diff --git a/src/main/java/umontreal/ssj/simevents/Simulator.java b/src/main/java/umontreal/ssj/simevents/Simulator.java
index 56f160463..672a29b12 100644
--- a/src/main/java/umontreal/ssj/simevents/Simulator.java
+++ b/src/main/java/umontreal/ssj/simevents/Simulator.java
@@ -26,7 +26,6 @@
 
 import umontreal.ssj.simevents.eventlist.EventList;
 import umontreal.ssj.simevents.eventlist.SplayTree;
-import java.util.ListIterator;
 
 /**
  * Represents the executive of a discrete-event simulator. This class
diff --git a/src/main/java/umontreal/ssj/simevents/eventlist/BinaryTree.java b/src/main/java/umontreal/ssj/simevents/eventlist/BinaryTree.java
index e58c0f755..2bd08be06 100644
--- a/src/main/java/umontreal/ssj/simevents/eventlist/BinaryTree.java
+++ b/src/main/java/umontreal/ssj/simevents/eventlist/BinaryTree.java
@@ -28,14 +28,9 @@
 import java.util.ListIterator;
 import java.util.ConcurrentModificationException;
 import java.util.NoSuchElementException;
-import umontreal.ssj.util.PrintfFormat;
 import umontreal.ssj.simevents.Event;
 import umontreal.ssj.util.PrintfFormat;
 
-/**
- *  Implantation de l'interface EventList ayant comme structure de
- *  donnees un arbre binaire.
- */
 
 /**
  * An implementation of  @ref EventList using a binary search tree. Every
@@ -55,24 +50,25 @@ public class BinaryTree implements EventList {
    // racine de l'arbre
    private Entry root = null;
 
-   // liste d'objets qui peuvent etre reutilises
-   private Entry freeEntries = null;
-
    // compteur de modifications sur l'iterateur.
    private int modCount = 0;
+   
+   @Override
    public boolean isEmpty() {
       return root == null;
    }
 
+   @Override
    public void clear() {
       while (root != null)
          remove (root);
    }
 
+   @Override
    public void add (Event ev) {
       // fonction qui ajoute un evenement dans l'arbre
       // note : si deux evenements ont le meme temps, alors il faut
-      //        toujours faire en sorte que ces evenements se retrouvement
+      //        toujours faire en sorte que ces evenements se retrouvent
       //        comme les fils droits les uns des autres
 
       Entry cursor = root;
@@ -101,6 +97,7 @@ public void add (Event ev) {
       ++modCount;
    }
 
+   @Override
    public void addFirst (Event ev) {
    /**
     * Ajoute "ev" comme premier evenement dans l'arbre.
@@ -128,6 +125,7 @@ public void addFirst (Event ev) {
       ++modCount;
    }
 
+   @Override
    public void addBefore (Event ev, Event other) {
       Entry otherEntry = findEntry (other);
       Entry evEntry    = add (ev , null);
@@ -164,6 +162,7 @@ public void addBefore (Event ev, Event other) {
       ++modCount;
    }
 
+   @Override
    public void addAfter (Event ev, Event other) {
       // on va chercher le "Entry" de other
       Entry otherEntry = findEntry (other);
@@ -183,6 +182,7 @@ public void addAfter (Event ev, Event other) {
       ++modCount;
    }
 
+   @Override
    public Event getFirst() {
       if (root==null)
          return null;
@@ -192,6 +192,7 @@ public Event getFirst() {
       return cursor.event;
    }
 
+   @Override
    public Event getFirstOfClass (String cl) {
       Entry cursor = root;
       if (root != null)
@@ -207,6 +208,7 @@ public Event getFirstOfClass (String cl) {
    }
 
    @SuppressWarnings("unchecked")
+   @Override
    public <E extends Event> E getFirstOfClass (Class<E> cl) {
       Entry cursor = root;
       if (root != null)
@@ -221,14 +223,17 @@ public <E extends Event> E getFirstOfClass (Class<E> cl) {
       return null;
    }
 
+   @Override
    public Iterator<Event> iterator() {
       return listIterator();
    }
 
+   @Override
    public ListIterator<Event> listIterator() {
       return new BTItr();
    }
 
