Java tutorial
/* * Copyright (C) 2016 The Guava Authors * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package com.google.common.collect; import static com.google.common.base.Preconditions.checkNotNull; import com.google.common.annotations.Beta; import com.google.common.annotations.GwtCompatible; import java.util.Comparator; import java.util.Iterator; /** * Provides static methods for working with {@link Comparator} instances. For many other helpful * comparator utilities, see either {@code Comparator} itself (for Java 8 or later), or * {@code com.google.common.collect.Ordering} (otherwise). * * <h3>Relationship to {@code Ordering}</h3> * * <p>In light of the significant enhancements to {@code Comparator} in Java 8, the overwhelming * majority of usages of {@code Ordering} can be written using only built-in JDK APIs. Because of * this, and because it's awkward to have to convert comparators into {@code Ordering} instances, * {@code Ordering} and its methods are planned for deletion. This class is intended to * "fill the gap" and provide those features of {@code Ordering} not already provided by the JDK. * * @since 21.0 * @author Louis Wasserman */ @Beta @GwtCompatible public final class Comparators { private Comparators() { } /** * Returns a new comparator which sorts iterables by comparing corresponding elements pairwise * until a nonzero result is found; imposes "dictionary order." If the end of one iterable is * reached, but not the other, the shorter iterable is considered to be less than the longer one. * For example, a lexicographical natural ordering over integers considers {@code * [] < [1] < [1, 1] < [1, 2] < [2]}. * * <p>Note that {@code Collections.reverseOrder(lexicographical(comparator))} is not * equivalent to {@code lexicographical(Collections.reverseOrder(comparator))} (consider how each * would order {@code [1]} and {@code [1, 1]}). */ // Note: 90% of the time we don't add type parameters or wildcards that serve only to "tweak" the // desired return type. However, *nested* generics introduce a special class of problems that we // think tip it over into being worthwhile. public static <T, S extends T> Comparator<Iterable<S>> lexicographical(Comparator<T> comparator) { return new LexicographicalOrdering<S>(checkNotNull(comparator)); } /** * Returns {@code true} if each element in {@code iterable} after the first is greater than or * equal to the element that preceded it, according to the specified comparator. Note that this * is always true when the iterable has fewer than two elements. */ public static <T> boolean isInOrder(Iterable<? extends T> iterable, Comparator<T> comparator) { checkNotNull(comparator); Iterator<? extends T> it = iterable.iterator(); if (it.hasNext()) { T prev = it.next(); while (it.hasNext()) { T next = it.next(); if (comparator.compare(prev, next) > 0) { return false; } prev = next; } } return true; } /** * Returns {@code true} if each element in {@code iterable} after the first is <i>strictly</i> * greater than the element that preceded it, according to the specified comparator. Note that * this is always true when the iterable has fewer than two elements. */ public static <T> boolean isInStrictOrder(Iterable<? extends T> iterable, Comparator<T> comparator) { checkNotNull(comparator); Iterator<? extends T> it = iterable.iterator(); if (it.hasNext()) { T prev = it.next(); while (it.hasNext()) { T next = it.next(); if (comparator.compare(prev, next) >= 0) { return false; } prev = next; } } return true; } }