, U> implements MultiAssociation<T, U> {
+
+	private final Object2ObjectOpenHashMap<T, ObjectOpenHashSet<U>> associations;
+	private final Object2ObjectOpenHashMap<U, ObjectOpenHashSet<T>> inverseAssociations;
+
+	public HashMultiAssociation() {
+		this.associations = new Object2ObjectOpenHashMap<>();
+		this.inverseAssociations = new Object2ObjectOpenHashMap<>();
+	}
+
+	private HashMultiAssociation(Object2ObjectOpenHashMap<T, ObjectOpenHashSet<U>> associations,
+			Object2ObjectOpenHashMap<U, ObjectOpenHashSet<T>> inverseAssociations) {
+		this.associations = associations;
+		this.inverseAssociations = inverseAssociations;
+	}
+
+	/**
+	 * Remove the source from this destination
+	 */
+	@Nullable
+	private boolean removeSourceFromDest(@NotNull U dest, @NotNull T src) {
+		var sources = inverseAssociations.get(dest);
+		if (sources != null) {
+			sources.remove(src);
+
+			// Keep the map clean
+			if (sources.isEmpty()) {
+				inverseAssociations.remove(dest);
+			}
+
+			return true;
+		}
+		return false;
+	}
+
+	/**
+	 * Remove the destination from this source
+	 */
+	@Nullable
+	private boolean removeDestFromSource(@NotNull T src, @NotNull U dest) {
+		var dests = associations.get(src);
+		if (dests != null) {
+			dests.remove(dest);
+
+			// Keep the map clean
+			if (dests.isEmpty()) {
+				associations.remove(src);
+			}
+
+			return true;
+		}
+		return false;
+	}
+
+	@Override
+	public boolean link(T src, U dest) {
+		return associations.computeIfAbsent(src, s -> new ObjectOpenHashSet<>()).add(dest);
+	}
+
+	@Override
+	public boolean unlink(T src, U dest) {
+		var removed = removeDestFromSource(src, dest);
+
+		if (removed) {
+			removeSourceFromDest(dest, src);
+			return true;
+		}
+
+		return false;
+	}
+
+	@Override
+	public Set<U> unlink(T src) {
+		var dests = associations.remove(src);
+
+		if (dests == null) {
+			return Set.of();
+		}
+
+		for (U dest : dests) {
+			removeSourceFromDest(dest, src);
+		}
+		return dests;
+	}
+
+	@Override
+	public Set<T> unlinkFromSource(U dest) {
+		var sources = inverseAssociations.remove(dest);
+
+		if (sources == null) {
+			return Set.of();
+		}
+
+		for (T source : sources) {
+			removeDestFromSource(source, dest);
+		}
+		return sources;
+	}
+
+	@Override
+	public boolean hasAnyLink(T src) {
+		var dests = associations.get(src);
+		if (dests == null) {
+			return false;
+		}
+		return !dests.isEmpty();
+	}
+
+	@Override
+	public boolean hasAnyLinkSource(U dest) {
+		var sources = inverseAssociations.get(dest);
+		if (sources == null) {
+			return false;
+		}
+		return !sources.isEmpty();
+	}
+
+	@Override
+	public boolean hasLink(T src, U dest) {
+		var dests = associations.get(src);
+		if (dests == null) {
+			return false;
+		}
+		return dests.contains(dest);
+	}
+
+	@Override
+	public Set<U> getLinks(T src) {
+		Objects.requireNonNull(src);
+		var dests = associations.get(src);
+		if (dests == null) return Set.of();
+		return dests.clone();
+	}
+
+	@Override
+	public Set<T> getLinkSources(U dest) {
+		Objects.requireNonNull(dest);
+		var sources = inverseAssociations.get(dest);
+		if (sources == null) return Set.of();
+		return sources.clone();
+	}
+
+	@Override
+	public void clear() {
+		associations.clear();
+		inverseAssociations.clear();
+	}
+
+	@Override
+	public int size() {
+		return Math.max(associations.size(), inverseAssociations.size());
+	}
+
+	@Override
+	public ObjectSet<T> getSources() {
+		return associations.clone().keySet();
+	}
+
+	@Override
+	public ObjectSet<U> getDestinations() {
+		return inverseAssociations.clone().keySet();
+	}
+
+	@SuppressWarnings("MethodDoesntCallSuperMethod")
+	@Override
+	public HashMultiAssociation<T, U> clone() {
+		var associationsClone = associations.clone();
+		associationsClone.replaceAll((item, destinations) -> destinations.clone());
+
+		var inverseAssociationsClone = inverseAssociations.clone();
+		inverseAssociationsClone.replaceAll((item, sources) -> sources.clone());
+
+		return new HashMultiAssociation<>(associationsClone, inverseAssociationsClone);
+	}
+}
diff --git a/src/main/java/org/warp/commonutils/type/MultiAssociation.java b/src/main/java/org/warp/commonutils/type/MultiAssociation.java
new file mode 100644
index 0000000..d926a05
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/MultiAssociation.java
@@ -0,0 +1,123 @@
+package org.warp.commonutils.type;
+
+import it.unimi.dsi.fastutil.objects.ObjectSet;
+import java.util.Set;
+
+/**
+ * One to many relationship
+ *
+ * o ------- o
+ *      +--- o
+ * o ---+--- o
+ *      +-|- o
+ * o -----|- o
+ * o ------- o
+ * o ---+--- o
+ *      +--- o
+ *
+ * @param <T> Source type
+ * @param <U> Destination type
+ */
+public interface MultiAssociation<T, U> {
