/*
 
* Copyright (c) 1997, 2014, Oracle and/or its affiliates. All rights reserved.
 
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 
*
 
* This code is free software; you can redistribute it and/or modify it
 
* under the terms of the GNU General Public License version 2 only, as
 
* published by the Free Software Foundation.
  
Oracle designates this
 
* particular file as subject to the "Classpath" exception as provided
 
* by Oracle in the LICENSE file that accompanied this code.
 
*
 
* This code is distributed in the hope that it will be useful, but WITHOUT
 
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 
* FITNESS FOR A PARTICULAR PURPOSE.
  
See the GNU General Public License
 
* version 2 for more details (a copy is included in the LICENSE file that
 
* accompanied this code).
 
*
 
* You should have received a copy of the GNU General Public License version
 
* 2 along with this work; if not, write to the Free Software Foundation,
 
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 
*
 
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 
* or visit www.oracle.com if you need additional information or have any
 
* questions.
 
*/

package java.util;

import java.io.Serializable;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.function.ToDoubleFunction;
import java.util.Comparators;

/**
 
* A comparison function, which imposes a <i>total ordering</i> on some
 
* collection of objects.
  
Comparators can be passed to a sort method (such
 
* as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
 
* Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
 
* over the sort order.
  
Comparators can also be used to control the order of
 
* certain data structures (such as {@link SortedSet sorted sets} or {@link
 
* SortedMap sorted maps}), or to provide an ordering for collections of
 
* objects that don't have a {@link Comparable natural ordering}.<p>
 
*
 
* The ordering imposed by a comparator <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) &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 <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
 
*
 
* @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 &quot;equal to&quot; 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
   
objthe 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
     
* @param
  
keyExtractor the function used to extract the long sort key
     
* @return a comparator that compares by an extracted key
     
* @see #comparing(Function)
     
* @throws NullPointerException if the argument is null
     
* @since 1.8
     
*/

    
public static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor) {
        
Objects.requireNonNull(keyExtractor);
        
return (Comparator<T> & Serializable)
            
(c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
    
}

    
/**
     
* Accepts a function that extracts a {@code double} sort key from a type
     
* {@code T}, and returns a {@code Comparator<T>} that compares by that
     
* sort key.
     
*
     
* <p>The returned comparator is serializable if the specified function
     
* is also serializable.
     
*
     
* @param
  
<T> the type of element to be compared
     
* @param
  
keyExtractor the function used to extract the double sort key
     
* @return a comparator that compares by an extracted key
     
* @see #comparing(Function)
     
* @throws NullPointerException if the argument is null
     
* @since 1.8
     
*/

    
public static<T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor) {
        
Objects.requireNonNull(keyExtractor);
        
return (Comparator<T> & Serializable)
            
(c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
    
}
}