LUCENE-1754: add EmptyDocIdSetIterator

git-svn-id: https://svn.apache.org/repos/asf/lucene/java/trunk@798091 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/lucene/search/EmptyDocIdSetIterator.java

@@ -0,0 +1,42 @@
+package org.apache.lucene.search;
+
+/**
+ * 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.io.IOException;
+
+/**
+ * An empty implementation for {@link DocIdSetIterator}. This can be used by
+ * classes which receive a null {@link DocIdSet} from
+ * {@link Filter#getDocIdSet(org.apache.lucene.index.IndexReader)}.
+ */
+public final class EmptyDocIdSetIterator extends DocIdSetIterator {
+
+  private static final DocIdSetIterator instance = new EmptyDocIdSetIterator();
+  
+  public static DocIdSetIterator getInstance() { return instance; }
+  
+  // to prevent instantiation
+  private EmptyDocIdSetIterator() { }
+  
+  public int advance(int target) throws IOException { return NO_MORE_DOCS; }
+  
+  public int docID() { return NO_MORE_DOCS; }
+  
+  public int nextDoc() throws IOException { return NO_MORE_DOCS; }
+  
+}