Index: /trunk/src/org/openstreetmap/josm/data/UndoRedoHandler.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/UndoRedoHandler.java	(revision 6412)
+++ /trunk/src/org/openstreetmap/josm/data/UndoRedoHandler.java	(revision 6413)
@@ -12,4 +12,5 @@
 import org.openstreetmap.josm.gui.layer.Layer;
 import org.openstreetmap.josm.gui.layer.OsmDataLayer.CommandQueueListener;
+import org.openstreetmap.josm.tools.CheckParameterUtil;
 
 public class UndoRedoHandler implements MapView.LayerChangeListener {
@@ -26,4 +27,7 @@
     private final LinkedList<CommandQueueListener> listenerCommands = new LinkedList<CommandQueueListener>();
 
+    /**
+     * Constructs a new {@code UndoRedoHandler}.
+     */
     public UndoRedoHandler() {
         MapView.addLayerChangeListener(this);
@@ -31,7 +35,9 @@
 
     /**
-     * Execute the command and add it to the intern command queue.
+     * Executes the command and add it to the intern command queue.
+     * @param c The command to execute. Must not be {@code null}.
      */
     public void addNoRedraw(final Command c) {
+        CheckParameterUtil.ensureParameterNotNull(c, "c");
         c.executeCommand();
         commands.add(c);
@@ -53,5 +59,6 @@
 
     /**
-     * Execute the command and add it to the intern command queue.
+     * Executes the command and add it to the intern command queue.
+     * @param c The command to execute. Must not be {@code null}.
      */
     synchronized public void add(final Command c) {
@@ -69,4 +76,5 @@
     /**
      * Undoes multiple commands.
+     * @param num The number of commands to undo
      */
     synchronized public void undo(int num) {
@@ -104,4 +112,5 @@
     /**
      * Redoes multiple commands.
+     * @param num The number of commands to redo
      */
     public void redo(int num) {
@@ -167,8 +176,17 @@
     public void activeLayerChange(Layer oldLayer, Layer newLayer) {}
 
+    /**
+     * Removes a command queue listener.
+     * @param l The command queue listener to remove
+     */
     public void removeCommandQueueListener(CommandQueueListener l) {
         listenerCommands.remove(l);
     }
 
+    /**
+     * Adds a command queue listener.
+     * @param l The commands queue listener to add
+     * @return {@code true} if the listener has been added, {@code false} otherwise
+     */
     public boolean addCommandQueueListener(CommandQueueListener l) {
         return listenerCommands.add(l);