+
+	/**
+	 * Link source to dest
+	 * @param src Source
+	 * @param dest Destination
+	 * @return true if linked, false if it was already linked with that destination
+	 */
+	boolean link(T src, U dest);
+
+	/**
+	 * Unlink only if src is linked with dest
+	 * @param src Source
+	 * @param dest Destination
+	 * @return true if unlinked, false if not present
+	 */
+	boolean unlink(T src, U dest);
+
+	/**
+	 * Unlink
+	 * @param src Source
+	 * @return previous linked destinations
+	 */
+	Set<U> unlink(T src);
+
+	/**
+	 * Unlink
+	 * @param dest Destination
+	 * @return previous linked source
+	 */
+	Set<T> unlinkFromSource(U dest);
+
+	/**
+	 * Check if link exists
+	 * @param src Source
+	 * @return true if source is linked with at least 1 destination
+	 */
+	default boolean hasAnyLink(T src) {
+		return !getLinks(src).isEmpty();
+	}
+
+	/**
+	 * Check if link exists
+	 * @param dest Destination
+	 * @return true if destination is linked with at least 1 destination
+	 */
+	default boolean hasAnyLinkSource(U dest) {
+		return !getLinkSources(dest).isEmpty();
+	}
+
+	/**
+	 * Check if link exists
+	 * @param src Source
+	 * @param dest Destination
+	 * @return true if source and destination are linked together
+	 */
+	boolean hasLink(T src, U dest);
+
+	/**
+	 * Get a link destination
+	 * @param src Source
+	 * @return Existing linked destinations
+	 */
+	Set<U> getLinks(T src);
+
+	/**
+	 * Get a link source
+	 * @param dest Source
+	 * @return Existing linked sources
+	 */
+	Set<T> getLinkSources(U dest);
+
+	/**
+	 * Delete all links
+	 */
+	void clear();
+
+	/**
+	 * Get the count of existing links
+	 * @return size
+	 */
+	int size();
+
+	/**
+	 * Get all the sources
+	 * @return Set of sources
+	 */
+	ObjectSet<T> getSources();
+
+	/**
+	 * Get all the destinations
+	 * @return Set of destinations
+	 */
+	ObjectSet<U> getDestinations();
+
+	static <T, U> MultiAssociation<T, U> synchronize(MultiAssociation<T, U> association) {
+		return new SynchronizedMultiAssociation<>(association);
+	}
+
+	static <T, U> MultiAssociation<T, U> synchronize(MultiAssociation<T, U> association, Object lock) {
+		return new SynchronizedMultiAssociation<>(association, lock);
+	}
+}
diff --git a/src/main/java/org/warp/commonutils/type/SynchronizedMultiAssociation.java b/src/main/java/org/warp/commonutils/type/SynchronizedMultiAssociation.java
new file mode 100644
index 0000000..a876f92
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/SynchronizedMultiAssociation.java
@@ -0,0 +1,111 @@
+package org.warp.commonutils.type;
+
+import it.unimi.dsi.fastutil.objects.ObjectSet;
+import java.util.Set;
+
+public class SynchronizedMultiAssociation<T, U> implements MultiAssociation<T, U> {
+
+	private final MultiAssociation<T, U> association;
+	private final Object lock;
+
+	SynchronizedMultiAssociation(MultiAssociation<T, U> association) {
+		this.association = association;
+		this.lock = new Object();
+	}
+
+	SynchronizedMultiAssociation(MultiAssociation<T, U> association, Object lock) {
+		this.association = association;
+		this.lock = lock;
+	}
+
+	@Override
+	public boolean link(T src, U dest) {
+		synchronized (lock) {
+			return association.link(src, dest);
+		}
+	}
+
+	@Override
+	public boolean unlink(T src, U dest) {
+		synchronized (lock) {
+			return association.unlink(src, dest);
+		}
+	}
+
+	@Override
+	public Set<U> unlink(T src) {
+		synchronized (lock) {
+			return association.unlink(src);
+		}
+	}
+
+	@Override
+	public Set<T> unlinkFromSource(U dest) {
+		synchronized (lock) {
+			return association.unlinkFromSource(dest);
+		}
+	}
+
+	@Override
+	public boolean hasAnyLink(T src) {
+		synchronized (lock) {
+			return association.hasAnyLink(src);
+		}
+	}
+
+	@Override
+	public boolean hasAnyLinkSource(U dest) {
+		synchronized (lock) {
+			return association.hasAnyLinkSource(dest);
+		}
+	}
+
+	@Override
+	public boolean hasLink(T src, U dest) {
+		synchronized (lock) {
+			return association.hasLink(src, dest);
+		}
+	}
+
+	@Override
+	public Set<U> getLinks(T src) {
+		synchronized (lock) {
+			return association.getLinks(src);
+		}
+	}
+
+	@Override
+	public Set<T> getLinkSources(U dest) {
+		synchronized (lock) {
+			return association.getLinkSources(dest);
+		}
+	}
+
+	@Override
+	public void clear() {
+		synchronized (lock) {
+			association.clear();
+		}
+	}
+
+	@Override
+	public int size() {
+		synchronized (lock) {
+			return association.size();
+		}
+	}
+
+	@Override
+	public ObjectSet<T> getSources() {
+		synchronized (lock) {
+			return association.getSources();
+		}
+	}
+
+	@Override
+	public ObjectSet<U> getDestinations() {
+		synchronized (lock) {
+			return association.getDestinations();
+		}
+	}
+}