280 lines
11 KiB
Java
280 lines
11 KiB
Java
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/*
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* Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.util;
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import java.io.Serializable;
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import java.util.function.BinaryOperator;
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import java.util.function.Function;
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import java.util.function.ToDoubleFunction;
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import java.util.function.ToIntFunction;
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import java.util.function.ToLongFunction;
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/**
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* This class consists of {@code static} utility methods for comparators. Mostly
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* factory method that returns a {@link Comparator}.
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*
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* <p> Unless otherwise noted, passing a {@code null} argument to a method in
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* this class will cause a {@link NullPointerException} to be thrown.
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*
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* @see Comparator
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* @since 1.8
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*/
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public class Comparators {
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private Comparators() {
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throw new AssertionError("no instances");
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}
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/**
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* Compares {@link Comparable} objects in natural order.
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*
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* @see Comparable
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*/
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private enum NaturalOrderComparator implements Comparator<Comparable<Object>> {
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INSTANCE;
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@Override
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public int compare(Comparable<Object> c1, Comparable<Object> c2) {
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return c1.compareTo(c2);
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}
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}
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/**
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* Returns a comparator that imposes the reverse of the <em>natural
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* ordering</em>.
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*
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* <p>The returned comparator is serializable.
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*
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* @param <T> {@link Comparable} type
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*
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* @return A comparator that imposes the reverse of the <i>natural
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* ordering</i> on a collection of objects that implement
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* the {@link Comparable} interface.
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* @see Comparable
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*/
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public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
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return Collections.reverseOrder();
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}
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/**
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* Returns a comparator that imposes the reverse ordering of the specified
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* {@link Comparator}.
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*
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* <p>The returned comparator is serializable (assuming the specified
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* comparator is also serializable).
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*
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* @param <T> the element type to be compared
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* @param cmp a comparator whose ordering is to be reversed by the returned
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* comparator
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* @return A comparator that imposes the reverse ordering of the
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* specified comparator.
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*/
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public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
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Objects.requireNonNull(cmp);
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return Collections.reverseOrder(cmp);
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}
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/**
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* Gets a comparator compares {@link Comparable} type in natural order.
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*
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* @param <T> {@link Comparable} type
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*/
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public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
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return (Comparator<T>) NaturalOrderComparator.INSTANCE;
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}
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/**
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* Gets a comparator compares {@link Map.Entry} in natural order on key.
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*
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* @param <K> {@link Comparable} key type
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* @param <V> value type
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*/
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public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> naturalOrderKeys() {
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return (Comparator<Map.Entry<K, V>> & Serializable)
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(c1, c2) -> c1.getKey().compareTo(c2.getKey());
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}
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/**
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* Gets a comparator compares {@link Map.Entry} in natural order on value.
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*
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* @param <K> key type
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* @param <V> {@link Comparable} value type
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*/
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public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> naturalOrderValues() {
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return (Comparator<Map.Entry<K, V>> & Serializable)
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(c1, c2) -> c1.getValue().compareTo(c2.getValue());
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}
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/**
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* Gets a comparator compares {@link Map.Entry} by key using the given
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* {@link Comparator}.
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*
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* <p>The returned comparator is serializable assuming the specified
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* comparators are also serializable.
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*
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* @param <K> key type
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* @param <V> value type
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* @param cmp the key {@link Comparator}
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*/
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public static <K, V> Comparator<Map.Entry<K, V>> byKey(Comparator<? super K> cmp) {
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Objects.requireNonNull(cmp);
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return (Comparator<Map.Entry<K, V>> & Serializable)
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(c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
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}
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/**
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* Gets a comparator compares {@link Map.Entry} by value using the given
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* {@link Comparator}.
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*
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* @param <K> key type
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* @param <V> value type
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* @param cmp the value {@link Comparator}
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*/
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public static <K, V> Comparator<Map.Entry<K, V>> byValue(Comparator<? super V> cmp) {
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Objects.requireNonNull(cmp);
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return (Comparator<Map.Entry<K, V>> & Serializable)
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(c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
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}
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/**
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* Accepts a function that extracts a {@link java.lang.Comparable
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* Comparable} sort key from a type {@code T}, and returns a {@code
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* Comparator<T>} that compares by that sort key. For example, if a class
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* {@code Person} has a {@code String}-valued getter {@code getLastName},
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* then {@code comparing(Person::getLastName)} would return a {@code
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* Comparator<Person>} that compares {@code Person} objects by their last
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* name.
