/*
    * 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.commons.collections4;
  
  import java.util.AbstractList;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Objects;
  
  import org.apache.commons.collections4.bag.HashBag;
  import org.apache.commons.collections4.functors.DefaultEquator;
  import org.apache.commons.collections4.list.FixedSizeList;
  import org.apache.commons.collections4.list.LazyList;
  import org.apache.commons.collections4.list.PredicatedList;
  import org.apache.commons.collections4.list.TransformedList;
  import org.apache.commons.collections4.list.UnmodifiableList;
  import org.apache.commons.collections4.sequence.CommandVisitor;
  import org.apache.commons.collections4.sequence.EditScript;
  import org.apache.commons.collections4.sequence.SequencesComparator;
  
  /**
   * Provides utility methods and decorators for {@link List} instances.
   *
   * @since 1.0
   */
  public class ListUtils {
  
      /**
       * A simple wrapper to use a CharSequence as List.
       */
      private static final class CharSequenceAsList extends AbstractList<Character> {
          private final CharSequence sequence;
  
          CharSequenceAsList(final CharSequence sequence) {
              this.sequence = sequence;
          }
  
          @Override
          public Character get(final int index) {
              return Character.valueOf(sequence.charAt(index));
          }
  
          @Override
          public int size() {
              return sequence.length();
          }
      }
  
      /**
       * A helper class used to construct the longest common subsequence.
       */
      private static final class LcsVisitor<E> implements CommandVisitor<E> {
          private final ArrayList<E> sequence;
  
          LcsVisitor() {
              sequence = new ArrayList<>();
          }
  
          public List<E> getSubSequence() {
              return sequence;
          }
  
          @Override
          public void visitDeleteCommand(final E object) {
              // noop
          }
  
          @Override
          public void visitInsertCommand(final E object) {
              // noop
          }
  
          @Override
          public void visitKeepCommand(final E object) {
              sequence.add(object);
          }
      }
  
      /**
       * Provides a partition view on a {@link List}.
       * @since 4.0
      */
     private static final class Partition<T> extends AbstractList<List<T>> {
         private final List<T> list;
         private final int size;
 
         private Partition(final List<T> list, final int size) {
             this.list = list;
             this.size = size;
         }
 
         @Override
         public List<T> get(final int index) {
             final int listSize = size();
             if (index < 0) {
                 throw new IndexOutOfBoundsException("Index " + index + " must not be negative");
             }
             if (index >= listSize) {
                 throw new IndexOutOfBoundsException("Index " + index + " must be less than size " +
                                                     listSize);
             }
             final int start = index * size;
             final int end = Math.min(start + size, list.size());
             return list.subList(start, end);
         }
 
         @Override
         public boolean isEmpty() {
             return list.isEmpty();
         }
 
         @Override
         public int size() {
             return (int) Math.ceil((double) list.size() / (double) size);
         }
     }
 
     /**
      * Returns either the passed in list, or if the list is {@code null},
      * the value of {@code defaultList}.
      *
      * @param <T> the element type
      * @param list  the list, possibly {@code null}
      * @param defaultList  the returned values if list is {@code null}
      * @return an empty list if the argument is {@code null}
      * @since 4.0
      */
     public static <T> List<T> defaultIfNull(final List<T> list, final List<T> defaultList) {
         return list == null ? defaultList : list;
     }
 
     /**
      * Returns an immutable empty list if the argument is {@code null},
      * or the argument itself otherwise.
      *
      * @param <T> the element type
      * @param list the list, possibly {@code null}
      * @return an empty list if the argument is {@code null}
      */
     public static <T> List<T> emptyIfNull(final List<T> list) {
         return list == null ? Collections.<T>emptyList() : list;
     }
 
     /**
      * Returns a fixed-sized list backed by the given list.
      * Elements may not be added or removed from the returned list, but
      * existing elements can be changed (for instance, via the
      * {@link List#set(int, Object)} method).
      *
      * @param <E>  the element type
      * @param list  the list whose size to fix, must not be null
      * @return a fixed-size list backed by that list
      * @throws NullPointerException  if the List is null
      */
     public static <E> List<E> fixedSizeList(final List<E> list) {
         return FixedSizeList.fixedSizeList(list);
     }
 
