Index: lucene/core/src/java/org/apache/lucene/codecs/lucene3x/Lucene3xNormsProducer.java
===================================================================
--- lucene/core/src/java/org/apache/lucene/codecs/lucene3x/Lucene3xNormsProducer.java	(revision 1242924)
+++ lucene/core/src/java/org/apache/lucene/codecs/lucene3x/Lucene3xNormsProducer.java	(working copy)
@@ -18,6 +18,7 @@
  */
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.IdentityHashMap;
 import java.util.Map;
@@ -37,7 +38,6 @@
 import org.apache.lucene.store.IndexInput;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.IOUtils;
-import org.apache.lucene.util.MapBackedSet;
 import org.apache.lucene.util.StringHelper;
 
 /**
@@ -60,7 +60,7 @@
   final Map<String,NormsDocValues> norms = new HashMap<String,NormsDocValues>();
   // any .nrm or .sNN files we have open at any time.
   // TODO: just a list, and double-close() separate norms files?
-  final Set<IndexInput> openFiles = new MapBackedSet<IndexInput>(new IdentityHashMap<IndexInput,Boolean>());
+  final Set<IndexInput> openFiles = Collections.newSetFromMap(new IdentityHashMap<IndexInput,Boolean>());
   // points to a singleNormFile
   IndexInput singleNormStream;
   final int maxdoc;
Index: lucene/core/src/java/org/apache/lucene/store/MMapDirectory.java
===================================================================
--- lucene/core/src/java/org/apache/lucene/store/MMapDirectory.java	(revision 1242924)
+++ lucene/core/src/java/org/apache/lucene/store/MMapDirectory.java	(working copy)
@@ -27,6 +27,7 @@
 import java.nio.channels.FileChannel;
 import java.nio.channels.FileChannel.MapMode;
 
+import java.util.Collections;
 import java.util.Set;
 import java.util.WeakHashMap;
 
@@ -35,7 +36,6 @@
 import java.security.PrivilegedActionException;
 import java.lang.reflect.Method;
 
-import org.apache.lucene.util.MapBackedSet;
 import org.apache.lucene.util.Constants;
 
 /** File-based {@link Directory} implementation that uses
@@ -261,7 +261,7 @@
     private ByteBuffer curBuf; // redundant for speed: buffers[curBufIndex]
   
     private boolean isClone = false;
-    private final Set<MMapIndexInput> clones = new MapBackedSet<MMapIndexInput>(new WeakHashMap<MMapIndexInput,Boolean>());
+    private final Set<MMapIndexInput> clones = Collections.newSetFromMap(new WeakHashMap<MMapIndexInput,Boolean>());
 
     MMapIndexInput(String resourceDescription, RandomAccessFile raf, long offset, long length, int chunkSizePower) throws IOException {
       super(resourceDescription);
Index: lucene/core/src/java/org/apache/lucene/util/MapBackedSet.java
===================================================================
--- lucene/core/src/java/org/apache/lucene/util/MapBackedSet.java	(revision 1242924)
+++ lucene/core/src/java/org/apache/lucene/util/MapBackedSet.java	(working copy)
@@ -1,69 +0,0 @@
-package org.apache.lucene.util;
-
-/**
- * Licensed to the Apache Software Foundation (ASF) under one or more
- * contributor license agreements.  See the NOTICE file distributed with
- * this work for additional information regarding copyright ownership.
- * The ASF licenses this file to You under the Apache License, Version 2.0
- * (the "License"); you may not use this file except in compliance with
- * the License.  You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-import java.util.AbstractSet;
-import java.util.Iterator;
-import java.util.Map;
-
-/**
- * A Set implementation that wraps an actual Map based
- * implementation.
- * 
- * @lucene.internal
- */
-public final class MapBackedSet<E> extends AbstractSet<E> {
-  private final Map<E, Boolean> map;
-
-  /**
-   * Creates a new instance which wraps the specified {@code map}.
-   */
-  public MapBackedSet(Map<E, Boolean> map) {
-    this.map = map;
-  }
-
-  @Override
-  public int size() {
-    return map.size();
-  }
-
-  @Override
-  public boolean contains(Object o) {
-    return map.containsKey(o);
-  }
-
-  @Override
-  public boolean add(E o) {
-    return map.put(o, Boolean.TRUE) == null;
-  }
-
-  @Override
-  public boolean remove(Object o) {
-    return map.remove(o) != null;
-  }
-
-  @Override
-  public void clear() {
-    map.clear();
-  }
-
-  @Override
-  public Iterator<E> iterator() {
-    return map.keySet().iterator();
-  }
-}
Index: modules/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestIndexClose.java
===================================================================
--- modules/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestIndexClose.java	(revision 1242924)
+++ modules/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestIndexClose.java	(working copy)
@@ -1,6 +1,7 @@
 package org.apache.lucene.facet.taxonomy.directory;
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.IdentityHashMap;
 import java.util.Set;
@@ -10,13 +11,11 @@
 import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.IndexWriterConfig;
 import org.apache.lucene.index.IndexWriterConfig.OpenMode;
-import org.apache.lucene.store.AlreadyClosedException;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.LockObtainFailedException;
 import org.junit.Test;
 
 import org.apache.lucene.util.LuceneTestCase;
-import org.apache.lucene.util.MapBackedSet;
 import org.apache.lucene.analysis.MockAnalyzer;
 import org.apache.lucene.analysis.MockTokenizer;
 import org.apache.lucene.facet.taxonomy.CategoryPath;
@@ -95,7 +94,7 @@
   }
 
   private static class LeakChecker {
-    Set<DirectoryReader> readers = new MapBackedSet<DirectoryReader>(new IdentityHashMap<DirectoryReader,Boolean>());
+    Set<DirectoryReader> readers = Collections.newSetFromMap(new IdentityHashMap<DirectoryReader,Boolean>());
 
     int iwriter=0;
     Set<Integer> openWriters = new HashSet<Integer>();