diff --git a/src/main/java/org/warp/commonutils/type/Association.java b/src/main/java/org/warp/commonutils/type/Association.java
new file mode 100644
index 0000000..6579138
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/Association.java
@@ -0,0 +1,107 @@
+package org.warp.commonutils.type;
+
+import java.util.Optional;
+import java.util.Set;
+
+/**
+ * One to many relationship (not overlapping)
+ *
+ * o ------- o
+ *      +--- o
+ * o ---+--- o
+ *      +--- o
+ * o ------- o
+ * o ------- o
+ * o ---+--- o
+ *      +--- o
+ *
+ * @param <T> Source type
+ * @param <U> Destination type
+ */
+public interface Association<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
+	 */
+	Optional<T> unlinkFromSource(U dest);
+
+	/**
+	 * Check if link exists
+	 * @param src Source
+	 * @return true if source it's 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 a source
+	 */
+	default boolean hasLinkSource(U dest) {
+		return getLinkSource(dest).isPresent();
+	}
+
+	/**
+	 * 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 Source if the link exists
+	 */
+	Optional<T> getLinkSource(U dest);
+
+	/**
+	 * Delete all links
+	 */
+	void clear();
+
+	/**
+	 * Get the count of existing links
+	 * @return size
+	 */
+	int size();
+
+	static <T, U> Association<T, U> synchronize(Association<T, U> association) {
+		return new SynchronizedAssociation(association);
+	}
+}
diff --git a/src/main/java/org/warp/commonutils/type/BiAssociation.java b/src/main/java/org/warp/commonutils/type/BiAssociation.java
new file mode 100644
index 0000000..f2ade49
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/BiAssociation.java
@@ -0,0 +1,106 @@
+package org.warp.commonutils.type;
+
+import java.util.Optional;
+
+/**
+ * One to one relationship
+ *
+ * o ------- o
+ * o ------- o
+ * o ------- o
+ * o ------- o
+ * o ------- o
+ * o ------- o
+ * o ------- o
+ * o ------- o
+ *
+ * @param <T> Source type
+ * @param <U> Destination type
+ */
+public interface BiAssociation<T, U> {
+
+	/**
+	 * Link source to dest
+	 * @param src Source
+	 * @param dest Destination
+	 * @return previous value if it was already linked
+	 */
+	Optional<U> 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 destination
+	 */
+	Optional<U> unlink(T src);
+
+	/**
+	 * Unlink
+	 * @param dest Destination
+	 * @return previous linked source
+	 */
+	Optional<T> unlinkFromSource(U dest);
+
+	/**
+	 * Check if link exists
+	 * @param src Source
+	 * @return true if source it's linked with a destination
+	 */
+	default boolean hasLink(T src) {
+		return getLink(src).isPresent();
+	}
+
+	/**
+	 * Check if link exists
+	 * @param dest Destination
+	 * @return true if destination is linked with a source
+	 */
+	default boolean hasLinkSource(U dest) {
+		return getLinkSource(dest).isPresent();
+	}
+
+	/**
+	 * 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 Destination if the link exists
+	 */
+	Optional<U> getLink(T src);
+
+	/**
+	 * Get a link source
+	 * @param dest Source
+	 * @return Source if the link exists
+	 */
+	Optional<T> getLinkSource(U dest);
+
+	/**
+	 * Delete all links
+	 */
+	void clear();
+
+	/**
+	 * Get the count of existing links
+	 * @return size
+	 */
+	int size();
+
+	static <T, U> BiAssociation<T, U> synchronize(BiAssociation<T, U> association) {
+		return new SynchronizedBiAssociation(association);
+	}
+}
diff --git a/src/main/java/org/warp/commonutils/type/HashAssociation.java b/src/main/java/org/warp/commonutils/type/HashAssociation.java
new file mode 100644
index 0000000..da9faeb
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/HashAssociation.java
@@ -0,0 +1,166 @@
+package org.warp.commonutils.type;
+
+import it.unimi.dsi.fastutil.objects.Object2ObjectOpenHashMap;