+   @Override
    public boolean remove (Event ev) {
       Entry evEntry = findEntry(ev);
       if (evEntry == null)
@@ -237,6 +242,7 @@ public boolean remove (Event ev) {
          return remove(evEntry);
    }
 
+   @Override
    public Event removeFirst() {
       if (root == null)
          return null;
@@ -251,8 +257,9 @@ public Event removeFirst() {
       return first;
    }
 
+   @Override
    public String toString() {
-      StringBuffer sb = new StringBuffer ("Contents of the event list BinaryTree:");
+      StringBuilder sb = new StringBuilder ("Contents of the event list BinaryTree:");
       Entry cursor = root;
 
       if (root != null)
@@ -271,19 +278,7 @@ public String toString() {
    }
 
    private Entry add (Event ev, Entry father) {
-      // On regarde la liste freeEntries
-      if (freeEntries != null) {
-         Entry tempo = freeEntries;
-         freeEntries = freeEntries.right;
-         tempo.event = ev;
-         tempo.left  = null;
-         tempo.right = null;
-         tempo.father = father;
-         return tempo;
-      }
-      // on cree un nouvel objet
-      else
-         return new Entry(ev, null, null, father);
+      return new Entry(ev, null, null, father);
    }
 
    private boolean remove (Entry e) {
@@ -345,9 +340,6 @@ else if (filsGauche)
             while (cursor.left != null)
                cursor = cursor.left;
 
-            // echange entre e et cursor et elimination de e
-            cursor.father.left = cursor.right;
-
             if (isRoot)
                root = cursor;
             else if (filsGauche)
@@ -355,6 +347,7 @@ else if (filsGauche)
             else
                e.father.right = cursor;
 
+            // echange entre e et cursor et elimination de e
             cursor.father.left = cursor.right;
             if (cursor.right != null)
                cursor.right.father = cursor.father;
@@ -369,11 +362,10 @@ else if (filsGauche)
       }
 
       // recupere l'espace du noeud
-      e.right = freeEntries;
+      e.right = null;
       e.left =  null;
       e.event = null;
-      freeEntries = e;
-      e = null;
+
       ++modCount;
       return true;
    }
@@ -465,6 +457,7 @@ private class BTItr implements ListIterator<Event> {
          nextIndex = 0;
       }
 
+      @Override
       public void add(Event ev) {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -519,18 +512,21 @@ public void add(Event ev) {
          ++expectedModCount;
       }
 
+      @Override
       public boolean hasNext() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
          return next != null;
       }
 
+      @Override
       public boolean hasPrevious() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
          return prev != null;
       }
 
+      @Override
       public Event next() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -543,6 +539,7 @@ public Event next() {
          return ev;
       }
 
+      @Override
       public int nextIndex() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -550,6 +547,7 @@ public int nextIndex() {
          return nextIndex;
       }
 
+      @Override
       public Event previous() {
          if (!hasPrevious())
             throw new NoSuchElementException();
@@ -562,6 +560,7 @@ public Event previous() {
          return ev;
       }
 
+      @Override
       public int previousIndex() {
          if (!hasPrevious())
             throw new NoSuchElementException();
@@ -569,6 +568,7 @@ public int previousIndex() {
          return nextIndex - 1;
       }
 
+      @Override
       public void remove() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -586,6 +586,7 @@ public void remove() {
          ++expectedModCount;
       }
 
+      @Override
       public void set (Event ev) {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
diff --git a/src/main/java/umontreal/ssj/simevents/eventlist/DoublyLinked.java b/src/main/java/umontreal/ssj/simevents/eventlist/DoublyLinked.java
index b4d883a6b..2824937d0 100644
--- a/src/main/java/umontreal/ssj/simevents/eventlist/DoublyLinked.java
+++ b/src/main/java/umontreal/ssj/simevents/eventlist/DoublyLinked.java
@@ -46,38 +46,27 @@ public class DoublyLinked implements EventList {
 
    // First and last elements in the list.
    private Node first = null, last = null;
-   private static Node free = null;     // Pointer to stack of free nodes.
+
+   @Override
    public boolean isEmpty() {
       return first == null;
    }
 
+   @Override
    public void clear() {
       if (first == null)
          return;
-      /*      Node p = first;
-              while (p != null) {
-              p.ev.setTime(-20.0);
-              p.ev = null;
-              p = p.succ;
-              }
-      */
-      synchronized (DoublyLinked.class) {
-         last.succ = free;   free = first;
-      }
+
+      last.succ = null;
       last = first = null;
       ++modCount;
    }
 
