Github user jparkie commented on a diff in the pull request:
https://github.com/apache/flink/pull/4652#discussion_r140828795
--- Diff:
flink-libraries/flink-table/src/main/java/org/apache/flink/table/runtime/functions/aggfunctions/cardinality/ICardinality.java
---
@@ -0,0 +1,80 @@
+/*
+ * 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.
+ */
+
+package org.apache.flink.table.runtime.functions.aggfunctions.cardinality;
+
+import java.io.IOException;
+
+/**
+ * An interface definition for implementation of cardinality.
+ */
+public interface ICardinality {
+
+ /**
+ * Check whether the element is impact estimate.
+ *
+ * @param o stream element
+ * @return false if the value returned by cardinality() is unaffected
by the appearance of o in the stream.
+ */
+ boolean offer(Object o);
+
+ /**
+ * Offer the value as a hashed long value.
+ *
+ * @param hashedLong - the hash of the item to offer to the estimator
+ * @return false if the value returned by cardinality() is unaffected
by the appearance of hashedLong in the stream
+ */
+ boolean offerHashed(long hashedLong);
+
+ /**
+ * Offer the value as a hashed long value.
+ *
+ * @param hashedInt - the hash of the item to offer to the estimator
+ * @return false if the value returned by cardinality() is unaffected
by the appearance of hashedInt in the stream
+ */
+ boolean offerHashed(int hashedInt);
+
+ /**
+ * @return the number of unique elements in the stream or an estimate
thereof.
+ */
+ long cardinality();
+
+ /**
+ * @return size in bytes needed for serialization.
+ */
+ int sizeof();
+
+ /**
+ * Get the byte array used for the calculation.
+ *
+ * @return The byte array used for the calculation
+ * @throws IOException
+ */
+ byte[] getBytes() throws IOException;
+
+ /**
+ * Merges estimators to produce a new estimator for the combined streams
+ * of this estimator and those passed as arguments.
+ * <p/>
+ * Nor this estimator nor the one passed as parameters are modified.
+ *
+ * @param estimators Zero or more compatible estimators
+ * @throws Exception If at least one of the estimators is not
compatible with this one
+ */
+ ICardinality merge(ICardinality... estimators) throws Exception;
--- End diff --
Wouldn't it be nicer to have a more specific Exception?
---