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*
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* @param <T> the original element type
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* @param <U> the {@link Comparable} type for comparison
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* @param keyExtractor the function used to extract the {@link Comparable} sort key
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*/
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public static <T, U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor) {
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Objects.requireNonNull(keyExtractor);
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return (Comparator<T> & Serializable)
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(c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
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}
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/**
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* Accepts a function that extracts an {@code int} value from a type {@code
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* T}, and returns a {@code Comparator<T>} that compares by that value.
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*
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* <p>The returned comparator is serializable assuming the specified
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* function is also serializable.
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*
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* @see #comparing(Function)
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* @param <T> the original element type
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* @param keyExtractor the function used to extract the integer value
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*/
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public static <T> Comparator<T> comparing(ToIntFunction<? super T> keyExtractor) {
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Objects.requireNonNull(keyExtractor);
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return (Comparator<T> & Serializable)
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(c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
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}
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/**
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* Accepts a function that extracts a {@code long} value from a type {@code
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* T}, and returns a {@code Comparator<T>} that compares by that value.
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*
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* <p>The returned comparator is serializable assuming the specified
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* function is also serializable.
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*
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* @see #comparing(Function)
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* @param <T> the original element type
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* @param keyExtractor the function used to extract the long value
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*/
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public static <T> Comparator<T> comparing(ToLongFunction<? super T> keyExtractor) {
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Objects.requireNonNull(keyExtractor);
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return (Comparator<T> & Serializable)
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(c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
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}
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/**
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* Accepts a function that extracts a {@code double} value from a type
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* {@code T}, and returns a {@code Comparator<T>} that compares by that
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* value.
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*
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* <p>The returned comparator is serializable assuming the specified
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* function is also serializable.
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*
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* @see #comparing(Function)
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* @param <T> the original element type
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* @param keyExtractor the function used to extract the double value
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*/
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public static<T> Comparator<T> comparing(ToDoubleFunction<? super T> keyExtractor) {
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Objects.requireNonNull(keyExtractor);
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return (Comparator<T> & Serializable)
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(c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
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}
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/**
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* Constructs a lexicographic order from two {@link Comparator}s. For
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* example, if you have comparators {@code byLastName} and {@code
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* byFirstName}, each of type {@code Comparator<Person>}, then {@code
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* compose(byLastName, byFirstName)} creates a {@code Comparator<Person>}
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* which sorts by last name, and for equal last names sorts by first name.
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*
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* <p>The returned comparator is serializable assuming the specified
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* comparators are also serializable.
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*
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* @param <T> the element type to be compared
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* @param first the first comparator
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* @param second the secondary comparator used when equals on the first
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*/
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public static<T> Comparator<T> compose(Comparator<? super T> first, Comparator<? super T> second) {
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Objects.requireNonNull(first);
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Objects.requireNonNull(second);
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return (Comparator<T> & Serializable) (c1, c2) -> {
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int res = first.compare(c1, c2);
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return (res != 0) ? res : second.compare(c1, c2);
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};
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}
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/**
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* Constructs a {@link BinaryOperator} which returns the lesser of two elements
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* according to the specified {@code Comparator}
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*
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* @param comparator A {@code Comparator} for comparing the two values
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* @param <T> the type of the elements to be compared
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* @return a {@code BinaryOperator} which returns the lesser of its operands,
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* according to the supplied {@code Comparator}
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*/
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public static<T> BinaryOperator<T> lesserOf(Comparator<? super T> comparator) {
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return (a, b) -> comparator.compare(a, b) <= 0 ? a : b;
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}
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/**
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* Constructs a {@link BinaryOperator} which returns the greater of two elements
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* according to the specified {@code Comparator}
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*
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* @param comparator A {@code Comparator} for comparing the two values
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* @param <T> the type of the elements to be compared
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* @return a {@code BinaryOperator} which returns the greater of its operands,
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* according to the supplied {@code Comparator}
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*/
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public static<T> BinaryOperator<T> greaterOf(Comparator<? super T> comparator) {
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return (a, b) -> comparator.compare(a, b) >= 0 ? a : b;
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}
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}
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