+import it.unimi.dsi.fastutil.objects.ObjectOpenHashSet;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+public class HashAssociation<T, U> implements Association<T, U> {
+
+	private final Object2ObjectOpenHashMap<T, ObjectOpenHashSet<U>> associations;
+	private final Object2ObjectOpenHashMap<U, T> inverseAssociations;
+
+	public HashAssociation() {
+		this.associations = new Object2ObjectOpenHashMap<>();
+		this.inverseAssociations = new Object2ObjectOpenHashMap<>();
+	}
+
+	private HashAssociation(Object2ObjectOpenHashMap<T, ObjectOpenHashSet<U>> associations,
+			Object2ObjectOpenHashMap<U, T> inverseAssociations) {
+		this.associations = associations;
+		this.inverseAssociations = inverseAssociations;
+	}
+
+	@Override
+	public boolean link(T src, U dest) {
+		Objects.requireNonNull(src);
+		Objects.requireNonNull(dest);
+		// If the destination is associated to null
+		var previousSrc = inverseAssociations.get(dest);
+
+		// Return if the association already exists
+		if (src.equals(previousSrc)) {
+			return false;
+		}
+
+		// Remove the previous association if present
+		if (previousSrc != null) {
+			inverseAssociations.remove(dest);
+			var destinations = associations.get(previousSrc);
+			destinations.remove(dest);
+			if (destinations.isEmpty()) {
+				associations.remove(previousSrc);
+			}
+		}
+
+		if (!associations.computeIfAbsent(src, s -> new ObjectOpenHashSet<>(0)).add(dest)) {
+			throw new IllegalStateException("Association was partially linked");
+		}
+		return true;
+	}
+
+	@Override
+	public boolean unlink(T src, U dest) {
+		Objects.requireNonNull(src);
+		Objects.requireNonNull(dest);
+
+		var linkedSrc = inverseAssociations.get(dest);
+
+		// Check if the link is different
+		if (!Objects.equals(src, linkedSrc)) {
+			return false;
+		}
+
+		inverseAssociations.remove(dest);
+		var destinations = associations.get(src);
+		if (!destinations.remove(dest)) {
+			throw new IllegalStateException("Association was partially linked");
+		}
+		if (destinations.isEmpty()) {
+			associations.remove(src);
+		}
+
+		return true;
+	}
+
+	@Override
+	public Set<U> unlink(T src) {
+		Objects.requireNonNull(src);
+
+		var destinations = associations.remove(src);
+
+		if (destinations == null) {
+			return Set.of();
+		}
+
+		for (U destination : destinations) {
+			if (!Objects.equals(src, inverseAssociations.remove(destination))) {
+				throw new IllegalStateException("Association was partially linked");
+			}
+		}
+
+		return destinations;
+	}
+
+	@Override
+	public Optional<T> unlinkFromSource(U dest) {
+		Objects.requireNonNull(dest);
+
+		var previousSrc = inverseAssociations.remove(dest);
+
+		if (previousSrc == null) {
+			return Optional.empty();
+		}
+
+		var destinations = associations.get(previousSrc);
+		destinations.remove(dest);
+		if (destinations.isEmpty()) {
+			associations.remove(previousSrc);
+		}
+
+		return Optional.of(previousSrc);
+	}
+
+	@Override
+	public boolean hasAnyLink(T src) {
+		Objects.requireNonNull(src);
+		var destinations = associations.get(src);
+		return destinations != null && !destinations.isEmpty();
+	}
+
+	@Override
+	public boolean hasLinkSource(U dest) {
+		Objects.requireNonNull(dest);
+		return inverseAssociations.containsKey(dest);
+	}
+
+	@Override
+	public boolean hasLink(T src, U dest) {
+		Objects.requireNonNull(src);
+		Objects.requireNonNull(dest);
+		return Objects.equals(src, inverseAssociations.get(dest));
+	}
+
+	@Override
+	public Set<U> getLinks(T src) {
+		Objects.requireNonNull(src);
+		return associations.get(src).clone();
+	}
+
+	@Override
+	public Optional<T> getLinkSource(U dest) {
+		Objects.requireNonNull(dest);
+
