diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdSubclassMapTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdSubclassMapTest.java
new file mode 100644
index 000000000..07561deac
--- /dev/null
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdSubclassMapTest.java
@@ -0,0 +1,221 @@
+/*
+ * Copyright (C) 2011, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.lib;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class ObjectIdSubclassMapTest {
+	private MutableObjectId idBuf;
+
+	private SubId id_1, id_2, id_3, id_a31, id_b31;
+
+	@Before
+	public void init() {
+		idBuf = new MutableObjectId();
+		id_1 = new SubId(id(1));
+		id_2 = new SubId(id(2));
+		id_3 = new SubId(id(3));
+		id_a31 = new SubId(id(31));
+		id_b31 = new SubId(id((1 << 8) + 31));
+	}
+
+	@Test
+	public void testEmptyMap() {
+		ObjectIdSubclassMap<SubId> m = new ObjectIdSubclassMap<SubId>();
+		assertTrue(m.isEmpty());
+		assertEquals(0, m.size());
+
+		Iterator<SubId> i = m.iterator();
+		assertNotNull(i);
+		assertFalse(i.hasNext());
+
+		assertFalse(m.contains(id(1)));
+	}
+
+	@Test
+	public void testAddGetAndContains() {
+		ObjectIdSubclassMap<SubId> m = new ObjectIdSubclassMap<SubId>();
+		m.add(id_1);
+		m.add(id_2);
+		m.add(id_3);
+		m.add(id_a31);
+		m.add(id_b31);
+		assertFalse(m.isEmpty());
+		assertEquals(5, m.size());
+
+		assertSame(id_1, m.get(id_1));
+		assertSame(id_1, m.get(id(1)));
+		assertSame(id_1, m.get(id(1).copy()));
+		assertSame(id_2, m.get(id(2).copy()));
+		assertSame(id_3, m.get(id(3).copy()));
+		assertSame(id_a31, m.get(id(31).copy()));
+		assertSame(id_b31, m.get(id_b31.copy()));
+
+		assertTrue(m.contains(id_1));
+	}
+
+	@Test
+	public void testClear() {
+		ObjectIdSubclassMap<SubId> m = new ObjectIdSubclassMap<SubId>();
+
+		m.add(id_1);
+		assertSame(id_1, m.get(id_1));
+
+		m.clear();
+		assertTrue(m.isEmpty());
+		assertEquals(0, m.size());
+
+		Iterator<SubId> i = m.iterator();
+		assertNotNull(i);
+		assertFalse(i.hasNext());
+
+		assertFalse(m.contains(id(1)));
+	}
+
+	@Test
+	public void testAddIfAbsent() {
+		ObjectIdSubclassMap<SubId> m = new ObjectIdSubclassMap<SubId>();
+		m.add(id_1);
+
+		assertSame(id_1, m.addIfAbsent(new SubId(id_1)));
+		assertEquals(1, m.size());
+
+		assertSame(id_2, m.addIfAbsent(id_2));
+		assertEquals(2, m.size());
+		assertSame(id_a31, m.addIfAbsent(id_a31));
+		assertSame(id_b31, m.addIfAbsent(id_b31));
+
+		assertSame(id_a31, m.addIfAbsent(new SubId(id_a31)));
+		assertSame(id_b31, m.addIfAbsent(new SubId(id_b31)));
+		assertEquals(4, m.size());
+	}
+
+	@Test
+	public void testAddGrowsWithObjects() {
+		ObjectIdSubclassMap<SubId> m = new ObjectIdSubclassMap<SubId>();
+		m.add(id_1);
+		for (int i = 32; i < 8000; i++)
+			m.add(new SubId(id(i)));
+		assertEquals(8000 - 32 + 1, m.size());
+
+		assertSame(id_1, m.get(id_1.copy()));
+		for (int i = 32; i < 8000; i++)
+			assertTrue(m.contains(id(i)));
+	}
+
+	@Test
+	public void testAddIfAbsentGrowsWithObjects() {
+		ObjectIdSubclassMap<SubId> m = new ObjectIdSubclassMap<SubId>();
+		m.add(id_1);
+		for (int i = 32; i < 8000; i++)
+			m.addIfAbsent(new SubId(id(i)));
+		assertEquals(8000 - 32 + 1, m.size());
+
+		assertSame(id_1, m.get(id_1.copy()));
+		for (int i = 32; i < 8000; i++)
+			assertTrue(m.contains(id(i)));
+	}
+
+	@Test
+	public void testIterator() {
+		ObjectIdSubclassMap<SubId> m = new ObjectIdSubclassMap<SubId>();
+		m.add(id_1);
+		m.add(id_2);
+		m.add(id_3);
+
+		Iterator<SubId> i = m.iterator();
+		assertTrue(i.hasNext());
+		assertSame(id_1, i.next());
+		assertTrue(i.hasNext());
+		assertSame(id_2, i.next());
+		assertTrue(i.hasNext());
+		assertSame(id_3, i.next());
+
+		assertFalse(i.hasNext());
+		try {
+			i.next();
+			fail("did not fail on next with no next");
+		} catch (NoSuchElementException expected) {
+			// OK
+		}
+
+		i = m.iterator();
+		assertSame(id_1, i.next());
+		try {
+			i.remove();
+			fail("did not fail on remove");
+		} catch (UnsupportedOperationException expected) {
+			// OK
+		}
+	}
+
+	private AnyObjectId id(int val) {
+		// Using bytes 2 and 3 positions our value at the low end of idBuf.w1,
+		// which is what ObjectIdSubclassMap uses for hashing. This makes
+		// collisions likely, making collision testing easier.
