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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 total ordering on some collection of objects. Comparators can be passed to a sort method (such as Collections.sort or 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 sorted sets or sorted maps), or to provide an ordering for collections of objects that don't have a natural ordering.
 The ordering imposed by a comparator c on a set of elements S is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value as e1.equals(e2) for every e1 and e2 in 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 c is used with elements (or keys) drawn from a set S. If the ordering imposed by c on 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 equals.
 For example, suppose one adds two elements a and b such that (a.equals(b) && c.compare(a, b) != 0) to an empty TreeSet with comparator c. The second add operation will return true (and the size of the tree set will increase) because a and b are not equivalent from the tree set's perspective, even though this is contrary to the specification of the Set.add method.
 Note: It is generally a good idea for comparators to also implement java.io.Serializable, as they may be used as ordering methods in serializable data structures (like TreeSet, TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implement Serializable.

For the mathematically inclined, the relation that defines the
imposed ordering that a given comparator c imposes on a given set of objects 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 compare that the quotient is an equivalence relation on S, and that the imposed ordering is a total order on S. When we say that the ordering imposed by c on S is consistent with
equals, we mean that the quotient for the ordering is the equivalence relation defined by the objects' 
equals(Object) method(s):
    {(x, y) such that x.equals(y)}. 


Unlike 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.

Author: Josh Bloch,  Neal Gafter
Type parameters:
  • <T> – the type of objects that may be compared by this comparator
See Also:
Since:1.2
/**
 * 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) &lt;= 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.
 The implementor must ensure that sgn(compare(x, y)) ==
-sgn(compare(y, x)) for all x and y. (This implies that compare(x, y) must throw an exception if and only if compare(y, x) throws an exception.)
 The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.
 Finally, the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.

It is generally the case, but not strictly required that (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."
 In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return one of -1, 0, or 1 according to whether the value of expression is negative, zero, or positive, respectively.

Params:
  • o1 – the first object to be compared.
  • o2 – the second object to be compared.
Throws:
  • NullPointerException – if an argument is null and this
        comparator does not permit null arguments
  • ClassCastException – if the arguments' types prevent them from
        being compared by this comparator.
Returns:a negative integer, zero, or a positive integer as the
        first argument is less than, equal to, or greater than the
        second.
/**
     * 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 Object.equals(Object). Additionally, this method can return true only if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus, comp1.equals(comp2) implies that sgn(comp1.compare(o1,
o2))==sgn(comp2.compare(o1, o2)) for every object reference o1 and o2.

Note that it is always safe not to override 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. 
Params:
  • obj –   the reference object with which to compare.
See Also:
Returns: true only if the specified object is also a comparator and it imposes the same ordering as this comparator.
/**
     * Indicates whether some other object is &quot;equal to&quot; 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.

Returns:a comparator that imposes the reverse ordering of this
        comparator.
Since:1.8
/**
     * 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 Comparator considers two elements equal, i.e. compare(a, b) == 0, other is used to determine the order. 
The returned comparator is serializable if the specified comparator
is also serializable.

Params:
  • other – the other comparator to be used when this comparator
        compares two objects that are equal.
Throws:
API Note: For example, to sort a collection of String based on the length and then case-insensitive natural ordering, the comparator can be composed using following code, 

    Comparator<String> cmp = Comparator.comparingInt(String::length)
            .thenComparing(String.CASE_INSENSITIVE_ORDER);

Returns:a lexicographic-order comparator composed of this and then the
        other comparator
Since:1.8
/**
     * 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 Comparator. 
Params:
  • keyExtractor – the function used to extract the sort key
  • keyComparator – the Comparator used to compare the sort key
Type parameters:
  • <U> –  the type of the sort key
Throws:
See Also:
Implementation Requirements:This default implementation behaves as if 
          thenComparing(comparing(keyExtractor, cmp)).
Returns:a lexicographic-order comparator composed of this comparator
        and then comparing on the key extracted by the keyExtractor function
Since:1.8
/**
     * 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 Comparable sort key. 
Params:
  • keyExtractor – the function used to extract the Comparable sort key
Type parameters:
Throws:
See Also:
Implementation Requirements:This default implementation behaves as if 
          thenComparing(comparing(keyExtractor)).
Returns:a lexicographic-order comparator composed of this and then the Comparable sort key.
Since:1.8
/**
     * 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 int sort key. 
Params:
  • keyExtractor – the function used to extract the integer sort key
Throws:
See Also:
Implementation Requirements:This default implementation behaves as if 
          thenComparing(comparingInt(keyExtractor)).
Returns:a lexicographic-order comparator composed of this and then the int sort key
Since:1.8
/**
     * 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 long sort key. 
Params:
  • keyExtractor – the function used to extract the long sort key
Throws:
See Also:
Implementation Requirements:This default implementation behaves as if 
          thenComparing(comparingLong(keyExtractor)).
Returns:a lexicographic-order comparator composed of this and then the long sort key
Since:1.8
/**
     * 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 double sort key. 
Params:
  • keyExtractor – the function used to extract the double sort key
Throws:
See Also:
Implementation Requirements:This default implementation behaves as if 
          thenComparing(comparingDouble(keyExtractor)).
Returns:a lexicographic-order comparator composed of this and then the double sort key
Since:1.8
/**
     * 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 natural
ordering.