+   @Override
    public void add (Event ev) {
       Node newNode;
-      synchronized (DoublyLinked.class) {
-         if (free == null)
-            newNode = new Node();
-         else {
-            newNode = free;
-            free = free.succ;
-         }
-      }
+      newNode = new Node();
+
       newNode.ev = ev;
       ++modCount;
       if (last == null) {     // Easy: the event list was empty.
@@ -104,16 +93,11 @@ public void add (Event ev) {
       }
    }
 
+   @Override
    public void addFirst (Event ev) {
       Node newNode;
-      synchronized (DoublyLinked.class) {
-         if (free == null)
-            newNode = new Node();
-         else {
-            newNode = free;
-            free = free.succ;
-         }
-      }
+      newNode = new Node();
+      
       newNode.ev = ev;
       newNode.prec = null;
       if (first == null) {
@@ -128,6 +112,7 @@ public void addFirst (Event ev) {
       ++modCount;
    }
 
+   @Override
    public void addBefore (Event ev, Event other) {
       Node node = last;
       while (node != null && node.ev.compareTo(other) >= 0 && node.ev != other)
@@ -136,14 +121,8 @@ public void addBefore (Event ev, Event other) {
          throw new IllegalArgumentException ("Event not in list.");
 
       Node newNode;
-      synchronized (DoublyLinked.class) {
-         if (free == null)
-            newNode = new Node();
-         else {
-            newNode = free;
-            free = free.succ;
-         }
-      }
+      newNode = new Node();
+      
       newNode.ev = ev;
 
       newNode.prec = node.prec;
@@ -156,6 +135,7 @@ public void addBefore (Event ev, Event other) {
       ++modCount;
    }
 
+   @Override
    public void addAfter (Event ev, Event other) {
       Node node = last;
       while (node != null && node.ev.compareTo(other) >= 0 && node.ev != other)
@@ -164,14 +144,8 @@ public void addAfter (Event ev, Event other) {
          throw new IllegalArgumentException ("Event not in list.");
 
       Node newNode;
-      synchronized (DoublyLinked.class) {
-         if (free == null)
-            newNode = new Node();
-         else {
-            newNode = free;
-            free = free.succ;
-         }
-      }
+      newNode = new Node();
+      
       newNode.ev = ev;
 
       newNode.prec = node;
@@ -184,10 +158,12 @@ public void addAfter (Event ev, Event other) {
       ++modCount;
    }
 
+   @Override
    public Event getFirst() {
       return first == null ? null : first.ev;
    }
 
+   @Override
    public Event getFirstOfClass (String cl) {
       Node node = first;
       while (node != null) {
@@ -199,6 +175,7 @@ public Event getFirstOfClass (String cl) {
    }
 
    @SuppressWarnings("unchecked")
+   @Override
    public <E extends Event> E getFirstOfClass (Class<E> cl) {
       Node node = first;
       while (node != null) {
@@ -209,6 +186,7 @@ public <E extends Event> E getFirstOfClass (Class<E> cl) {
       return null;
    }
 
+   @Override
    public boolean remove (Event ev) {
       // Find the node corresponding to this event ev.
       Node node = last;
@@ -236,13 +214,13 @@ public boolean remove (Event ev) {
          }
       }
       node.ev = null;
-      synchronized (DoublyLinked.class) {
-         node.succ = free;  free = node;  // Recycle node.
-      }
+      node.succ = null;
+              
       ++modCount;
       return true;
    }
 
+   @Override
    public Event removeFirst() {
       if (first == null)
          return null;
@@ -256,23 +234,25 @@ public Event removeFirst() {
          first.prec = null;
 
       temp.ev = null;
-      synchronized (DoublyLinked.class) {
-         temp.succ = free;  free = temp;   // Recycle node.
-      }
+      temp.succ = null;
+
       ++modCount;
       return ev;
    }
 
+   @Override
    public Iterator<Event> iterator() {
       return listIterator();
    }
 
+   @Override
    public ListIterator<Event> listIterator() {
       return new DLItr();
    }
 