+
+		val <<= 1;
+
+		idBuf.setByte(2, (val >>> 8) & 0xff);
+		idBuf.setByte(3, val & 0xff);
+		return idBuf;
+	}
+
+	private static class SubId extends ObjectId {
+		SubId(AnyObjectId id) {
+			super(id);
+		}
+	}
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectIdSubclassMap.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectIdSubclassMap.java
index 612e00ea2..ad78acdb8 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectIdSubclassMap.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectIdSubclassMap.java
@@ -114,12 +114,12 @@ public class ObjectIdSubclassMap<V extends ObjectId> implements Iterable<V> {
 	 * <p>
 	 * An existing mapping for <b>must not</b> be in this map. Callers must
 	 * first call {@link #get(AnyObjectId)} to verify there is no current
-	 * mapping prior to adding a new mapping.
+	 * mapping prior to adding a new mapping, or use
+	 * {@link #addIfAbsent(ObjectId)}.
 	 *
 	 * @param newValue
 	 *            the object to store.
-	 * @param
-	 *            <Q>
+	 * @param <Q>
 	 *            type of instance to store.
 	 */
 	public <Q extends V> void add(final Q newValue) {
@@ -129,6 +129,47 @@ public class ObjectIdSubclassMap<V extends ObjectId> implements Iterable<V> {
 		size++;
 	}
 
+	/**
+	 * Store an object for future lookup.
+	 * <p>
+	 * Stores {@code newValue}, but only if there is not already an object for
+	 * the same object name. Callers can tell if the value is new by checking
+	 * the return value with reference equality:
+	 *
+	 * <pre>
+	 * V obj = ...;
+	 * boolean wasNew = map.addIfAbsent(obj) == obj;
+	 * </pre>
+	 *
+	 * @param newValue
+	 *            the object to store.
+	 * @return {@code newValue} if stored, or the prior value already stored and
+	 *         that would have been returned had the caller used
+	 *         {@code get(newValue)} first.
+	 * @param <Q>
+	 *            type of instance to store.
+	 */
+	public <Q extends V> V addIfAbsent(final Q newValue) {
+		int i = index(newValue);
+		V obj;
+
+		while ((obj = obj_hash[i]) != null) {
+			if (AnyObjectId.equals(obj, newValue))
+				return obj;
+			if (++i == obj_hash.length)
+				i = 0;
+		}
+
+		if (obj_hash.length - 1 <= size * 2) {
+			grow();
+			insert(newValue);
+		} else {
+			obj_hash[i] = newValue;
+		}
+		size++;
+		return newValue;
+	}
+
 	/**
 	 * @return number of objects in map
 	 */
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/CachedObjectDirectory.java b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/CachedObjectDirectory.java
index 001ae288a..d5f2ac53c 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/CachedObjectDirectory.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/CachedObjectDirectory.java
@@ -228,8 +228,7 @@ class CachedObjectDirectory extends FileObjectDatabase {
 		switch (result) {
 		case INSERTED:
 		case EXISTS_LOOSE:
-			if (!unpackedObjects.contains(objectId))
-				unpackedObjects.add(objectId);
+			unpackedObjects.addIfAbsent(objectId);
 			break;
 
 		case EXISTS_PACKED: