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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.
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/*
* 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 {
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 implements CommandVisitor {
private final ArrayList sequence;
LcsVisitor() {
sequence = new ArrayList<>();
}
public List 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 extends AbstractList> {
private final List list;
private final int size;
private Partition(final List list, final int size) {
this.list = list;
this.size = size;
}
@Override
public List 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 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 List defaultIfNull(final List list, final List defaultList) {
return list == null ? defaultList : list;
}
/**
* Returns an immutable empty list if the argument is {@code null},
* or the argument itself otherwise.
*
* @param the element type
* @param list the list, possibly {@code null}
* @return an empty list if the argument is {@code null}
*/
public static List emptyIfNull(final List list) {
return list == null ? Collections.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 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 List fixedSizeList(final List list) {
return FixedSizeList.fixedSizeList(list);
}
/**
* Gets the first element of a list.
*
* Shorthand for {@code list.get(0)}
*
*
* @param 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 getFirst(final List list) {
return Objects.requireNonNull(list, "list").get(0);
}
/**
* Gets the last element of a list.
*
* Shorthand for {@code list.get(list.size() - 1)}
*
*
* @param 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 getLast(final List list) {
return Objects.requireNonNull(list, "list").get(list.size() - 1);
}
/**
* Generates a hash code using the algorithm specified in
* {@link java.util.List#hashCode()}.
*
* 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.
*
*
* @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.
*
* If the input List or predicate is null, or no element of the List
* matches the predicate, -1 is returned.
*
*
* @param 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 int indexOf(final List list, final Predicate 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 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 List intersection(final List list1, final List list2) {
final List result = new ArrayList<>();
List smaller = list1;
List larger = list2;
if (list1.size() > list2.size()) {
smaller = list2;
larger = list1;
}
final HashSet 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)}.
*
* 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.
*
*
* The relevant text (slightly paraphrased as this is a static method) is:
*
*
* 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 equal. (Two elements {@code e1} and
* {@code e2} are equal 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.
*
*
* Note: The behavior of this method is undefined if the lists are
* modified during the equals comparison.
*
*
* @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.
*
* 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.
*
*
* For instance:
*
*
* Factory<Date> factory = new Factory<Date>() {
* public Date create() {
* return new Date();
* }
* }
* List<Date> lazy = ListUtils.lazyList(new ArrayList<Date>(), factory);
* Date date = lazy.get(3);
*
*
* 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}.
*
*
* @param 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 List lazyList(final List list, final Factory factory) {
return LazyList.lazyList(list, factory);
}
/**
* Returns a "lazy" list whose elements will be created on demand.
*
* 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.
*
*
* For instance:
*
*
* List<Integer> hours = Arrays.asList(7, 5, 8, 2);
* Transformer<Integer,Date> transformer = input -> LocalDateTime.now().withHour(hours.get(input));
* List<LocalDateTime> lazy = ListUtils.lazyList(new ArrayList<LocalDateTime>(), transformer);
* Date date = lazy.get(3);
*
*
* 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}.
*
*
* @param 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 List lazyList(final List list, final Transformer transformer) {
return LazyList.lazyList(list, transformer);
}
/**
* Returns the longest common subsequence (LCS) of two {@link CharSequence} objects.
*
* This is a convenience method for using {@link #longestCommonSubsequence(List, List)}
* with {@link CharSequence} instances.
*
*
* @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 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 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 List longestCommonSubsequence(final List a, final List b) {
return longestCommonSubsequence(a, b, DefaultEquator.defaultEquator());
}
/**
* Returns the longest common subsequence (LCS) of two sequences (lists).
*
* @param 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 List longestCommonSubsequence(final List listA, final List listB,
final Equator equator) {
Objects.requireNonNull(listA, "listA");
Objects.requireNonNull(listB, "listB");
Objects.requireNonNull(equator, "equator");
final SequencesComparator comparator = new SequencesComparator<>(listA, listB, equator);
final EditScript script = comparator.getScript();
final LcsVisitor 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.
*
* 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.
*
*
* Adapted from https://github.com/google/guava
*
*
* @param 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 List> partition(final List 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.
*
* 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.
*
*
* @param 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 List predicatedList(final List list, final Predicate 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);}.
*
* 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 (e.g. O(1)) implementation of
* {@link Collection#contains(Object)}.
*
*
* @param 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 List removeAll(final Collection collection, final Collection remove) {
Objects.requireNonNull(collection, "collection");
Objects.requireNonNull(remove, "remove");
final List 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);}.
*
* 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 (e.g. O(1)) implementation of
* {@link Collection#contains(Object)}.
*
*
* @param 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 List retainAll(final Collection collection, final Collection retain) {
final List 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.
*
* A {@code null} predicate matches no elements.
*
*
* @param 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 List select(final Collection inputCollection,
final Predicate 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.
*
* If the input predicate is {@code null}, the result is an empty list.
*
*
* @param 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 not matching the predicate (new list)
* @throws NullPointerException if the input collection is null
* @since 4.0
* @see CollectionUtils#selectRejected(Iterable, Predicate)
*/
public static List selectRejected(final Collection inputCollection,
final Predicate 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.
*
* This differs from {@link List#removeAll(Collection)} in that
* cardinality is respected; if list1 contains two
* occurrences of null and list2 only
* contains one occurrence, then the returned list will still contain
* one occurrence.
*
*
* @param 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 List subtract(final List list1, final List list2) {
final ArrayList result = new ArrayList<>();
final HashBag 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 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 List sum(final List list1, final List list2) {
return subtract(union(list1, list2), intersection(list1, list2));
}
/**
* Returns a synchronized list backed by the given list.
*
* You must manually synchronize on the returned list's iterator to
* avoid non-deterministic behavior:
*
*
* List list = ListUtils.synchronizedList(myList);
* synchronized (list) {
* Iterator i = list.iterator();
* while (i.hasNext()) {
* process (i.next());
* }
* }
*
*
* This method is just a wrapper for {@link Collections#synchronizedList(List)}.
*
*
* @param 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 List synchronizedList(final List list) {
return Collections.synchronizedList(list);
}
/**
* Returns a transformed list backed by the given list.
*
* 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.
*
*
* 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.
*
*
* Existing entries in the specified list will not be transformed.
* If you want that behavior, see {@link TransformedList#transformedList}.
*
*
* @param 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 List transformedList(final List list,
final Transformer 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 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 List union(final List list1, final List list2) {
final ArrayList result = new ArrayList<>(list1.size() + list2.size());
result.addAll(list1);
result.addAll(list2);
return result;
}
/**
* Returns an unmodifiable list backed by the given list.
*
* This method uses the implementation in the decorators subpackage.
*
*
* @param 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 List unmodifiableList(final List list) {
return UnmodifiableList.unmodifiableList(list);
}
/**
* Don't allow instances.
*/
private ListUtils() {
// empty
}
}
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