+   @Override
    public String toString() {
-      StringBuffer sb = new StringBuffer ("Contents of the event list DoublyLinked:");
+      StringBuilder sb = new StringBuilder ("Contents of the event list DoublyLinked:");
       Node node = first;
       while (node != null) {
          sb.append (PrintfFormat.NEWLINE +
@@ -306,6 +286,7 @@ private class DLItr implements ListIterator<Event> {
          nextIndex = 0;
       }
 
+      @Override
       public void add(Event ev) {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -321,14 +302,8 @@ public void add(Event ev) {
          }
 
          Node newNode;
-         synchronized (DoublyLinked.class) {
-            if (free == null)
-               newNode = new Node();
-            else {
-               newNode = free;
-               free = free.succ;
-            }
-         }
+         newNode = new Node();
+         
          newNode.ev = ev;
          ++nextIndex;
          ++modCount;
@@ -362,18 +337,21 @@ else if (prev == null) {             // Must be inserted first.
          }
       }
 
+      @Override
       public boolean hasNext() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
          return next != null;
       }
 
+      @Override
       public boolean hasPrevious() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
          return prev != null;
       }
 
+      @Override
       public Event next() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -386,6 +364,7 @@ public Event next() {
          return ev;
       }
 
+      @Override
       public int nextIndex() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -393,6 +372,7 @@ public int nextIndex() {
          return nextIndex;
       }
 
+      @Override
       public Event previous() {
          if (!hasPrevious())
             throw new NoSuchElementException();
@@ -405,6 +385,7 @@ public Event previous() {
          return ev;
       }
 
+      @Override
       public int previousIndex() {
          if (!hasPrevious())
             throw new NoSuchElementException();
@@ -412,6 +393,7 @@ public int previousIndex() {
          return nextIndex - 1;
       }
 
+      @Override
       public void remove() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -445,14 +427,14 @@ public void remove() {
             }
          }
          lastRet.ev = null;
-         synchronized (DoublyLinked.class) {
-            lastRet.succ = free;  free = lastRet;  // Recycle node.
-         }
+         lastRet.succ = null;
+
          lastRet = null;
          ++modCount;
          ++expectedModCount;
       }
 
+      @Override
       public void set (Event ev) {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
diff --git a/src/main/java/umontreal/ssj/simevents/eventlist/Henriksen.java b/src/main/java/umontreal/ssj/simevents/eventlist/Henriksen.java
index d5c229620..b28df1440 100644
--- a/src/main/java/umontreal/ssj/simevents/eventlist/Henriksen.java
+++ b/src/main/java/umontreal/ssj/simevents/eventlist/Henriksen.java
@@ -37,7 +37,7 @@
  * of Henriksen @cite sKIN86a&thinsp; (see also @cite sFIS01a&thinsp; (p.
  * 207)).
  *
- * Events are stored in a normal doubly-linked list. An additionnal index
+ * Events are stored in a normal doubly-linked list. An additional index
  * array is added to the structure to allow quick access to the events.
  *
  * <div class="SSJ-bigskip"></div>
@@ -63,7 +63,6 @@ public class Henriksen implements EventList {
      Ceci permet un certain balancement des elements presents dans l'index,
      et donc une meilleure efficacite de la recherche binaire sans avoir
      a mettre tous les elements dans l'index.
-
    */
 
    private static final double MIN_VALUE = -10E38; //n'importe quoi < 0
@@ -96,10 +95,12 @@ public Henriksen() {
       entryVec[0] = lastEntry;
    }
 
+   @Override
    public boolean isEmpty() {
       return firstEntry.right == entryVec[0];
    }
 
+   @Override
    public void clear() {
       if(isEmpty())
          return;
@@ -116,6 +117,7 @@ public void clear() {
    }
 
 
+   @Override
    public void add (Event ev) {
       Entry prec = findEntry(ev, false);
 
@@ -125,6 +127,7 @@ public void add (Event ev) {
       modCount++;
    }
 
+   @Override
    public void addFirst (Event ev) {
       Entry e = new Entry(ev, firstEntry, firstEntry.right, ev.time());
       firstEntry.right.left = e;
@@ -133,6 +136,7 @@ public void addFirst (Event ev) {
       modCount++;
    }
 
+   @Override
    public void addBefore (Event ev, Event other) {
       Entry otherEntry = findEntry(other, true);
       if(otherEntry == null)
@@ -144,6 +148,7 @@ public void addBefore (Event ev, Event other) {
       modCount++;
    }
 
