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core module for querydsl

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/*
 * Copyright 2015, The Querydsl Team (http://www.querydsl.com/team)
 *
 * 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 com.querydsl.core.group;

import com.querydsl.core.types.Expression;

/**
 * Defines the way results of a given expression are grouped. GroupExpressions are also used to
 * access values of a given GroupExpression within a Group. GroupExpressions are stateless wrappers
 * for Expressions that know how to collect row values into a group.
 *
 * @param  Element type
 * @param  Target type (e.g. List, Set)
 * @author sasa
 * @author tiwe
 */
public interface GroupExpression extends Expression {

  /**
   * Get the expression wrapped by this group definition
   *
   * @return wrapped expression
   */
  Expression getExpression();

  /**
   * Create a new GroupCollector to collect values belonging to this group.
   *
   * @return new GroupCollector
   */
  GroupCollector createGroupCollector();
}