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/***********************************************************************************************************************
 *
 * Copyright (C) 2010-2013 by the Stratosphere project (http://stratosphere.eu)
 *
 * Licensed 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 eu.stratosphere.api.java.operators;

import eu.stratosphere.api.common.InvalidProgramException;
import eu.stratosphere.api.java.DataSet;

/**
 * Grouping is an intermediate step for a transformation on a grouped DataSet.

 * The following transformation can be applied on Grouping:
 * 
 * 	{@link Grouping#reduce(ReduceFunction)},
 * {@link Grouping#reduceGroup(GroupReduceFunction)}, and
 * {@link Grouping#aggregate(Aggregations, int)}.
 * 
 *
 * @param  The type of the elements of the grouped DataSet.
 * 
 * @see DataSet
 */
public abstract class Grouping {
	
	protected final DataSet dataSet;
	
	protected final Keys keys;

	public Grouping(DataSet set, Keys keys) {
		if (set == null || keys == null) {
			throw new NullPointerException();
		}
		
		if (keys.isEmpty()) {
			throw new InvalidProgramException("The grouping keys must not be empty.");
		}

		this.dataSet = set;
		this.keys = keys;
	}
	
	
	protected DataSet getDataSet() {
		return this.dataSet;
	}
	
	protected Keys getKeys() {
		return this.keys;
	}

}