The returned comparator is serializable and throws NullPointerException when comparing null. 
Type parameters:
See Also:
Returns:a comparator that imposes the reverse of the natural
        ordering on Comparable objects.
Since:1.8
/**
     * 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 Comparable objects in natural order. 
The returned comparator is serializable and throws NullPointerException when comparing null. 
Type parameters:
See Also:
Returns:a comparator that imposes the natural ordering on 
        Comparable objects.
Since:1.8
/**
     * 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 null to be less than non-null. When both are null, they are considered equal. If both are non-null, the specified Comparator is used to determine the order. If the specified comparator is null, then the returned comparator considers all non-null values to be equal. 
The returned comparator is serializable if the specified comparator
is serializable.

Params:
  • comparator – a Comparator for comparing non-null values
Type parameters:
  • <T> – the type of the elements to be compared
Returns:a comparator that considers null to be less than non-null, and compares non-null objects with the supplied Comparator.
Since:1.8
/**
     * 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 null to be greater than non-null. When both are null, they are considered equal. If both are non-null, the specified Comparator is used to determine the order. If the specified comparator is null, then the returned comparator considers all non-null values to be equal. 
The returned comparator is serializable if the specified comparator
is serializable.

Params:
  • comparator – a Comparator for comparing non-null values
Type parameters:
  • <T> – the type of the elements to be compared
Returns:a comparator that considers null to be greater than non-null, and compares non-null objects with the supplied Comparator.
Since:1.8
/**
     * 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 T, and returns a Comparator<T> that compares by that sort key using the specified Comparator. 
The returned comparator is serializable if the specified function
and comparator are both serializable.

Params:
  • keyExtractor – the function used to extract the sort key
  • keyComparator – the Comparator used to compare the sort key
Type parameters:
  • <T> – the type of element to be compared
  • <U> – the type of the sort key
Throws:
API Note: For example, to obtain a Comparator that compares 
Person objects by their last name ignoring case differences, 

    Comparator<Person> cmp = Comparator.comparing(
            Person::getLastName,
            String.CASE_INSENSITIVE_ORDER);

Returns:a comparator that compares by an extracted key using the specified Comparator
Since:1.8
/**
     * 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 
Comparable sort key from a type T, and returns a 
Comparator<T> that compares by that sort key. 
The returned comparator is serializable if the specified function
is also serializable.

Params:
  • keyExtractor – the function used to extract the Comparable sort key
Type parameters:
  • <T> – the type of element to be compared
  • <U> – the type of the Comparable sort key
Throws:
API Note: For example, to obtain a Comparator that compares 
Person objects by their last name, 

    Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);

Returns:a comparator that compares by an extracted key
Since:1.8
/**
     * 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 int sort key from a type T, and returns a Comparator<T> that compares by that sort key. 
The returned comparator is serializable if the specified function
is also serializable.

Params:
  • keyExtractor – the function used to extract the integer sort key
Type parameters:
  • <T> – the type of element to be compared
Throws:
See Also:
Returns:a comparator that compares by an extracted key
Since:1.8
/**
     * 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 long sort key from a type T, and returns a Comparator<T> that compares by that sort key. 
The returned comparator is serializable if the specified function is
also serializable.

Params:
  • keyExtractor – the function used to extract the long sort key
Type parameters:
  • <T> – the type of element to be compared
Throws:
See Also:
Returns:a comparator that compares by an extracted key
Since:1.8
/**
     * 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 double sort key from a type T, and returns a Comparator<T> that compares by that sort key. 
The returned comparator is serializable if the specified function
is also serializable.

Params:
  • keyExtractor – the function used to extract the double sort key
Type parameters:
  • <T> – the type of element to be compared
Throws:
See Also:
Returns:a comparator that compares by an extracted key
Since:1.8
/**
     * 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));
    }
}