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/* * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.util; import java.io.Serializable; import java.util.function.Function; import java.util.function.ToIntFunction; import java.util.function.ToLongFunction; import java.util.function.ToDoubleFunction; import java.util.Comparators; /** * A comparison function, which imposes a <i>total ordering</i> on some * collection of objects. Comparators can be passed to a sort method (such * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control * over the sort order. Comparators can also be used to control the order of * certain data structures (such as {@link SortedSet sorted sets} or {@link * SortedMap sorted maps}), or to provide an ordering for collections of * objects that don't have a {@link Comparable natural ordering}.<p> * * The ordering imposed by a comparator {@code c} on a set of elements * {@code S} is said to be <i>consistent with equals</i> if and only if * {@code c.compare(e1, e2)==0} has the same boolean value as * {@code e1.equals(e2)} for every {@code e1} and {@code e2} in * {@code S}.<p> * * Caution should be exercised when using a comparator capable of imposing an * ordering inconsistent with equals to order a sorted set (or sorted map). * Suppose a sorted set (or sorted map) with an explicit comparator {@code c} * is used with elements (or keys) drawn from a set {@code S}. If the * ordering imposed by {@code c} on {@code S} is inconsistent with equals, * the sorted set (or sorted map) will behave "strangely." In particular the * sorted set (or sorted map) will violate the general contract for set (or * map), which is defined in terms of {@code equals}.<p> * * For example, suppose one adds two elements {@code a} and {@code b} such that * {@code (a.equals(b) && c.compare(a, b) != 0)} * to an empty {@code TreeSet} with comparator {@code c}. * The second {@code add} operation will return * true (and the size of the tree set will increase) because {@code a} and * {@code b} are not equivalent from the tree set's perspective, even though * this is contrary to the specification of the * {@link Set#add Set.add} method.<p> * * Note: It is generally a good idea for comparators to also implement * {@code java.io.Serializable}, as they may be used as ordering methods in * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In * order for the data structure to serialize successfully, the comparator (if * provided) must implement {@code Serializable}.<p> * * For the mathematically inclined, the <i>relation</i> that defines the * <i>imposed ordering</i> that a given comparator {@code c} imposes on a * given set of objects {@code S} is:<pre> * {(x, y) such that c.compare(x, y) <= 0}. * </pre> The <i>quotient</i> for this total order is:<pre> * {(x, y) such that c.compare(x, y) == 0}. * </pre> * * It follows immediately from the contract for {@code compare} that the * quotient is an <i>equivalence relation</i> on {@code S}, and that the * imposed ordering is a <i>total order</i> on {@code S}. When we say that * the ordering imposed by {@code c} on {@code S} is <i>consistent with * equals</i>, we mean that the quotient for the ordering is the equivalence * relation defined by the objects' {@link Object#equals(Object) * equals(Object)} method(s):<pre> * {(x, y) such that x.equals(y)}. </pre> * * <p>Unlike {@code Comparable}, a comparator may optionally permit * comparison of null arguments, while maintaining the requirements for * an equivalence relation. * * <p>This interface is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @param <T> the type of objects that may be compared by this comparator * * @author Josh Bloch * @author Neal Gafter * @see Comparable * @see java.io.Serializable * @since 1.2 */ @FunctionalInterface public interface Comparator<T> { /** * Compares its two arguments for order. Returns a negative integer, * zero, or a positive integer as the first argument is less than, equal * to, or greater than the second.<p> * * The implementor must ensure that {@code sgn(compare(x, y)) == * -sgn(compare(y, x))} for all {@code x} and {@code y}. (This * implies that {@code compare(x, y)} must throw an exception if and only * if {@code compare(y, x)} throws an exception.)<p> * * The implementor must also ensure that the relation is transitive: * {@code ((compare(x, y)>0) && (compare(y, z)>0))} implies * {@code compare(x, z)>0}.<p> * * Finally, the implementor must ensure that {@code compare(x, y)==0} * implies that {@code sgn(compare(x, z))==sgn(compare(y, z))} for all * {@code z}.