CollectionUtils.java

package org.springframework.roo.support.util;

/*
 * Copyright 2002-2008 the original author or authors.
 * 
 * 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.
 */

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.Map;

/**
 * Miscellaneous collection utility methods. Mainly for internal use within the
 * framework.
 * 
 * @author Alan Stewart
 * @author Andrew Swan
 * @since 1.1.3
 */
public final class CollectionUtils {

  /**
   * Convert the supplied array into a List. A primitive array gets converted
   * into a List of the appropriate wrapper type.
   * <p>
   * A <code>null</code> source value will be converted to an empty List.
   * 
   * @param source the (potentially primitive) array
   * @return the converted List result
   * @see ObjectUtils#toObjectArray(Object)
   */
  public static List<?> arrayToList(final Object source) {
    return Arrays.asList(ObjectUtils.toObjectArray(source));
  }

  /**
   * Filters (removes elements from) the given {@link Iterable} using the
   * given filter.
   * 
   * @param <T> the type of object being filtered
   * @param unfiltered the iterable to filter; can be <code>null</code>
   * @param filter the filter to apply; can be <code>null</code> for none
   * @return a non-<code>null</code> list
   */
  public static <T> List<T> filter(final Iterable<? extends T> unfiltered, final Filter<T> filter) {
    final List<T> filtered = new ArrayList<T>();
    if (unfiltered != null) {
      for (final T element : unfiltered) {
        if (filter == null || filter.include(element)) {
          filtered.add(element);
        }
      }
    }
    return filtered;
  }

  /**
   * Returns the first element of the given collection
   * 
   * @param <T>
   * @param collection
   * @return <code>null</code> if the first element is <code>null</code> or
   *         the collection is <code>null</code> or empty
   */
  public static <T> T firstElementOf(final Collection<? extends T> collection) {
    if (isEmpty(collection)) {
      return null;
    }
    return collection.iterator().next();
  }

  /**
   * Return <code>true</code> if the supplied Collection is <code>null</code>
   * or empty. Otherwise, return <code>false</code>.
   * 
   * @param collection the Collection to check
   * @return whether the given Collection is empty
   */
  public static boolean isEmpty(final Collection<?> collection) {
    return collection == null || collection.isEmpty();
  }

  /**
   * Return <code>true</code> if the supplied Map is <code>null</code> or
   * empty. Otherwise, return <code>false</code>.
   * 
   * @param map the Map to check
   * @return whether the given Map is empty
   */
  public static boolean isEmpty(final Map<?, ?> map) {
    return map == null || map.isEmpty();
  }

  /**
   * Populates the given collection by replacing any existing contents with
   * the given elements, in a null-safe way.
   * 
   * @param <T> the type of element in the collection
   * @param collection the collection to populate (can be <code>null</code>)
   * @param items the items with which to populate the collection (can be
   *            <code>null</code> or empty for none)
   * @return the given collection (useful if it was anonymous)
   */
  public static <T> Collection<T> populate(final Collection<T> collection,
      final Collection<? extends T> items) {
    if (collection != null) {
      collection.clear();
      if (items != null) {
        collection.addAll(items);
      }
    }
    return collection;
  }

  /**
   * Constructor is private to prevent instantiation
   * 
   * @since 1.2.0
   */
  private CollectionUtils() {}
}