Eugene Susla | 6a7006a | 2017-03-13 12:57:58 -0700 | [diff] [blame] | 1 | /* |
| 2 | * Copyright (C) 2017 The Android Open Source Project |
| 3 | * |
| 4 | * Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | * you may not use this file except in compliance with the License. |
| 6 | * You may obtain a copy of the License at |
| 7 | * |
| 8 | * http://www.apache.org/licenses/LICENSE-2.0 |
| 9 | * |
| 10 | * Unless required by applicable law or agreed to in writing, software |
| 11 | * distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | * See the License for the specific language governing permissions and |
| 14 | * limitations under the License. |
| 15 | */ |
| 16 | |
| 17 | package com.android.internal.util; |
| 18 | |
| 19 | import android.annotation.NonNull; |
| 20 | import android.annotation.Nullable; |
| 21 | |
| 22 | import java.util.ArrayList; |
| 23 | import java.util.Collection; |
| 24 | import java.util.Collections; |
| 25 | import java.util.List; |
| 26 | import java.util.function.Function; |
| 27 | import java.util.stream.Stream; |
| 28 | |
| 29 | /** |
| 30 | * Utility methods for dealing with (typically {@link Nullable}) {@link Collection}s |
| 31 | * |
| 32 | * Unless a method specifies otherwise, a null value for a collection is treated as an empty |
| 33 | * collection of that type. |
| 34 | */ |
| 35 | public class CollectionUtils { |
| 36 | private CollectionUtils() { /* cannot be instantiated */ } |
| 37 | |
| 38 | /** |
| 39 | * Returns a list of items from the provided list that match the given condition. |
| 40 | * |
| 41 | * This is similar to {@link Stream#filter} but without the overhead of creating an intermediate |
| 42 | * {@link Stream} instance |
| 43 | */ |
| 44 | public static @NonNull <T> List<T> filter(@Nullable List<T> list, |
| 45 | java.util.function.Predicate<? super T> predicate) { |
| 46 | ArrayList<T> result = null; |
| 47 | for (int i = 0; i < size(list); i++) { |
| 48 | final T item = list.get(i); |
| 49 | if (predicate.test(item)) { |
| 50 | result = ArrayUtils.add(result, item); |
| 51 | } |
| 52 | } |
| 53 | return emptyIfNull(result); |
| 54 | } |
| 55 | |
| 56 | /** |
| 57 | * Returns a list of items resulting from applying the given function to each element of the |
| 58 | * provided list. |
| 59 | * |
| 60 | * The resulting list will have the same {@link #size} as the input one. |
| 61 | * |
| 62 | * This is similar to {@link Stream#map} but without the overhead of creating an intermediate |
| 63 | * {@link Stream} instance |
| 64 | */ |
| 65 | public static @NonNull <I, O> List<O> map(@Nullable List<I> cur, |
| 66 | Function<? super I, ? extends O> f) { |
| 67 | if (cur == null || cur.isEmpty()) return Collections.emptyList(); |
| 68 | final ArrayList<O> result = new ArrayList<>(); |
| 69 | for (int i = 0; i < cur.size(); i++) { |
| 70 | result.add(f.apply(cur.get(i))); |
| 71 | } |
| 72 | return result; |
| 73 | } |
| 74 | |
| 75 | /** |
| 76 | * Returns the given list, or an immutable empty list if the provided list is null |
| 77 | * |
| 78 | * This can be used to guaranty null-safety without paying the price of extra allocations |
| 79 | * |
| 80 | * @see Collections#emptyList |
| 81 | */ |
| 82 | public static @NonNull <T> List<T> emptyIfNull(@Nullable List<T> cur) { |
| 83 | return cur == null ? Collections.emptyList() : cur; |
| 84 | } |
| 85 | |
| 86 | /** |
| 87 | * Returns the size of the given list, or 0 if the list is null |
| 88 | */ |
| 89 | public static int size(@Nullable Collection<?> cur) { |
| 90 | return cur != null ? cur.size() : 0; |
| 91 | } |
| 92 | |
| 93 | /** |
| 94 | * Returns the elements of the given list that are of type {@code c} |
| 95 | */ |
| 96 | public static @NonNull <T> List<T> filter(@Nullable List<?> list, Class<T> c) { |
| 97 | if (ArrayUtils.isEmpty(list)) return Collections.emptyList(); |
| 98 | ArrayList<T> result = null; |
| 99 | for (int i = 0; i < list.size(); i++) { |
| 100 | final Object item = list.get(i); |
| 101 | if (c.isInstance(item)) { |
| 102 | result = ArrayUtils.add(result, (T) item); |
| 103 | } |
| 104 | } |
| 105 | return emptyIfNull(result); |
| 106 | } |
| 107 | |
| 108 | /** |
| 109 | * Returns whether there exists at least one element in the list for which |
| 110 | * condition {@code predicate} is true |
| 111 | */ |
| 112 | public static <T> boolean any(@Nullable List<T> items, |
| 113 | java.util.function.Predicate<T> predicate) { |
| 114 | return find(items, predicate) != null; |
| 115 | } |
| 116 | |
| 117 | /** |
| 118 | * Returns the first element from the list for which |
| 119 | * condition {@code predicate} is true, or null if there is no such element |
| 120 | */ |
| 121 | public static @Nullable <T> T find(@Nullable List<T> items, |
| 122 | java.util.function.Predicate<T> predicate) { |
| 123 | if (ArrayUtils.isEmpty(items)) return null; |
| 124 | for (int i = 0; i < items.size(); i++) { |
| 125 | final T item = items.get(i); |
| 126 | if (predicate.test(item)) return item; |
| 127 | } |
| 128 | return null; |
| 129 | } |
Eugene Susla | 4df89bc | 2017-03-28 20:27:28 -0700 | [diff] [blame^] | 130 | |
| 131 | /** |
| 132 | * Similar to {@link List#add}, but with support for list values of {@code null} and |
| 133 | * {@link Collections#emptyList} |
| 134 | */ |
| 135 | public static @NonNull <T> List<T> add(@Nullable List<T> cur, T val) { |
| 136 | if (cur == null || cur == Collections.emptyList()) { |
| 137 | cur = new ArrayList<>(); |
| 138 | } |
| 139 | cur.add(val); |
| 140 | return cur; |
| 141 | } |
| 142 | |
| 143 | /** |
| 144 | * Similar to {@link List#remove}, but with support for list values of {@code null} and |
| 145 | * {@link Collections#emptyList} |
| 146 | */ |
| 147 | public static @NonNull <T> List<T> remove(@Nullable List<T> cur, T val) { |
| 148 | if (cur == null || cur == Collections.emptyList()) { |
| 149 | return Collections.emptyList(); |
| 150 | } |
| 151 | cur.remove(val); |
| 152 | return cur; |
| 153 | } |
| 154 | |
Eugene Susla | 6a7006a | 2017-03-13 12:57:58 -0700 | [diff] [blame] | 155 | } |