+   @Override
    public void addAfter (Event ev, Event other) {
       Entry otherEntry = findEntry(other, true);
       if(otherEntry == null)
@@ -156,10 +161,12 @@ public void addAfter (Event ev, Event other) {
    }
 
 
+   @Override
    public Event getFirst() {
       return firstEntry.right.event;
    }
 
+   @Override
    public Event getFirstOfClass (String cl) {
       Entry e = firstEntry.right;
       while(e.right != null) {
@@ -171,6 +178,7 @@ public Event getFirstOfClass (String cl) {
    }
 
    @SuppressWarnings("unchecked")
+   @Override
    public <E extends Event> E getFirstOfClass (Class<E> cl) {
       Entry e = firstEntry.right;
       while(e.right != null) {
@@ -181,14 +189,17 @@ public <E extends Event> E getFirstOfClass (Class<E> cl) {
       return null;
    }
 
+   @Override
    public Iterator<Event> iterator() {
       return listIterator();
    }
 
+   @Override
    public ListIterator<Event> listIterator() {
       return new HItr();
    }
 
+   @Override
    public boolean remove (Event ev) {
       Entry e = findEntry (ev, true);
       if (e == null)
@@ -218,6 +229,7 @@ public boolean remove (Event ev) {
       return true;
    }
 
+   @Override
    public Event removeFirst() {
       // si la premiere moitie de l'index est composee d'entrees perimees,
       // on coupe de moitie l'index
@@ -245,8 +257,9 @@ public Event removeFirst() {
    }
 
 
+   @Override
    public String toString() {
-      StringBuffer sb = new StringBuffer
+      StringBuilder sb = new StringBuilder
                         ("Contents of the event list Henriksen:");
       Entry e = firstEntry.right;
       while (e.right != null) {
@@ -273,6 +286,7 @@ private static class Entry {
          this.time = time;
       }
 
+      @Override
       public String toString() {
          return "[" + event + " |" + time + "|]";
       }
@@ -294,6 +308,7 @@ private HItr() {
          nextIndex = 0;
       }
 
+      @Override
       public void add (Event ev) {
          if(modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -319,18 +334,21 @@ public void add (Event ev) {
          expectedModCount++;
       }
 
+      @Override
       public boolean hasNext() {
          if(modCount != expectedModCount)
             throw new ConcurrentModificationException();
          return next != entryVec[0];
       }
 
+      @Override
       public boolean hasPrevious() {
          if(modCount != expectedModCount)
             throw new ConcurrentModificationException();
          return next != firstEntry;
       }
 
+      @Override
       public Event next() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -343,6 +361,7 @@ public Event next() {
          return ev;
       }
 
+      @Override
       public int nextIndex() {
          if(!hasNext())
             throw new NoSuchElementException();
@@ -350,6 +369,7 @@ public int nextIndex() {
          return nextIndex;
       }
 
+      @Override
       public Event previous() {
          if(!hasPrevious())
             throw new NoSuchElementException();
@@ -362,6 +382,7 @@ public Event previous() {
          return ev;
       }
 
+      @Override
       public int previousIndex() {
          if(!hasPrevious())
             throw new NoSuchElementException();
@@ -370,6 +391,7 @@ public int previousIndex() {
       }
 
 
+      @Override
       public void remove() {
          if(modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -407,6 +429,7 @@ public void remove() {
          expectedModCount++;
       }
 
+      @Override
       public void set (Event ev) {
          if(modCount != expectedModCount)
             throw new ConcurrentModificationException();
diff --git a/src/main/java/umontreal/ssj/simevents/eventlist/RedblackTree.java b/src/main/java/umontreal/ssj/simevents/eventlist/RedblackTree.java
index c1becd661..2e4e8715a 100644
--- a/src/main/java/umontreal/ssj/simevents/eventlist/RedblackTree.java
+++ b/src/main/java/umontreal/ssj/simevents/eventlist/RedblackTree.java
@@ -33,7 +33,6 @@
 import java.util.NoSuchElementException;
 import umontreal.ssj.util.PrintfFormat;
 import umontreal.ssj.simevents.Event;
-import umontreal.ssj.simevents.Sim;
 
