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