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;
025
026/**
027 * Provides static methods for working with {@link Comparator} instances. For many other helpful
028 * comparator utilities, see either {@code Comparator} itself (for Java 8 or later), or
029 * {@code com.google.common.collect.Ordering} (otherwise).
030 *
031 * <h3>Relationship to {@code Ordering}</h3>
032 *
033 * <p>In light of the significant enhancements to {@code Comparator} in Java 8, the overwhelming
034 * majority of usages of {@code Ordering} can be written using only built-in JDK APIs. Because of
035 * this, and because it's awkward to have to convert comparators into {@code Ordering} instances,
036 * {@code Ordering} and its methods are planned for deletion. This class is intended to
037 * "fill the gap" and provide those features of {@code Ordering} not already provided by the JDK.
038 *
039 * @since 21.0
040 * @author Louis Wasserman
041 */
042@Beta
043@GwtCompatible
044public final class Comparators {
045  private Comparators() {}
046
047  /**
048   * Returns a new comparator which sorts iterables by comparing corresponding elements pairwise
049   * until a nonzero result is found; imposes "dictionary order." If the end of one iterable is
050   * reached, but not the other, the shorter iterable is considered to be less than the longer one.
051   * For example, a lexicographical natural ordering over integers considers {@code
052   * [] < [1] < [1, 1] < [1, 2] < [2]}.
053   *
054   * <p>Note that {@code Collections.reverseOrder(lexicographical(comparator))} is not
055   * equivalent to {@code lexicographical(Collections.reverseOrder(comparator))} (consider how each
056   * would order {@code [1]} and {@code [1, 1]}).
057   */
058  // Note: 90% of the time we don't add type parameters or wildcards that serve only to "tweak" the
059  // desired return type. However, *nested* generics introduce a special class of problems that we
060  // think tip it over into being worthwhile.
061  public static <T, S extends T> Comparator<Iterable<S>> lexicographical(Comparator<T> comparator) {
062    return new LexicographicalOrdering<S>(checkNotNull(comparator));
063  }
064
065  /**
066   * Returns {@code true} if each element in {@code iterable} after the first is greater than or
067   * equal to the element that preceded it, according to the specified comparator. Note that this
068   * is always true when the iterable has fewer than two elements.
069   */
070  public static <T> boolean isInOrder(Iterable<? extends T> iterable, Comparator<T> comparator) {
071    checkNotNull(comparator);
072    Iterator<? extends T> it = iterable.iterator();
073    if (it.hasNext()) {
074      T prev = it.next();
075      while (it.hasNext()) {
076        T next = it.next();
077        if (comparator.compare(prev, next) > 0) {
078          return false;
079        }
080        prev = next;
081      }
082    }
083    return true;
084  }
085
086  /**
087   * Returns {@code true} if each element in {@code iterable} after the first is <i>strictly</i>
088   * greater than the element that preceded it, according to the specified comparator. Note that
089   * this is always true when the iterable has fewer than two elements.
090   */
091  public static <T> boolean isInStrictOrder(
092      Iterable<? extends T> iterable, Comparator<T> comparator) {
093    checkNotNull(comparator);
094    Iterator<? extends T> it = iterable.iterator();
095    if (it.hasNext()) {
096      T prev = it.next();
097      while (it.hasNext()) {
098        T next = it.next();
099        if (comparator.compare(prev, next) >= 0) {
100          return false;
101        }
102        prev = next;
103      }
104    }
105    return true;
106  }
107}