 /**
  * An implementation of  @ref EventList using a *red black* tree, which is
@@ -52,23 +51,22 @@
  * <div class="SSJ-bigskip"></div>
  */
 public class RedblackTree implements EventList {
-   private TreeMap<Event, Node> tree = new TreeMap<Event, Node>(new EventComparator());
-   private static Node free = null;
+   private final TreeMap<Event, Node> tree = new TreeMap<Event, Node>(new EventComparator());
    private int modCount = 0;
+   
+   @Override
    public void clear() {
       Iterator<Node> itr = tree.values().iterator();
       while (itr.hasNext()) {
          Node node = itr.next();
          node.events.clear();
          itr.remove();
-         synchronized (RedblackTree.class) {
-            node.nextNode = free;
-            free = node;
-         }
+         node.nextNode = null;
       }
       ++modCount;
    }
 
+   @Override
    public void add (Event ev) {
       Node node = tree.get (ev);
       if (node != null)
@@ -78,8 +76,8 @@ public void add (Event ev) {
       ++modCount;
    }
 
+   @Override
    public void addFirst (Event ev) {
- //     ev.setTime (Sim.time());   // necessaire si eventime n'est pas deja a 0
       Node node = tree.get (ev);
       if (node != null)
          node.events.add (ev);
@@ -88,29 +86,31 @@ public void addFirst (Event ev) {
       ++modCount;
    }
 
+   @Override
    public void addBefore (Event ev, Event other) {
       Node node = tree.get (other);
       if (node == null)
          throw new IllegalArgumentException ("Event not in list.");
-   //   ev.setTime (other.time());
       node.addBefore (ev, other);
       ++modCount;
    }
 
+   @Override
    public void addAfter (Event ev, Event other) {
       Node node = tree.get (other);
       if (node == null)
          throw new IllegalArgumentException ("Event not in list.");
-   //   ev.setTime (other.time());
       node.addAfter (ev, other);
       ++modCount;
    }
 
+   @Override
    public Event getFirst() {
       return isEmpty() ? null :
          tree.get (tree.firstKey()).events.get (0);
    }
 
+   @Override
    public Event getFirstOfClass (String cl) {
       Iterator<Node> itr = tree.values().iterator();
       while (itr.hasNext()) {
@@ -122,6 +122,7 @@ public Event getFirstOfClass (String cl) {
       return null;
    }
 
+   @Override
    public <E extends Event> E getFirstOfClass (Class<E> cl) {
       Iterator<Node> itr = tree.values().iterator();
       while (itr.hasNext()) {
@@ -133,20 +134,20 @@ public <E extends Event> E getFirstOfClass (Class<E> cl) {
       return null;
    }
 
+   @Override
    public boolean remove (Event ev) {
       Node node = tree.get (ev);
       if (node == null)
          return false;
       if (node.remove (ev)) {
          tree.remove (ev);
-         synchronized (RedblackTree.class) {
-            node.nextNode = free; free = node;
-         }
+         node.nextNode = null;
       }
       ++modCount;
       return true;
    }
 
+   @Override
    public Event removeFirst() {
       if (tree.isEmpty())
          return null;
@@ -156,16 +157,15 @@ public Event removeFirst() {
       node.events.remove (0);
       if (node.events.isEmpty()) {
          tree.remove (evKey);
-         synchronized (RedblackTree.class) {
-            node.nextNode = free; free = node;
-         }
+         node.nextNode = null;
       }
       ++modCount;
       return first;
    }
 
+   @Override
    public String toString() {
-      StringBuffer sb = new StringBuffer ("Contents of the event list RedblackTree:");
+      StringBuilder sb = new StringBuilder ("Contents of the event list RedblackTree:");
       for (Node node : tree.values()) {
          for (Event ev : node.events)
             sb.append (PrintfFormat.NEWLINE +
@@ -176,14 +176,17 @@ public String toString() {
       return sb.toString();
    }
 
+   @Override
    public Iterator<Event> iterator() {
       return listIterator();
    }
 
+   @Override
    public ListIterator<Event> listIterator() {
       return new RBItr();
    }
 
+   @Override
    public boolean isEmpty() {
       return tree.isEmpty();
    }
@@ -261,8 +264,9 @@ public boolean remove (Event ev) {
          throw new IllegalArgumentException ("Event not in node.");
       }
 
