Index: /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java	(revision 10671)
+++ /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java	(revision 10672)
@@ -636,5 +636,5 @@
 
         @Override
-        public boolean equals(Object obj) {
+        public synchronized boolean equals(Object obj) {
             return super.equals(obj)
                     || (obj instanceof TagCheck && rule.equals(((TagCheck) obj).rule))
@@ -643,5 +643,5 @@
 
         @Override
-        public int hashCode() {
+        public synchronized int hashCode() {
             return Objects.hash(super.hashCode(), rule);
         }
Index: /trunk/src/org/openstreetmap/josm/gui/widgets/QuadStateCheckBox.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/widgets/QuadStateCheckBox.java	(revision 10671)
+++ /trunk/src/org/openstreetmap/josm/gui/widgets/QuadStateCheckBox.java	(revision 10672)
@@ -92,5 +92,5 @@
     /** Do not let anyone add mouse listeners */
     @Override
-    public void addMouseListener(MouseListener l) {
+    public synchronized void addMouseListener(MouseListener l) {
         // Do nothing
     }