linq4j/src/main/java/org/apache/calcite/linq4j/Lookup.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.calcite.linq4j;

import org.apache.calcite.linq4j.function.Function2;

import java.util.Map;

/**
 * Represents a collection of keys each mapped to one or more values.
 *
 * @param <K> Key type
 * @param <V> Value type
 */
public interface Lookup<K, V>
    extends Map<K, Enumerable<V>>, Enumerable<Grouping<K, V>> {
  /**
   * Applies a transform function to each key and its associated values and
   * returns the results.
   *
   * @param resultSelector Result selector
   * @param <TResult> Result type
   *
   * @return Enumerable over results
   */
  <TResult> Enumerable<TResult> applyResultSelector(
      Function2<K, Enumerable<V>, TResult> resultSelector);
}