+      @Override
       public String toString() {
-         StringBuffer sb = new StringBuffer();
+         StringBuilder sb = new StringBuilder();
          boolean first = true;
          Iterator<Event> itr = events.iterator();
          while (itr.hasNext()) {
@@ -277,11 +281,10 @@ public String toString() {
    }
 
    private static class EventComparator implements Comparator<Event> {
+      @Override
       public int compare (Event ev1, Event ev2) {
          return ev1.compareTo(ev2);
       }
-
-      public boolean equals (Object obj) { return true; }
    }
 
    private class RBItr implements ListIterator<Event> {
@@ -313,10 +316,12 @@ private class RBItr implements ListIterator<Event> {
          }
       }
 
+      @Override
       public void add(Event ev) {
          throw new UnsupportedOperationException();
       }
 
+      @Override
       public boolean hasNext() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -324,6 +329,7 @@ public boolean hasNext() {
             nextNodeIndex < nextNode.events.size();
       }
 
+      @Override
       public boolean hasPrevious() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -331,6 +337,7 @@ public boolean hasPrevious() {
             prevNodeIndex >= 0;
       }
 
+      @Override
       public Event next() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -347,6 +354,7 @@ public Event next() {
          return ev;
       }
 
+      @Override
       public int nextIndex() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -354,6 +362,7 @@ public int nextIndex() {
          return nextIndex;
       }
 
+      @Override
       public Event previous() {
          if (!hasPrevious())
             throw new NoSuchElementException();
@@ -371,6 +380,7 @@ public Event previous() {
          return ev;
       }
 
+      @Override
       public int previousIndex() {
          if (!hasPrevious())
             throw new NoSuchElementException();
@@ -378,26 +388,19 @@ public int previousIndex() {
          return nextIndex - 1;
       }
 
+      @Override
       public void remove() {
          throw new UnsupportedOperationException();
       }
 
+      @Override
       public void set (Event ev) {
          throw new UnsupportedOperationException();
       }
    }
 
    private Node newNode (Event ev) {
-      Node temp;
-      synchronized (RedblackTree.class) {
-         if (free == null)
-            return new Node (ev);
-
-         temp = free;
-         free = free.nextNode;
-      }
-      temp.events.add (ev);
-      return temp;
+      return new Node (ev);
    }
 
    private class EventMapKey extends Event {
@@ -406,6 +409,7 @@ public EventMapKey(Event ev) {
         this.priority = ev.priority();
       }
 
+      @Override
       public void actions() { }
    }
 }
\ No newline at end of file
diff --git a/src/main/java/umontreal/ssj/simevents/eventlist/SplayTree.java b/src/main/java/umontreal/ssj/simevents/eventlist/SplayTree.java
index 2a8c0ec91..296a078f7 100644
--- a/src/main/java/umontreal/ssj/simevents/eventlist/SplayTree.java
+++ b/src/main/java/umontreal/ssj/simevents/eventlist/SplayTree.java
@@ -44,7 +44,6 @@
  */
 public class SplayTree implements EventList {
    private Entry root = null;
-   private static Entry free = null;
    private int modCount = 0;
 
    private int myCompareTo (Event ev, Event other) {
@@ -59,10 +58,12 @@ private int myCompareTo (Event ev, Event other) {
          return 1;
       return 0;
    }
+   @Override
    public boolean isEmpty() {
       return root == null;
    }
 
+   @Override
    public void clear() {
       // Simply root = null would be more efficient but the
       // entries would not be recuperated.
@@ -70,6 +71,7 @@ public void clear() {
          remove (root);
    }
 
+   @Override
    public void add (Event ev) {
       //ajoute un element dans l'arbre en faisant un "splay"
 
@@ -173,6 +175,7 @@ else if (myCompareTo(ev, temp.event) > 0) {
       ++modCount;
    }
 
+   @Override
    public void addFirst (Event ev) {
       if (root == null)
          root = add (ev, null);
@@ -185,6 +188,7 @@ public void addFirst (Event ev) {
       ++modCount;
    }
 
+   @Override
    public void addBefore (Event ev, Event other) {
       Entry otherEntry = findEntry (other);
       if (otherEntry == null)
@@ -215,6 +219,7 @@ public void addBefore (Event ev, Event other) {
       ++modCount;
    }
 