<p> * * It is generally the case, but <i>not</i> strictly required that * {@code (compare(x, y)==0) == (x.equals(y))}. Generally speaking, * any comparator that violates this condition should clearly indicate * this fact. The recommended language is "Note: this comparator * imposes orderings that are inconsistent with equals."<p> * * In the foregoing description, the notation * {@code sgn(}<i>expression</i>{@code )} designates the mathematical * <i>signum</i> function, which is defined to return one of {@code -1}, * {@code 0}, or {@code 1} according to whether the value of * <i>expression</i> is negative, zero, or positive, respectively. * * @param o1 the first object to be compared. * @param o2 the second object to be compared. * @return a negative integer, zero, or a positive integer as the * first argument is less than, equal to, or greater than the * second. * @throws NullPointerException if an argument is null and this * comparator does not permit null arguments * @throws ClassCastException if the arguments' types prevent them from * being compared by this comparator. */ int compare(T o1, T o2); /** * Indicates whether some other object is "equal to" this * comparator. This method must obey the general contract of * {@link Object#equals(Object)}. Additionally, this method can return * {@code true} <i>only</i> if the specified object is also a comparator * and it imposes the same ordering as this comparator. Thus, * {@code comp1.equals(comp2)} implies that {@code sgn(comp1.compare(o1, * o2))==sgn(comp2.compare(o1, o2))} for every object reference * {@code o1} and {@code o2}.<p> * * Note that it is <i>always</i> safe <i>not</i> to override * {@code Object.equals(Object)}. However, overriding this method may, * in some cases, improve performance by allowing programs to determine * that two distinct comparators impose the same order. * * @param obj the reference object with which to compare. * @return {@code true} only if the specified object is also * a comparator and it imposes the same ordering as this * comparator. * @see Object#equals(Object) * @see Object#hashCode() */ boolean equals(Object obj); /** * Returns a comparator that imposes the reverse ordering of this * comparator. * * @return a comparator that imposes the reverse ordering of this * comparator. * @since 1.8 */ default Comparator<T> reversed() { return Collections.reverseOrder(this); } /** * Returns a lexicographic-order comparator with another comparator. * If this {@code Comparator} considers two elements equal, i.e. * {@code compare(a, b) == 0}, {@code other} is used to determine the order. * * <p>The returned comparator is serializable if the specified comparator * is also serializable. * * @apiNote * For example, to sort a collection of {@code String} based on the length * and then case-insensitive natural ordering, the comparator can be * composed using following code, * * <pre>{@code * Comparator<String> cmp = Comparator.comparingInt(String::length) * .thenComparing(String.CASE_INSENSITIVE_ORDER); * }</pre> * * @param other the other comparator to be used when this comparator * compares two objects that are equal. * @return a lexicographic-order comparator composed of this and then the * other comparator * @throws NullPointerException if the argument is null. * @since 1.8 */ default Comparator<T> thenComparing(Comparator<? super T> other) { Objects.requireNonNull(other); return (Comparator<T> & Serializable) (c1, c2) -> { int res = compare(c1, c2); return (res != 0) ? res : other.compare(c1, c2); }; } /** * Returns a lexicographic-order comparator with a function that * extracts a key to be compared with the given {@code Comparator}. * * @implSpec This default implementation behaves as if {@code * thenComparing(comparing(keyExtractor, cmp))}. * * @param <U> the type of the sort key * @param keyExtractor the function used to extract the sort key * @param keyComparator the {@code Comparator} used to compare the sort key * @return a lexicographic-order comparator composed of this comparator * and then comparing on the key extracted by the keyExtractor function * @throws NullPointerException if either argument is null. * @see #comparing(Function, Comparator) * @see #thenComparing(Comparator) * @since 1.8 */ default <U> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator) { return thenComparing(comparing(keyExtractor, keyComparator)); } /** * Returns a lexicographic-order comparator with a function that * extracts a {@code Comparable} sort key. * * @implSpec This default implementation behaves as if {@code * thenComparing(comparing(keyExtractor))}. * * @param <U> the type of the {@link Comparable} sort key * @param keyExtractor the function used to extract the {@link * Comparable} sort key * @return a lexicographic-order comparator composed of this and then the * {@link Comparable} sort key. * @throws