/* * Copyright (c) 1997, 2021, 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 total ordering 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 {@linkplain SortedSet sorted sets} or * {@linkplain SortedMap sorted maps}), or to provide an ordering for * collections of objects that don't have a {@linkplain Comparable * natural ordering}.

* * The ordering imposed by a comparator {@code c} on a set of elements * {@code S} is said to be consistent with equals 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}.

* * 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}.

* * 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.

* * 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}.

* * For the mathematically inclined, the relation that defines the * imposed ordering that a given comparator {@code c} imposes on a * given set of objects {@code S} is:

 *       {(x, y) such that c.compare(x, y) <= 0}.
 *
The quotient for this total order is:
 *       {(x, y) such that c.compare(x, y) == 0}.
 *
* * It follows immediately from the contract for {@code compare} that the * quotient is an equivalence relation on {@code S}, and that the * imposed ordering is a total order on {@code S}. When we say that * the ordering imposed by {@code c} on {@code S} is consistent with * equals, we mean that the quotient for the ordering is the equivalence * relation defined by the objects' {@link Object#equals(Object) * equals(Object)} method(s):
 *     {(x, y) such that x.equals(y)}.
* * In other words, when the imposed ordering is consistent with * equals, the equivalence classes defined by the equivalence relation * of the {@code equals} method and the equivalence classes defined by * the quotient of the {@code compare} method are the same. *

Unlike {@code Comparable}, a comparator may optionally permit * comparison of null arguments, while maintaining the requirements for * an equivalence relation. * *

This interface is a member of the * * Java Collections Framework. * * @param 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 { /** * 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.

* * The implementor must ensure that {@link Integer#signum * signum}{@code (compare(x, y)) == -signum(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.)

* * 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}.

* * Finally, the implementor must ensure that {@code compare(x, * y)==0} implies that {@code signum(compare(x, * z))==signum(compare(y, z))} for all {@code z}. * * @apiNote * It is generally the case, but not 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." * * @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} only if the specified object is also * a comparator and it imposes the same ordering as this * comparator. Thus, {@code comp1.equals(comp2)} implies that * {@link Integer#signum signum}{@code (comp1.compare(o1, * o2))==signum(comp2.compare(o1, o2))} for every object reference * {@code o1} and {@code o2}.

* * Note that it is always safe not 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 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. * *

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, * * 

{@code
     *     Comparator cmp = Comparator.comparingInt(String::length)
     *             .thenComparing(String.CASE_INSENSITIVE_ORDER);
     * }
* * @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 thenComparing(Comparator other) { Objects.requireNonNull(other); return (Comparator & 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 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 Comparator thenComparing( Function keyExtractor, Comparator 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 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 > Comparator thenComparing( Function 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 thenComparingInt(ToIntFunction 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 thenComparingLong(ToLongFunction 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 thenComparingDouble(ToDoubleFunction keyExtractor) { return thenComparing(comparingDouble(keyExtractor)); } /** * Returns a comparator that imposes the reverse of the natural * ordering. * *

The returned comparator is serializable and throws {@link * NullPointerException} when comparing {@code null}. * * @param the {@link Comparable} type of element to be compared * @return a comparator that imposes the reverse of the natural * ordering on {@code Comparable} objects. * @see Comparable * @since 1.8 */ public static > Comparator reverseOrder() { return Collections.reverseOrder(); } /** * Returns a comparator that compares {@link Comparable} objects in natural * order. * *

The returned comparator is serializable and throws {@link * NullPointerException} when comparing {@code null}. * * @param the {@link Comparable} type of element to be compared * @return a comparator that imposes the natural ordering on {@code * Comparable} objects. * @see Comparable * @since 1.8 */ @SuppressWarnings("unchecked") public static > Comparator naturalOrder() { return (Comparator) 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. * *

The returned comparator is serializable if the specified comparator * is serializable. * * @param 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 Comparator nullsFirst(Comparator 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. * *

The returned comparator is serializable if the specified comparator * is serializable. * * @param 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 Comparator nullsLast(Comparator 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} that compares by that sort key using * the specified {@link Comparator}. * *

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, * * 

{@code
     *     Comparator cmp = Comparator.comparing(
     *             Person::getLastName,
     *             String.CASE_INSENSITIVE_ORDER);
     * }
* * @param the type of element to be compared * @param 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 Comparator comparing( Function keyExtractor, Comparator keyComparator) { Objects.requireNonNull(keyExtractor); Objects.requireNonNull(keyComparator); return (Comparator & 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} that compares by that sort key. * *

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, * * 

{@code
     *     Comparator byLastName = Comparator.comparing(Person::getLastName);
     * }
* * @param the type of element to be compared * @param 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 > Comparator comparing( Function keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator & 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} that compares by that * sort key. * *

The returned comparator is serializable if the specified function * is also serializable. * * @param 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 Comparator comparingInt(ToIntFunction keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator & 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} that compares by that * sort key. * *

The returned comparator is serializable if the specified function is * also serializable. * * @param 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 Comparator comparingLong(ToLongFunction keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator & 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} that compares by that * sort key. * *

The returned comparator is serializable if the specified function * is also serializable. * * @param 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 Comparator comparingDouble(ToDoubleFunction keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator & Serializable) (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2)); } }