+   @Override
    public void addAfter (Event ev, Event other) {
       Entry otherEntry = findEntry (other);
       if (otherEntry == null)
@@ -228,6 +233,7 @@ public void addAfter (Event ev, Event other) {
       ++modCount;
    }
 
+   @Override
    public Event getFirst() {
       if (root == null)
          return null;
@@ -237,6 +243,7 @@ public Event getFirst() {
       return cursor.event;
    }
 
+   @Override
    public Event getFirstOfClass (String cl) {
       Entry cursor = root;
       if (root != null)
@@ -251,6 +258,7 @@ public Event getFirstOfClass (String cl) {
    }
 
    @SuppressWarnings("unchecked")
+   @Override
    public <E extends Event> E getFirstOfClass (Class<E> cl) {
       Entry cursor = root;
       if (root != null)
@@ -264,14 +272,17 @@ public <E extends Event> E getFirstOfClass (Class<E> cl) {
       return null;
    }
 
+   @Override
    public Iterator<Event> iterator() {
       return listIterator();
    }
 
+   @Override
    public ListIterator<Event> listIterator() {
       return new SPItr();
    }
 
+   @Override
    public boolean remove (Event ev) {
       //on trouve le noeud correspondant a l'evenement pour l'enlever
       if (root == null)
@@ -282,6 +293,7 @@ public boolean remove (Event ev) {
       return remove (e);
    }
 
+   @Override
    public Event removeFirst() {
       if (root == null)
          return null;
@@ -293,8 +305,9 @@ public Event removeFirst() {
       return first;
    }
 
+   @Override
    public String toString() {
-      StringBuffer sb = new StringBuffer
+      StringBuilder sb = new StringBuilder
           ("Contents of the event list SplayTree:");
       Entry cursor = root;
       if (root != null)
@@ -343,6 +356,7 @@ private class SPItr implements ListIterator<Event> {
          nextIndex = 0;
       }
 
+      @Override
       public void add(Event ev) {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -399,18 +413,21 @@ public void add(Event ev) {
          ++expectedModCount;
       }
 
+      @Override
       public boolean hasNext() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
          return next != null;
       }
 
+      @Override
       public boolean hasPrevious() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
          return prev != null;
       }
 
+      @Override
       public Event next() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -423,6 +440,7 @@ public Event next() {
          return ev;
       }
 
+      @Override
       public int nextIndex() {
          if (!hasNext())
             throw new NoSuchElementException();
@@ -430,6 +448,7 @@ public int nextIndex() {
          return nextIndex;
       }
 
+      @Override
       public Event previous() {
          if (!hasPrevious())
             throw new NoSuchElementException();
@@ -442,6 +461,7 @@ public Event previous() {
          return ev;
       }
 
+      @Override
       public int previousIndex() {
          if (!hasPrevious())
             throw new NoSuchElementException();
@@ -449,6 +469,7 @@ public int previousIndex() {
          return nextIndex - 1;
       }
 
+      @Override
       public void remove() {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -466,6 +487,7 @@ public void remove() {
          ++expectedModCount;
       }
 
+      @Override
       public void set (Event ev) {
          if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
@@ -492,18 +514,7 @@ public void set (Event ev) {
     * Creates a new entry object.
     */
    private Entry add (Event ev, Entry father) {
-      Entry e;
-      synchronized (SplayTree.class) {
-         if (free == null)
-            return new Entry (ev, null, null, father);
-
-         e = free;
-         free = free.right;
-      }
-      e.event = ev;
-      e.left = e.right = null;
-      e.father = father;
-      return e;
+      return new Entry (ev, null, null, father);
    }
 
 
@@ -669,14 +680,9 @@ private boolean remove(Entry e)
       if(rightTree != null)
          rightTree.father = null;
 
-      // Recupere l'espace de l'avis d'evenement
       e.left = null;
       e.event = null;
-      synchronized (SplayTree.class) {
-         e.right = free;
-         free = e;
-      }
-
+      e.right = null;
 
       if(rightTree == null)
          root = leftTree;
@@ -697,7 +703,6 @@ else if(leftTree == null)
          root = newRoot;
       }
 
-
       ++modCount;
 
       return true;
@@ -870,4 +875,4 @@ public String toString() {
       }
    }
 */
-}
\ No newline at end of file
+}