     /**
      * Gets the first element of a list.
      * <p>
      * Shorthand for {@code list.get(0)}
      * </p>
      *
      * @param <T> The list type.
      * @param list The list.
      * @return the first element of a list.
      * @see List#get(int)
      * @since 4.5.0-M1
      */
     public static <T> T getFirst(final List<T> list) {
         return Objects.requireNonNull(list, "list").get(0);
     }
 
     /**
      * Gets the last element of a list.
      * <p>
      * Shorthand for {@code list.get(list.size() - 1)}
      * </p>
      *
      * @param <T> The list type.
      * @param list The list.
      * @return the last element of a list.
      * @see List#get(int)
      * @since 4.5.0-M1
      */
     public static <T> T getLast(final List<T> list) {
         return Objects.requireNonNull(list, "list").get(list.size() - 1);
     }
 
     /**
      * Generates a hash code using the algorithm specified in
      * {@link java.util.List#hashCode()}.
      * <p>
      * This method is useful for implementing {@code List} when you cannot
      * extend AbstractList. The method takes Collection instances to enable other
      * collection types to use the List implementation algorithm.
      * </p>
      *
      * @see java.util.List#hashCode()
      * @param list  the list to generate the hashCode for, may be null
      * @return the hash code
      */
     public static int hashCodeForList(final Collection<?> list) {
         if (list == null) {
             return 0;
         }
         int hashCode = 1;
 
         for (final Object obj : list) {
             hashCode = 31 * hashCode + (obj == null ? 0 : obj.hashCode());
         }
         return hashCode;
     }
 
     /**
      * Finds the first index in the given List which matches the given predicate.
      * <p>
      * If the input List or predicate is null, or no element of the List
      * matches the predicate, -1 is returned.
      * </p>
      *
      * @param <E>  the element type
      * @param list the List to search, may be null
      * @param predicate  the predicate to use, may be null
      * @return the first index of an Object in the List which matches the predicate or -1 if none could be found
      */
     public static <E> int indexOf(final List<E> list, final Predicate<E> predicate) {
         if (list != null && predicate != null) {
             for (int i = 0; i < list.size(); i++) {
                 final E item = list.get(i);
                 if (predicate.test(item)) {
                     return i;
                 }
             }
         }
         return CollectionUtils.INDEX_NOT_FOUND;
     }
 
     /**
      * Returns a new list containing all elements that are contained in
      * both given lists.
      *
      * @param <E> the element type
      * @param list1  the first list
      * @param list2  the second list
      * @return  the intersection of those two lists
      * @throws NullPointerException if either list is null
      */
     public static <E> List<E> intersection(final List<? extends E> list1, final List<? extends E> list2) {
         final List<E> result = new ArrayList<>();
 
         List<? extends E> smaller = list1;
         List<? extends E> larger = list2;
         if (list1.size() > list2.size()) {
             smaller = list2;
             larger = list1;
         }
 
         final HashSet<E> hashSet = new HashSet<>(smaller);
 
         for (final E e : larger) {
             if (hashSet.contains(e)) {
                 result.add(e);
                 hashSet.remove(e);
             }
         }
         return result;
     }
 
     /**
      * Tests two lists for value-equality as per the equality contract in
      * {@link java.util.List#equals(Object)}.
      * <p>
      * This method is useful for implementing {@code List} when you cannot
      * extend AbstractList. The method takes Collection instances to enable other
      * collection types to use the List implementation algorithm.
      * </p>
      * <p>
      * The relevant text (slightly paraphrased as this is a static method) is:
      * </p>
      * <blockquote>
      * Compares the two list objects for equality.  Returns
      * {@code true} if and only if both
      * lists have the same size, and all corresponding pairs of elements in
      * the two lists are <em>equal</em>.  (Two elements {@code e1} and
      * {@code e2} are <em>equal</em> if {@code (e1==null ? e2==null :
      * e1.equals(e2))}.)  In other words, two lists are defined to be
      * equal if they contain the same elements in the same order.  This
      * definition ensures that the equals method works properly across
      * different implementations of the {@code List} interface.
      * </blockquote>
      * <p>
      * <strong>Note:</strong> The behavior of this method is undefined if the lists are
      * modified during the equals comparison.
      * </p>
      *
      * @see java.util.List
      * @param list1  the first list, may be null
      * @param list2  the second list, may be null
      * @return whether the lists are equal by value comparison
      */
     public static boolean isEqualList(final Collection<?> list1, final Collection<?> list2) {
         if (list1 == list2) {
             return true;
         }
         if (list1 == null || list2 == null || list1.size() != list2.size()) {
             return false;
         }
 
