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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 <tt>c</tt> on a set of elements

 * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if

 * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as

 * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in

 * <tt>S</tt>.<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 <tt>c</tt>

 * is used with elements (or keys) drawn from a set <tt>S</tt>.  If the

 * ordering imposed by <tt>c</tt> on <tt>S</tt> 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 <tt>equals</tt>.<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

 * <tt>java.io.Serializable</tt>, 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 <tt>Serializable</tt>.<p>

 *

 * For the mathematically inclined, the <i>relation</i> that defines the

 * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a

 * given set of objects <tt>S</tt> 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 <tt>compare</tt> that the

 * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the

 * imposed ordering is a <i>total order</i> on <tt>S</tt>.  When we say that

 * the ordering imposed by <tt>c</tt> on <tt>S</tt> 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}/../technotes/guides/collections/index.html">

 * 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>

     *

     * In the foregoing description, the notation

     * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical

     * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,

     * <tt>0</tt>, or <tt>1</tt> according to whether the value of

     * <i>expression</i> is negative, zero or positive.<p>

     *

     * The implementor must ensure that <tt>sgn(compare(x, y)) ==

     * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This

     * implies that <tt>compare(x, y)</tt> must throw an exception if and only

     * if <tt>compare(y, x)</tt> throws an exception.)<p>

     *

     * The implementor must also ensure that the relation is transitive:

     * <tt>((compare(x, y)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies

     * <tt>compare(x, z)&gt;0</tt>.<p>

     *

     * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>

     * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all

     * <tt>z</tt>.<p>

     *

     * It is generally the case, but <i>not</i> strictly required that

     * <tt>(compare(x, y)==0) == (x.equals(y))</tt>.  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

     * <tt>true</tt> <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)</code> implies that <tt>sgn(comp1.compare(o1,

     * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference

     * <tt>o1</tt> and <tt>o2</tt>.<p>

     *

     * Note that it is <i>always</i> safe <i>not</i> to override

     * <tt>Object.equals(Object)</tt>.  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</code> 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 a {@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