--- /dev/null
+package org.apache.lucene.facet.index.attributes;
+
+import java.io.Serializable;
+
+import org.apache.lucene.facet.index.CategoryContainer;
+
+/**
+ * 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.
+ */
+
+/**
+ * Property that can be added to {@link CategoryAttribute}s during indexing.
+ * Note that properties are put in a map and could be shallow copied during
+ * {@link CategoryAttributeImpl#clone()}, therefore reuse of
+ * {@link CategoryProperty} objects is not recommended. Also extends
+ * {@link Serializable}, making the {@link CategoryContainer} serialization more
+ * elegant.
+ *
+ * @lucene.experimental
+ */
+public interface CategoryProperty extends Serializable {
+
+ /**
+ * When adding categories with properties to a certain document, it is
+ * possible that the same category will be added more than once with
+ * different instances of the same property. This method defined how to
+ * treat such cases, by merging the newly added property into the one
+ * previously added. Implementing classes can assume that this method will
+ * be called only with a property of the same class.
+ *
+ * @param other
+ * The category property to merge.
+ * @throws UnsupportedOperationException
+ * If merging is prohibited for this property.
+ */
+ public void merge(CategoryProperty other)
+ throws UnsupportedOperationException;
+}