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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 {@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) <= 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}/../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
* {@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.<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."
*
* @param o1 the first object to be compared.
* @param o2 the second object to be compared.
* @return a negative integer, zero, or a positive integer as the
* first argument is less than, equal to, or greater than the
* second.
* @throws NullPointerException if an argument is null and this
* comparator does not permit null arguments
* @throws ClassCastException if the arguments' types prevent them from
* being compared by this comparator.
*/
int compare(T o1, T o2);
/**
* Indicates whether some other object is "equal to" this
* comparator. This method must obey the general contract of
* {@link Object#equals(Object)}. Additionally, this method can return
* {@code true} <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.
*
* @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 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 {@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));
}
}