         final Iterator<?> it1 = list1.iterator();
         final Iterator<?> it2 = list2.iterator();
 
         while (it1.hasNext() && it2.hasNext()) {
             final Object obj1 = it1.next();
             final Object obj2 = it2.next();
 
             if (!Objects.equals(obj1, obj2)) {
                 return false;
             }
         }
 
         return !(it1.hasNext() || it2.hasNext());
     }
 
     /**
      * Returns a "lazy" list whose elements will be created on demand.
      * <p>
      * When the index passed to the returned list's {@link List#get(int) get}
      * method is greater than the list's size, then the factory will be used
      * to create a new object and that object will be inserted at that index.
      * </p>
      * <p>
      * For instance:
      * </p>
      * <pre>
      * Factory&lt;Date&gt; factory = new Factory&lt;Date&gt;() {
      *     public Date create() {
      *         return new Date();
      *     }
      * }
      * List&lt;Date&gt; lazy = ListUtils.lazyList(new ArrayList&lt;Date&gt;(), factory);
      * Date date = lazy.get(3);
      * </pre>
      * <p>
      * After the above code is executed, {@code date} will refer to
      * a new {@code Date} instance. Furthermore, that {@code Date}
      * instance is the fourth element in the list.  The first, second,
      * and third element are all set to {@code null}.
      * </p>
      *
      * @param <E> the element type
      * @param list  the list to make lazy, must not be null
      * @param factory  the factory for creating new objects, must not be null
      * @return a lazy list backed by the given list
      * @throws NullPointerException if the List or Factory is null
      */
     public static <E> List<E> lazyList(final List<E> list, final Factory<? extends E> factory) {
         return LazyList.lazyList(list, factory);
     }
 
     /**
      * Returns a "lazy" list whose elements will be created on demand.
      * <p>
      * When the index passed to the returned list's {@link List#get(int) get}
      * method is greater than the list's size, then the transformer will be used
      * to create a new object and that object will be inserted at that index.
      * </p>
      * <p>
      * For instance:
      * </p>
      * <pre>
      * List&lt;Integer&gt; hours = Arrays.asList(7, 5, 8, 2);
      * Transformer&lt;Integer,Date&gt; transformer = input -&gt; LocalDateTime.now().withHour(hours.get(input));
      * List&lt;LocalDateTime&gt; lazy = ListUtils.lazyList(new ArrayList&lt;LocalDateTime&gt;(), transformer);
      * Date date = lazy.get(3);
      * </pre>
      * <p>
      * After the above code is executed, {@code date} will refer to
      * a new {@code Date} instance. Furthermore, that {@code Date}
      * instance is the fourth element in the list.  The first, second,
      * and third element are all set to {@code null}.
      * </p>
      *
      * @param <E> the element type
      * @param list  the list to make lazy, must not be null
      * @param transformer  the transformer for creating new objects, must not be null
      * @return a lazy list backed by the given list
      * @throws NullPointerException if the List or Transformer is null
      */
     public static <E> List<E> lazyList(final List<E> list, final Transformer<Integer, ? extends E> transformer) {
         return LazyList.lazyList(list, transformer);
     }
 
     /**
      * Returns the longest common subsequence (LCS) of two {@link CharSequence} objects.
      * <p>
      * This is a convenience method for using {@link #longestCommonSubsequence(List, List)}
      * with {@link CharSequence} instances.
      * </p>
      *
      * @param charSequenceA  the first sequence
      * @param charSequenceB  the second sequence
      * @return the longest common subsequence as {@link String}
      * @throws NullPointerException if either sequence is {@code null}
      * @since 4.0
      */
     public static String longestCommonSubsequence(final CharSequence charSequenceA, final CharSequence charSequenceB) {
         Objects.requireNonNull(charSequenceA, "charSequenceA");
         Objects.requireNonNull(charSequenceB, "charSequenceB");
         final List<Character> lcs = longestCommonSubsequence(new CharSequenceAsList(charSequenceA),
                 new CharSequenceAsList(charSequenceB));
         final StringBuilder sb = new StringBuilder();
         for (final Character ch : lcs) {
             sb.append(ch);
         }
         return sb.toString();
     }
 