+		return Optional.ofNullable(inverseAssociations.get(dest));
+	}
+
+	@Override
+	public void clear() {
+		associations.clear();
+		inverseAssociations.clear();
+	}
+
+	@Override
+	public int size() {
+		return inverseAssociations.size();
+	}
+
+	@SuppressWarnings("MethodDoesntCallSuperMethod")
+	@Override
+	public HashAssociation<T, U> clone() {
+		var associationsClone = associations.clone();
+		associationsClone.replaceAll((item, destinations) -> destinations.clone());
+		return new HashAssociation<>(associationsClone, inverseAssociations.clone());
+	}
+}
diff --git a/src/main/java/org/warp/commonutils/type/HashBiAssociation.java b/src/main/java/org/warp/commonutils/type/HashBiAssociation.java
new file mode 100644
index 0000000..87e4f95
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/HashBiAssociation.java
@@ -0,0 +1,142 @@
+package org.warp.commonutils.type;
+
+import it.unimi.dsi.fastutil.objects.Object2ObjectOpenHashMap;
+import java.util.Objects;
+import java.util.Optional;
+
+public class HashBiAssociation<T, U> implements BiAssociation<T, U> {
+
+	private final Object2ObjectOpenHashMap<T, U> associations;
+	private final Object2ObjectOpenHashMap<U, T> inverseAssociations;
+
+	public HashBiAssociation() {
+		this.associations = new Object2ObjectOpenHashMap<>();
+		this.inverseAssociations = new Object2ObjectOpenHashMap<>();
+	}
+
+	private HashBiAssociation(Object2ObjectOpenHashMap<T, U> associations,
+			Object2ObjectOpenHashMap<U, T> inverseAssociations) {
+		this.associations = associations;
+		this.inverseAssociations = inverseAssociations;
+	}
+
+	@Override
+	public Optional<U> link(T src, U dest) {
+		Objects.requireNonNull(src);
+		Objects.requireNonNull(dest);
+
+		var previousSrc = inverseAssociations.put(dest, src);
+
+		// Return immediately if the link already exists
+		if (Objects.equals(src, previousSrc)) {
+			return Optional.of(dest);
+		}
+
+		// Remove the previous association
+		if (previousSrc != null) {
+			associations.remove(previousSrc);
+		}
+
+		var previousDest = associations.put(src, dest);
+
+		// Remove the previous association
+		if (previousDest != null) {
+			inverseAssociations.remove(previousDest);
+		}
+
+		return Optional.ofNullable(previousDest);
+	}
+
+	@Override
+	public boolean unlink(T src, U dest) {
+		Objects.requireNonNull(src);
+		Objects.requireNonNull(dest);
+
+		if (!Objects.equals(dest, associations.get(src))) {
+			return false;
+		}
+
+		associations.remove(src);
+		inverseAssociations.remove(dest);
+		return true;
+	}
+
+	@Override
+	public Optional<U> unlink(T src) {
+		Objects.requireNonNull(src);
+
+		var dest = associations.remove(src);
+
+		if (dest != null) {
+			inverseAssociations.remove(dest);
+		}
+
+		return Optional.ofNullable(dest);
+	}
+
+	@Override
+	public Optional<T> unlinkFromSource(U dest) {
+		Objects.requireNonNull(dest);
+
+		var src = inverseAssociations.remove(dest);
+
+		if (src != null) {
+			associations.remove(src);
+		}
+
+		return Optional.ofNullable(src);
+	}
+
+	@Override
+	public boolean hasLink(T src) {
+		Objects.requireNonNull(src);
+
+		return associations.containsKey(src);
+	}
+
+	@Override
+	public boolean hasLinkSource(U dest) {
+		Objects.requireNonNull(dest);
+
+		return inverseAssociations.containsKey(dest);
+	}
+
+	@Override
+	public boolean hasLink(T src, U dest) {
+		Objects.requireNonNull(src);
+		Objects.requireNonNull(dest);
+
+		return Objects.equals(dest, associations.get(src));
+	}
+
+	@Override
+	public Optional<U> getLink(T src) {
+		Objects.requireNonNull(src);
+
+		return Optional.ofNullable(associations.get(src));
+	}
+
+	@Override
+	public Optional<T> getLinkSource(U dest) {
+		Objects.requireNonNull(dest);
+
