001/*
002 * Copyright (C) 2016 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkNotNull;
020
021import com.google.common.annotations.Beta;
022import com.google.common.annotations.GwtCompatible;
023import java.util.Comparator;
024import java.util.Iterator;
025import org.checkerframework.checker.nullness.qual.Nullable;
026
027/**
028 * Provides static methods for working with {@link Comparator} instances. For many other helpful
029 * comparator utilities, see either {@code Comparator} itself (for Java 8 or later), or {@code
030 * com.google.common.collect.Ordering} (otherwise).
031 *
032 * <h3>Relationship to {@code Ordering}</h3>
033 *
034 * <p>In light of the significant enhancements to {@code Comparator} in Java 8, the overwhelming
035 * majority of usages of {@code Ordering} can be written using only built-in JDK APIs. This class is
036 * intended to "fill the gap" and provide those features of {@code Ordering} not already provided by
037 * the JDK.
038 *
039 * @since 21.0
040 * @author Louis Wasserman
041 */
042@Beta
043@GwtCompatible
044@ElementTypesAreNonnullByDefault
045public final class Comparators {
046  private Comparators() {}
047
048  /**
049   * Returns a new comparator which sorts iterables by comparing corresponding elements pairwise
050   * until a nonzero result is found; imposes "dictionary order." If the end of one iterable is
051   * reached, but not the other, the shorter iterable is considered to be less than the longer one.
052   * For example, a lexicographical natural ordering over integers considers {@code [] < [1] < [1,
053   * 1] < [1, 2] < [2]}.
054   *
055   * <p>Note that {@code Collections.reverseOrder(lexicographical(comparator))} is not equivalent to
056   * {@code lexicographical(Collections.reverseOrder(comparator))} (consider how each would order
057   * {@code [1]} and {@code [1, 1]}).
058   */
059  // Note: 90% of the time we don't add type parameters or wildcards that serve only to "tweak" the
060  // desired return type. However, *nested* generics introduce a special class of problems that we
061  // think tip it over into being worthwhile.
062  public static <T extends @Nullable Object, S extends T> Comparator<Iterable<S>> lexicographical(
063      Comparator<T> comparator) {
064    return new LexicographicalOrdering<S>(checkNotNull(comparator));
065  }
066
067  /**
068   * Returns {@code true} if each element in {@code iterable} after the first is greater than or
069   * equal to the element that preceded it, according to the specified comparator. Note that this is
070   * always true when the iterable has fewer than two elements.
071   */
072  public static <T extends @Nullable Object> boolean isInOrder(
073      Iterable<? extends T> iterable, Comparator<T> comparator) {
074    checkNotNull(comparator);
075    Iterator<? extends T> it = iterable.iterator();
076    if (it.hasNext()) {
077      T prev = it.next();
078      while (it.hasNext()) {
079        T next = it.next();
080        if (comparator.compare(prev, next) > 0) {
081          return false;
082        }
083        prev = next;
084      }
085    }
086    return true;
087  }
088
089  /**
090   * Returns {@code true} if each element in {@code iterable} after the first is <i>strictly</i>
091   * greater than the element that preceded it, according to the specified comparator. Note that
092   * this is always true when the iterable has fewer than two elements.
093   */
094  public static <T extends @Nullable Object> boolean isInStrictOrder(
095      Iterable<? extends T> iterable, Comparator<T> comparator) {
096    checkNotNull(comparator);
097    Iterator<? extends T> it = iterable.iterator();
098    if (it.hasNext()) {
099      T prev = it.next();
100      while (it.hasNext()) {
101        T next = it.next();
102        if (comparator.compare(prev, next) >= 0) {
103          return false;
104        }
105        prev = next;
106      }
107    }
108    return true;
109  }
110
111  /**
112   * Returns the minimum of the two values. If the values compare as 0, the first is returned.
113   *
114   * <p>The recommended solution for finding the {@code minimum} of some values depends on the type
115   * of your data and the number of elements you have. Read more in the Guava User Guide article on
116   * <a href="https://github.com/google/guava/wiki/CollectionUtilitiesExplained#comparators">{@code
117   * Comparators}</a>.
118   *
119   * @param a first value to compare, returned if less than or equal to b.
120   * @param b second value to compare.
121   * @throws ClassCastException if the parameters are not <i>mutually comparable</i>.
122   * @since 30.0
123   */
124  @Beta
125  public static <T extends Comparable<? super T>> T min(T a, T b) {
126    return (a.compareTo(b) <= 0) ? a : b;
127  }
128
129  /**
130   * Returns the minimum of the two values, according to the given comparator. If the values compare
131   * as equal, the first is returned.
132   *
133   * <p>The recommended solution for finding the {@code minimum} of some values depends on the type
134   * of your data and the number of elements you have. Read more in the Guava User Guide article on
135   * <a href="https://github.com/google/guava/wiki/CollectionUtilitiesExplained#comparators">{@code
136   * Comparators}</a>.
137   *
138   * @param a first value to compare, returned if less than or equal to b
139   * @param b second value to compare.
140   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> using the given
141   *     comparator.
142   * @since 30.0
143   */
144  @Beta
145  @ParametricNullness
146  public static <T extends @Nullable Object> T min(
147      @ParametricNullness T a, @ParametricNullness T b, Comparator<T> comparator) {
148    return (comparator.compare(a, b) <= 0) ? a : b;
149  }
150
151  /**
152   * Returns the maximum of the two values. If the values compare as 0, the first is returned.
153   *
154   * <p>The recommended solution for finding the {@code maximum} of some values depends on the type
155   * of your data and the number of elements you have. Read more in the Guava User Guide article on
156   * <a href="https://github.com/google/guava/wiki/CollectionUtilitiesExplained#comparators">{@code
157   * Comparators}</a>.
158   *
159   * @param a first value to compare, returned if greater than or equal to b.
160   * @param b second value to compare.
161   * @throws ClassCastException if the parameters are not <i>mutually comparable</i>.
162   * @since 30.0
163   */
164  @Beta
165  public static <T extends Comparable<? super T>> T max(T a, T b) {
166    return (a.compareTo(b) >= 0) ? a : b;
167  }
168
169  /**
170   * Returns the maximum of the two values, according to the given comparator. If the values compare
171   * as equal, the first is returned.
172   *
173   * <p>The recommended solution for finding the {@code maximum} of some values depends on the type
174   * of your data and the number of elements you have. Read more in the Guava User Guide article on
175   * <a href="https://github.com/google/guava/wiki/CollectionUtilitiesExplained#comparators">{@code
176   * Comparators}</a>.
177   *
178   * @param a first value to compare, returned if greater than or equal to b.
179   * @param b second value to compare.
180   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> using the given
181   *     comparator.
182   * @since 30.0
183   */
184  @Beta
185  @ParametricNullness
186  public static <T extends @Nullable Object> T max(
187      @ParametricNullness T a, @ParametricNullness T b, Comparator<T> comparator) {
188    return (comparator.compare(a, b) >= 0) ? a : b;
189  }
190}