     /**
      * Returns the longest common subsequence (LCS) of two sequences (lists).
      *
      * @param <E>  the element type
      * @param a  the first list
      * @param b  the second list
      * @return the longest common subsequence
      * @throws NullPointerException if either list is {@code null}
      * @since 4.0
      */
     public static <E> List<E> longestCommonSubsequence(final List<E> a, final List<E> b) {
         return longestCommonSubsequence(a, b, DefaultEquator.defaultEquator());
     }
 
     /**
      * Returns the longest common subsequence (LCS) of two sequences (lists).
      *
      * @param <E>  the element type
      * @param listA  the first list
      * @param listB  the second list
      * @param equator  the equator used to test object equality
      * @return the longest common subsequence
      * @throws NullPointerException if either list or the equator is {@code null}
      * @since 4.0
      */
     public static <E> List<E> longestCommonSubsequence(final List<E> listA, final List<E> listB,
                                                        final Equator<? super E> equator) {
         Objects.requireNonNull(listA, "listA");
         Objects.requireNonNull(listB, "listB");
         Objects.requireNonNull(equator, "equator");
 
         final SequencesComparator<E> comparator = new SequencesComparator<>(listA, listB, equator);
         final EditScript<E> script = comparator.getScript();
         final LcsVisitor<E> visitor = new LcsVisitor<>();
         script.visit(visitor);
         return visitor.getSubSequence();
     }
 
     /**
      * Returns consecutive {@link List#subList(int, int) sublists} of a
      * list, each of the same size (the final list may be smaller). For example,
      * partitioning a list containing {@code [a, b, c, d, e]} with a partition
      * size of 3 yields {@code [[a, b, c], [d, e]]} -- an outer list containing
      * two inner lists of three and two elements, all in the original order.
      * <p>
      * The outer list is unmodifiable, but reflects the latest state of the
      * source list. The inner lists are sublist views of the original list,
      * produced on demand using {@link List#subList(int, int)}, and are subject
      * to all the usual caveats about modification as explained in that API.
      * </p>
      * <p>
      * Adapted from https://github.com/google/guava
      * </p>
      *
      * @param <T> the element type
      * @param list  the list to return consecutive sublists of
      * @param size  the desired size of each sublist (the last may be smaller)
      * @return a list of consecutive sublists
      * @throws NullPointerException if list is null
      * @throws IllegalArgumentException if size is not strictly positive
      * @since 4.0
      */
     public static <T> List<List<T>> partition(final List<T> list, final int size) {
         Objects.requireNonNull(list, "list");
         if (size <= 0) {
             throw new IllegalArgumentException("Size must be greater than 0");
         }
         return new Partition<>(list, size);
     }
 
     /**
      * Returns a predicated (validating) list backed by the given list.
      * <p>
      * Only objects that pass the test in the given predicate can be added to the list.
      * Trying to add an invalid object results in an IllegalArgumentException.
      * It is important not to use the original list after invoking this method,
      * as it is a backdoor for adding invalid objects.
      * </p>
      *
      * @param <E> the element type
      * @param list  the list to predicate, must not be null
      * @param predicate  the predicate for the list, must not be null
      * @return a predicated list backed by the given list
      * @throws NullPointerException if the List or Predicate is null
      */
     public static <E> List<E> predicatedList(final List<E> list, final Predicate<E> predicate) {
         return PredicatedList.predicatedList(list, predicate);
     }
 
