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Java 1.6+ Library with tons of utility classes required in all projects
There is a newer version: 9.5.5
/**
 * Copyright (C) 2014-2016 Philip Helger (www.helger.com)
 * philip[at]helger[dot]com
 *
 * 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.helger.commons.aggregate;

import java.util.Collection;
import java.util.function.Function;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

import com.helger.commons.ValueEnforcer;
import com.helger.commons.collection.CollectionHelper;
import com.helger.commons.collection.ext.CommonsArrayList;
import com.helger.commons.string.StringHelper;

/**
 * Aggregate a list of input objects to an output object.
 *
 * @author Philip Helger
 * @param 
 *        The input type.
 * @param 
 *        The output type.
 */
@FunctionalInterface
public interface IAggregator  extends Function , DSTTYPE>
{
  /**
   * Aggregate a collection of input objects to a single output object.
   *
   * @param aObjects
   *        Source objects. May not be null.
   * @return The aggregated object. May be null.
   */
  @Nullable
  @SuppressWarnings ("unchecked")
  default DSTTYPE apply (@Nonnull final SRCTYPE... aObjects)
  {
    return apply (new CommonsArrayList <> (aObjects));
  }

  @Nonnull
  static  IAggregator  createNull ()
  {
    return aCollection -> null;
  }

  @Nonnull
  static  IAggregator  createConstant (@Nullable final DSTTYPE aValue)
  {
    return aCollection -> aValue;
  }

  @Nonnull
  static  IAggregator > createUseAll ()
  {
    return aCollection -> aCollection;
  }

  @Nonnull
  static  IAggregator  createUseFirst ()
  {
    return aCollection -> CollectionHelper.getFirstElement (aCollection);
  }

  @Nonnull
  static  IAggregator  createUseLast ()
  {
    return aCollection -> CollectionHelper.getLastElement (aCollection);
  }

  @Nonnull
  static IAggregator  createStringAll ()
  {
    return aCollection -> StringHelper.getImploded (aCollection);
  }

  @Nonnull
  static IAggregator  createStringAll (final char cSep)
  {
    return aCollection -> StringHelper.getImploded (cSep, aCollection);
  }

  @Nonnull
  static IAggregator  createStringAll (@Nonnull final String sSep)
  {
    ValueEnforcer.notNull (sSep, "Separator");
    return aCollection -> StringHelper.getImploded (sSep, aCollection);
  }

  @Nonnull
  static IAggregator  createStringIgnoreEmpty ()
  {
    return aCollection -> StringHelper.getImplodedNonEmpty (aCollection);
  }

  @Nonnull
  static IAggregator  createStringIgnoreEmpty (final char cSep)
  {
    return aCollection -> StringHelper.getImplodedNonEmpty (cSep, aCollection);
  }

  @Nonnull
  static IAggregator  createStringIgnoreEmpty (@Nonnull final String sSep)
  {
    ValueEnforcer.notNull (sSep, "Separator");
    return aCollection -> StringHelper.getImplodedNonEmpty (sSep, aCollection);
  }
}