001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2026 the original author or authors.
004//
005// This library is free software; you can redistribute it and/or
006// modify it under the terms of the GNU Lesser General Public
007// License as published by the Free Software Foundation; either
008// version 2.1 of the License, or (at your option) any later version.
009//
010// This library is distributed in the hope that it will be useful,
011// but WITHOUT ANY WARRANTY; without even the implied warranty of
012// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
013// Lesser General Public License for more details.
014//
015// You should have received a copy of the GNU Lesser General Public
016// License along with this library; if not, write to the Free Software
017// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
018///////////////////////////////////////////////////////////////////////////////////////////////
019
020package com.puppycrawl.tools.checkstyle.utils;
021
022import java.util.Arrays;
023import java.util.Collection;
024import java.util.Collections;
025import java.util.List;
026import java.util.Map;
027import java.util.Set;
028
029/**
030 * <div>Note: it simply wraps the existing JDK methods to provide a workaround
031 * for Pitest survival on mutation for removal of immutable wrapping,
032 * see #13127 for more details.
033 * </div>
034 *
035 */
036public final class UnmodifiableCollectionUtil {
037
038    /**
039     * Private constructor for UnmodifiableCollectionUtil.
040     *
041     */
042    private UnmodifiableCollectionUtil() {
043    }
044
045    /**
046     * Creates an unmodifiable list based on the provided collection.
047     *
048     * @param collection the collection to create an unmodifiable list from
049     * @param <T> the type of elements in the set
050     * @return an unmodifiable list containing the elements from the provided collection
051     */
052    public static <T> List<T> unmodifiableList(List<T> collection) {
053        return Collections.unmodifiableList(collection);
054    }
055
056    /**
057     * Creates an unmodifiable list from the provided collection.
058     * If the collection is null, returns an empty unmodifiable list.
059     *
060     * @param collection the collection to create an unmodifiable list from
061     * @param <T> the type of elements in the list
062     * @return an unmodifiable list containing the elements from the provided collection,
063     *         or an empty list if the collection is null
064     */
065    public static <T> List<T> unmodifiableList(Collection<? extends T> collection) {
066        final List<T> result;
067        if (collection == null) {
068            result = Collections.emptyList();
069        }
070        else {
071            result = List.copyOf(collection);
072        }
073        return result;
074    }
075
076    /**
077     * Returns an unmodifiable view of a List containing elements of a specific type.
078     *
079     * @param items The List of items to make unmodifiable.
080     * @param elementType The Class object representing the type of elements in the list.
081     * @param <S> The generic type of elements in the input Collection.
082     * @param <T> The type of elements in the resulting unmodifiable List.
083     * @return An unmodifiable List containing elements of the specified type.
084     */
085    public static <S, T> List<T> unmodifiableList(Collection<S> items, Class<T> elementType) {
086        return items.stream()
087                .map(elementType::cast)
088                .toList();
089    }
090
091    /**
092     * Creates a copy of array.
093     *
094     * @param array Array to create a copy of
095     * @param length length of array
096     * @param <T> The type of array
097     * @return copy of array
098     */
099    public static <T> T[] copyOfArray(T[] array, int length) {
100        return Arrays.copyOf(array, length);
101    }
102
103    /**
104     * Creates a copy of Map.
105     *
106     * @param map map to create a copy of
107     * @param <K> the type of keys in the map
108     * @param <V> the type of values in the map
109     * @return an immutable copy of the input map
110     */
111    public static <K, V> Map<K, V> copyOfMap(Map<? extends K, ? extends V> map) {
112        return Map.copyOf(map);
113    }
114
115    /**
116     * Returns an immutable set containing only the specified object.
117     *
118     * @param obj the type of object in the set
119     * @param <T> the type of object
120     * @return immutable set
121     */
122    public static <T> Set<T> singleton(T obj) {
123        return Collections.singleton(obj);
124    }
125}