     /**
      * Removes the elements in {@code remove} from {@code collection}. That is, this
      * method returns a list containing all the elements in {@code collection}
      * that are not in {@code remove}. The cardinality of an element {@code e}
      * in the returned collection is the same as the cardinality of {@code e}
      * in {@code collection} unless {@code remove} contains {@code e}, in which
      * case the cardinality is zero. This method is useful if you do not wish to modify
      * {@code collection} and thus cannot call {@code collection.removeAll(remove);}.
      * <p>
      * This implementation iterates over {@code collection}, checking each element in
      * turn to see if it's contained in {@code remove}. If it's not contained, it's added
      * to the returned list. As a consequence, it is advised to use a collection type for
      * {@code remove} that provides a fast (for example O(1)) implementation of
      * {@link Collection#contains(Object)}.
      * </p>
      *
      * @param <E>  the element type
      * @param collection  the collection from which items are removed (in the returned collection)
      * @param remove  the items to be removed from the returned {@code collection}
      * @return a {@code List} containing all the elements of {@code c} except
      * any elements that also occur in {@code remove}.
      * @throws NullPointerException if either parameter is null
      * @since 3.2
      */
     public static <E> List<E> removeAll(final Collection<E> collection, final Collection<?> remove) {
         Objects.requireNonNull(collection, "collection");
         Objects.requireNonNull(remove, "remove");
         final List<E> list = new ArrayList<>();
         for (final E obj : collection) {
             if (!remove.contains(obj)) {
                 list.add(obj);
             }
         }
         return list;
     }
 
     /**
      * Returns a List containing all the elements in {@code collection}
      * that are also in {@code retain}. The cardinality of an element {@code e}
      * in the returned list is the same as the cardinality of {@code e}
      * in {@code collection} unless {@code retain} does not contain {@code e}, in which
      * case the cardinality is zero. This method is useful if you do not wish to modify
      * the collection {@code c} and thus cannot call {@code collection.retainAll(retain);}.
      * <p>
      * This implementation iterates over {@code collection}, checking each element in
      * turn to see if it's contained in {@code retain}. If it's contained, it's added
      * to the returned list. As a consequence, it is advised to use a collection type for
      * {@code retain} that provides a fast (for example O(1)) implementation of
      * {@link Collection#contains(Object)}.
      * </p>
      *
      * @param <E>  the element type
      * @param collection  the collection whose contents are the target of the #retailAll operation
      * @param retain  the collection containing the elements to be retained in the returned collection
      * @return a {@code List} containing all the elements of {@code c}
      * that occur at least once in {@code retain}.
      * @throws NullPointerException if either parameter is null
      * @since 3.2
      */
     public static <E> List<E> retainAll(final Collection<E> collection, final Collection<?> retain) {
         final List<E> list = new ArrayList<>(Math.min(collection.size(), retain.size()));
 
         for (final E obj : collection) {
             if (retain.contains(obj)) {
                 list.add(obj);
             }
         }
         return list;
     }
 
     /**
      * Selects all elements from input collection which match the given
      * predicate into an output list.
      * <p>
      * A {@code null} predicate matches no elements.
      * </p>
      *
      * @param <E> the element type
      * @param inputCollection  the collection to get the input from, may not be null
      * @param predicate  the predicate to use, may be null
      * @return the elements matching the predicate (new list)
      * @throws NullPointerException if the input list is null
      * @since 4.0
      * @see CollectionUtils#select(Iterable, Predicate)
      */
     public static <E> List<E> select(final Collection<? extends E> inputCollection,
             final Predicate<? super E> predicate) {
         return CollectionUtils.select(inputCollection, predicate, new ArrayList<>(inputCollection.size()));
     }
 
     /**
      * Selects all elements from inputCollection which don't match the given
      * predicate into an output collection.
      * <p>
      * If the input predicate is {@code null}, the result is an empty list.
      * </p>
      *
      * @param <E> the element type
      * @param inputCollection the collection to get the input from, may not be null
      * @param predicate the predicate to use, may be null
      * @return the elements <strong>not</strong> matching the predicate (new list)
      * @throws NullPointerException if the input collection is null
      * @since 4.0
      * @see CollectionUtils#selectRejected(Iterable, Predicate)
      */
     public static <E> List<E> selectRejected(final Collection<? extends E> inputCollection,
             final Predicate<? super E> predicate) {
         return CollectionUtils.selectRejected(inputCollection, predicate, new ArrayList<>(inputCollection.size()));
     }
 