NullPointerException if the argument is null. * @see #comparing(Function) * @see #thenComparing(Comparator) * @since 1.8 */ default <U extends Comparable<? super U>> Comparator<T> thenComparing( Function<? super T, ? extends U> keyExtractor) { return thenComparing(comparing(keyExtractor)); } /** * Returns a lexicographic-order comparator with a function that * extracts an {@code int} sort key. * * @implSpec This default implementation behaves as if {@code * thenComparing(comparingInt(keyExtractor))}. * * @param keyExtractor the function used to extract the integer sort key * @return a lexicographic-order comparator composed of this and then the * {@code int} sort key * @throws NullPointerException if the argument is null. * @see #comparingInt(ToIntFunction) * @see #thenComparing(Comparator) * @since 1.8 */ default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor) { return thenComparing(comparingInt(keyExtractor)); } /** * Returns a lexicographic-order comparator with a function that * extracts a {@code long} sort key. * * @implSpec This default implementation behaves as if {@code * thenComparing(comparingLong(keyExtractor))}. * * @param keyExtractor the function used to extract the long sort key * @return a lexicographic-order comparator composed of this and then the * {@code long} sort key * @throws NullPointerException if the argument is null. * @see #comparingLong(ToLongFunction) * @see #thenComparing(Comparator) * @since 1.8 */ default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor) { return thenComparing(comparingLong(keyExtractor)); } /** * Returns a lexicographic-order comparator with a function that * extracts a {@code double} sort key. * * @implSpec This default implementation behaves as if {@code * thenComparing(comparingDouble(keyExtractor))}. * * @param keyExtractor the function used to extract the double sort key * @return a lexicographic-order comparator composed of this and then the * {@code double} sort key * @throws NullPointerException if the argument is null. * @see #comparingDouble(ToDoubleFunction) * @see #thenComparing(Comparator) * @since 1.8 */ default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) { return thenComparing(comparingDouble(keyExtractor)); } /** * Returns a comparator that imposes the reverse of the <em>natural * ordering</em>. * * <p>The returned comparator is serializable and throws {@link * NullPointerException} when comparing {@code null}. * * @param <T> the {@link Comparable} type of element to be compared * @return a comparator that imposes the reverse of the <i>natural * ordering</i> on {@code Comparable} objects. * @see Comparable * @since 1.8 */ public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() { return Collections.reverseOrder(); } /** * Returns a comparator that compares {@link Comparable} objects in natural * order. * * <p>The returned comparator is serializable and throws {@link * NullPointerException} when comparing {@code null}. * * @param <T> the {@link Comparable} type of element to be compared * @return a comparator that imposes the <i>natural ordering</i> on {@code * Comparable} objects. * @see Comparable * @since 1.8 */ @SuppressWarnings("unchecked") public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() { return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE; } /** * Returns a null-friendly comparator that considers {@code null} to be * less than non-null. When both are {@code null}, they are considered * equal. If both are non-null, the specified {@code Comparator} is used * to determine the order. If the specified comparator is {@code null}, * then the returned comparator considers all non-null values to be equal. * * <p>The returned comparator is serializable if the specified comparator * is serializable. * * @param <T> the type of the elements to be compared * @param comparator a {@code Comparator} for comparing non-null values * @return a comparator that considers {@code null} to be less than * non-null, and compares non-null objects with the supplied * {@code Comparator}. * @since 1.8 */ public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) { return new Comparators.NullComparator<>(true, comparator); } /** * Returns a null-friendly comparator that considers {@code null} to be * greater than non-null. When both are {@code null}, they are considered * equal. If both are non-null, the specified {@code Comparator} is used * to determine the order. If the specified comparator is {@code null}, * then the returned comparator considers all non-null values to be equal. * * <p>The returned comparator is serializable if the specified comparator * is serializable. * * @param <T> the type of the elements to be compared * @param comparator