+		return Optional.ofNullable(inverseAssociations.get(dest));
+	}
+
+	@Override
+	public void clear() {
+		associations.clear();
+		inverseAssociations.clear();
+	}
+
+	@Override
+	public int size() {
+		return inverseAssociations.size();
+	}
+
+	@SuppressWarnings("MethodDoesntCallSuperMethod")
+	@Override
+	public HashBiAssociation<T, U> clone() {
+		return new HashBiAssociation<>(associations.clone(), inverseAssociations.clone());
+	}
+}
diff --git a/src/main/java/org/warp/commonutils/type/SynchronizedAssociation.java b/src/main/java/org/warp/commonutils/type/SynchronizedAssociation.java
new file mode 100644
index 0000000..c93bb0f
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/SynchronizedAssociation.java
@@ -0,0 +1,97 @@
+package org.warp.commonutils.type;
+
+import java.util.Optional;
+import java.util.Set;
+
+public class SynchronizedAssociation<T, U> implements Association<T, U> {
+
+	private final Association<T,U> association;
+	private final Object lock;
+
+	public SynchronizedAssociation(Association<T, U> association) {
+		this.association = association;
+		this.lock = new Object();
+	}
+
+	public SynchronizedAssociation(Association<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 Optional<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 hasLinkSource(U dest) {
+		synchronized (lock) {
+			return association.hasLinkSource(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 Optional<T> getLinkSource(U dest) {
+		synchronized (lock) {
+			return association.getLinkSource(dest);
+		}
+	}
+
+	@Override
+	public void clear() {
+		synchronized (lock) {
+			association.clear();
+		}
+	}
+
+	@Override
+	public int size() {
+		synchronized (lock) {
+			return association.size();
+		}
+	}
+}
diff --git a/src/main/java/org/warp/commonutils/type/SynchronizedBiAssociation.java b/src/main/java/org/warp/commonutils/type/SynchronizedBiAssociation.java
new file mode 100644
index 0000000..bb0475e
--- /dev/null
+++ b/src/main/java/org/warp/commonutils/type/SynchronizedBiAssociation.java
@@ -0,0 +1,96 @@
+package org.warp.commonutils.type;
+
+import java.util.Optional;
+
+public class SynchronizedBiAssociation<T, U> implements BiAssociation<T, U> {
+
+	private final BiAssociation<T,U> association;
+	private final Object lock;
+
+	public SynchronizedBiAssociation(BiAssociation<T, U> association) {
+		this.association = association;
+		this.lock = new Object();
+	}
+
+	public SynchronizedBiAssociation(BiAssociation<T, U> association, Object lock) {
+		this.association = association;
+		this.lock = lock;
+	}
+
+	@Override
+	public Optional<U> 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 Optional<U> unlink(T src) {
+		synchronized (lock) {
+			return association.unlink(src);
+		}
+	}
+
+	@Override
+	public Optional<T> unlinkFromSource(U dest) {
+		synchronized (lock) {
+			return association.unlinkFromSource(dest);
+		}
+	}
+
+	@Override
+	public boolean hasLink(T src) {
+		synchronized (lock) {
+			return association.hasLink(src);
+		}
+	}
+
+	@Override
+	public boolean hasLinkSource(U dest) {
+		synchronized (lock) {
+			return association.hasLinkSource(dest);
+		}
+	}
+
+	@Override
+	public boolean hasLink(T src, U dest) {
+		synchronized (lock) {
+			return association.hasLink(src, dest);
+		}
+	}
+
+	@Override
+	public Optional<U> getLink(T src) {
+		synchronized (lock) {
+			return association.getLink(src);
+		}
+	}
+
+	@Override
+	public Optional<T> getLinkSource(U dest) {
+		synchronized (lock) {
+			return association.getLinkSource(dest);
+		}
+	}
+
+	@Override
+	public void clear() {
+		synchronized (lock) {
+			association.clear();
+		}
+	}
+
+	@Override
+	public int size() {
+		synchronized (lock) {
+			return association.size();
+		}
+	}
+}