     /**
      * Subtracts all elements in the second list from the first list,
      * placing the results in a new list.
      * <p>
      * This differs from {@link List#removeAll(Collection)} in that
      * cardinality is respected; if <Code>list1</Code> contains two
      * occurrences of <Code>null</Code> and <Code>list2</Code> only
      * contains one occurrence, then the returned list will still contain
      * one occurrence.
      * </p>
      *
      * @param <E> the element type
      * @param list1  the list to subtract from
      * @param list2  the list to subtract
      * @return a new list containing the results
      * @throws NullPointerException if either list is null
      */
     public static <E> List<E> subtract(final List<E> list1, final List<? extends E> list2) {
         final ArrayList<E> result = new ArrayList<>();
         final HashBag<E> bag = new HashBag<>(list2);
         for (final E e : list1) {
             if (!bag.remove(e, 1)) {
                 result.add(e);
             }
         }
         return result;
     }
 
     /**
      * Returns the sum of the given lists.  This is their intersection
      * subtracted from their union.
      *
      * @param <E> the element type
      * @param list1  the first list
      * @param list2  the second list
      * @return  a new list containing the sum of those lists
      * @throws NullPointerException if either list is null
      */
     public static <E> List<E> sum(final List<? extends E> list1, final List<? extends E> list2) {
         return subtract(union(list1, list2), intersection(list1, list2));
     }
 
     /**
      * Returns a synchronized list backed by the given list.
      * <p>
      * You must manually synchronize on the returned list's iterator to
      * avoid non-deterministic behavior:
      * </p>
      * <pre>
      * List list = ListUtils.synchronizedList(myList);
      * synchronized (list) {
      *     Iterator i = list.iterator();
      *     while (i.hasNext()) {
      *         process (i.next());
      *     }
      * }
      * </pre>
      * <p>
      * This method is just a wrapper for {@link Collections#synchronizedList(List)}.
      * </p>
      *
      * @param <E> the element type
      * @param list  the list to synchronize, must not be null
      * @return a synchronized list backed by the given list
      * @throws NullPointerException if the list is null
      */
     public static <E> List<E> synchronizedList(final List<E> list) {
         return Collections.synchronizedList(list);
     }
 
     /**
      * Returns a transformed list backed by the given list.
      * <p>
      * This method returns a new list (decorating the specified list) that
      * will transform any new entries added to it.
      * Existing entries in the specified list will not be transformed.
      * </p>
      * <p>
      * Each object is passed through the transformer as it is added to the
      * List. It is important not to use the original list after invoking this
      * method, as it is a backdoor for adding untransformed objects.
      * </p>
      * <p>
      * Existing entries in the specified list will not be transformed.
      * If you want that behavior, see {@link TransformedList#transformedList}.
      * </p>
      *
      * @param <E> the element type
      * @param list  the list to predicate, must not be null
      * @param transformer  the transformer for the list, must not be null
      * @return a transformed list backed by the given list
      * @throws NullPointerException if the List or Transformer is null
      */
     public static <E> List<E> transformedList(final List<E> list,
                                               final Transformer<? super E, ? extends E> transformer) {
         return TransformedList.transformingList(list, transformer);
     }
 
     /**
      * Returns a new list containing the second list appended to the
      * first list.  The {@link List#addAll(Collection)} operation is
      * used to append the two given lists into a new list.
      *
      * @param <E> the element type
      * @param list1  the first list
      * @param list2  the second list
      * @return a new list containing the union of those lists
      * @throws NullPointerException if either list is null
      */
     public static <E> List<E> union(final List<? extends E> list1, final List<? extends E> list2) {
         final ArrayList<E> result = new ArrayList<>(list1.size() + list2.size());
         result.addAll(list1);
         result.addAll(list2);
         return result;
     }
 
     /**
      * Returns an unmodifiable list backed by the given list.
      * <p>
      * This method uses the implementation in the decorators subpackage.
      * </p>
      *
      * @param <E>  the element type
      * @param list  the list to make unmodifiable, must not be null
      * @return an unmodifiable list backed by the given list
      * @throws NullPointerException if the list is null
      */
     public static <E> List<E> unmodifiableList(final List<? extends E> list) {
         return UnmodifiableList.unmodifiableList(list);
     }
 
     /**
      * Don't allow instances.
      */
     private ListUtils() {
         // empty
     }
 
 }