[streaming] Added interface for policy helpers. Such helpers will allow to use...

[streaming] Added interface for policy helpers. Such helpers will allow to use policy based windowing through a simpler API.

flink-addons/flink-streaming/flink-streaming-core/src/main/java/org/apache/flink/streaming/api/windowing/helper/WindowingHelper.java

+/*
+ * 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.streaming.api.windowing.helper;
+
+import org.apache.flink.streaming.api.windowing.policy.EvictionPolicy;
+import org.apache.flink.streaming.api.windowing.policy.TriggerPolicy;
+
+/**
+ * A helper representing a count or eviction policy. Such helper classes are
+ * used to provide a nice and well readable API.
+ * 
+ * @param <DATA>
+ *            the type of input data handled by this helper
+ * @see Count
+ * @see Time
+ * @see Delta
+ */
+public interface WindowingHelper<DATA> {
+
+	public EvictionPolicy<DATA> toEvict();
+
+	public TriggerPolicy<DATA> toTrigger();
+
+}