a {@code Comparator} for comparing non-null values * @return a comparator that considers {@code null} to be greater than * non-null, and compares non-null objects with the supplied * {@code Comparator}. * @since 1.8 */ public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) { return new Comparators.NullComparator<>(false, comparator); } /** * Accepts a function that extracts a sort key from a type {@code T}, and * returns a {@code Comparator<T>} that compares by that sort key using * the specified {@link Comparator}. * * <p>The returned comparator is serializable if the specified function * and comparator are both serializable. * * @apiNote * For example, to obtain a {@code Comparator} that compares {@code * Person} objects by their last name ignoring case differences, * * <pre>{@code * Comparator<Person> cmp = Comparator.comparing( * Person::getLastName, * String.CASE_INSENSITIVE_ORDER); * }</pre> * * @param <T> the type of element to be compared * @param <U> the type of the sort key * @param keyExtractor the function used to extract the sort key * @param keyComparator the {@code Comparator} used to compare the sort key * @return a comparator that compares by an extracted key using the * specified {@code Comparator} * @throws NullPointerException if either argument is null * @since 1.8 */ public static <T, U> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator) { Objects.requireNonNull(keyExtractor); Objects.requireNonNull(keyComparator); return (Comparator<T> & Serializable) (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1), keyExtractor.apply(c2)); } /** * Accepts a function that extracts a {@link java.lang.Comparable * Comparable} sort key from a type {@code T}, and returns a {@code * Comparator<T>} that compares by that sort key. * * <p>The returned comparator is serializable if the specified function * is also serializable. * * @apiNote * For example, to obtain a {@code Comparator} that compares {@code * Person} objects by their last name, * * <pre>{@code * Comparator<Person> byLastName = Comparator.comparing(Person::getLastName); * }</pre> * * @param <T> the type of element to be compared * @param <U> the type of the {@code Comparable} sort key * @param keyExtractor the function used to extract the {@link * Comparable} sort key * @return a comparator that compares by an extracted key * @throws NullPointerException if the argument is null * @since 1.8 */ public static <T, U extends Comparable<? super U>> Comparator<T> comparing( Function<? super T, ? extends U> keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator<T> & Serializable) (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2)); } /** * Accepts a function that extracts an {@code int} sort key from a type * {@code T}, and returns a {@code Comparator<T>} that compares by that * sort key. * * <p>The returned comparator is serializable if the specified function * is also serializable. * * @param <T> the type of element to be compared * @param keyExtractor the function used to extract the integer sort key * @return a comparator that compares by an extracted key * @see #comparing(Function) * @throws NullPointerException if the argument is null * @since 1.8 */ public static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator<T> & Serializable) (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2)); } /** * Accepts a function that extracts a {@code long} sort key from a type * {@code T}, and returns a {@code Comparator<T>} that compares by that * sort key. * * <p>The returned comparator is serializable if the specified function is * also serializable. * * @param <T> the type of element to be compared * @param keyExtractor the function used to extract the long sort key * @return a comparator that compares by an extracted key * @see #comparing(Function) * @throws NullPointerException if the argument is null * @since 1.8 */ public static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator<T> & Serializable) (c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2)); } /** * Accepts a function that extracts a {@code double} sort key from a type * {@code T}, and returns a {@code Comparator<T>} that compares by that * sort key. * * <p>The returned comparator is serializable if the specified function * is also serializable. * * @param <T> the type of element to be compared * @param keyExtractor the function used to extract the double sort key * @return a comparator that compares by an extracted key * @see #comparing(Function) * @throws NullPointerException if the argument is null * @since 1.8 */ public static <T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator<T> & Serializable) (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2)); } }