diff --git a/src/java/com/threerings/util/DefaultMap.java b/src/java/com/threerings/util/DefaultMap.java
new file mode 100644
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+++ b/src/java/com/threerings/util/DefaultMap.java
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+//
+// $Id$
+//
+// Narya library - tools for developing networked games
+// Copyright (C) 2002-2009 Three Rings Design, Inc., All Rights Reserved
+// http://www.threerings.net/code/narya/
+//
+// This library is free software; you can redistribute it and/or modify it
+// under the terms of the GNU Lesser General Public License as published
+// by the Free Software Foundation; either version 2.1 of the License, or
+// (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+
+package com.threerings.util;
+
+import java.lang.reflect.Constructor;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import com.google.common.collect.ForwardingMap;
+import com.google.common.collect.Maps;
+
+/**
+ * Provides a map implementation that automatically creates, and inserts into the map, a default
+ * value for keys with no value when a value is fetched via the {@link #get} method. Note that this
+ * map must assume that all keys supplied to {@link #get} are valid instances of the type specified
+ * by K as those keys will be used in a subsequent call to {@link #put}. Thus you must not call
+ * {@link #get} with a key of invalid type or you will cause your map to become unsound.
+ */
+public class DefaultMap<K, V> extends ForwardingMap<K, V>
+{
+    /** Used to create default values. */
+    public interface Creator<K, V>
+    {
+        /** Creates a new default value for the specified key. */
+        V create (K key);
+    }
+
+    /**
+     * Creates a default map backed by a {@link HashMap} that creates instances of the supplied
+     * class (using its no-args constructor) as defaults.
+     */
+    public static <K, V> DefaultMap<K, V> newInstanceHashMap (Class<V> clazz)
+    {
+        return newInstanceMap(Maps.<K, V>newHashMap(), clazz);
+    }
+
+    /**
+     * Creates a default map backed by the supplied map that creates instances of the supplied
+     * class (using its no-args constructor) as defaults.
+     */
+    public static <K, V> DefaultMap<K, V> newInstanceMap (Map<K, V> delegate, Class<V> clazz)
+    {
+        final Constructor<V> ctor;
+        try {
+            ctor = clazz.getConstructor();
+        } catch (NoSuchMethodException nsme) {
+            throw new IllegalArgumentException(clazz + " must have a no-args constructor.");
+        }
+        return newMap(delegate, new Creator<K, V>() {
+            public V create (K key) {
+                try {
+                    return ctor.newInstance();
+                } catch (Exception e) {
+                    throw new RuntimeException(e);
+                }
+            }
+        });
+    }
+
+    /**
+     * Creates a default map backed by a {@link HashMap} using the supplied default creator.
+     */
+    public static <K, V> DefaultMap<K, V> newHashMap (Creator<K, V> creator)
+    {
+        return newMap(Maps.<K, V>newHashMap(), creator);
+    }
+
+    /**
+     * Creates a default map backed by the supplied map using the supplied default creator.
+     */
+    public static <K, V> DefaultMap<K, V> newMap (Map<K, V> delegate, Creator<K, V> creator)
+    {
+        return new DefaultMap<K, V>(delegate, creator);
+    }
+
+    @Override // from Map
+    public V get (Object key)
+    {
+        @SuppressWarnings("unchecked") K tkey = (K)key;
+        V value = super.get(key);
+        // null is a valid value, so we check containsKey() before creating a default
+        if (value == null && !containsKey(key)) {
+            put(tkey, value = _creator.create(tkey));
+        }
+        return value;
+    }
+
+    protected DefaultMap (Map<K, V> delegate, Creator<K, V> creator)
+    {
+        _delegate = delegate;
+        _creator = creator;
+    }
+
+    @Override // from ForwardingMap
+    protected Map<K, V> delegate()
+    {
+        return _delegate;
+    }
+
+    protected final Map<K, V> _delegate;
+    protected final Creator<K, V> _creator;
+}