content: /*
   * 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.beam.sdk.state;
  
  import org.apache.beam.sdk.transforms.Combine;
  import org.apache.beam.sdk.transforms.GroupByKey;
  
  /**
   * A {@link ReadableState} cell that combines multiple input values and outputs a single value of a
   * different type.
   *
   * <p>This generalizes {@link GroupByKey} and {@link Combine} styles of grouping.
   *
   * @param <InputT> the type of values added to the state
   * @param <OutputT> the type of value extracted from the state
   */
  public interface GroupingState<InputT, OutputT> extends ReadableState<OutputT>, State {
    /**
     * Add a value to the buffer.
     *
     * <p>Elements added will not be reflected in {@code OutputT} objects returned by previous calls
     * to {@link #read}.
     */
    void add(InputT value);
  
    /**
     * Returns a {@link ReadableState} whose {@link #read} method will return true if this state is
     * empty at the point when that {@link #read} call returns.
     */
    ReadableState<Boolean> isEmpty();
  
    @Override
    GroupingState<InputT, OutputT> readLater();
  }
  