Module java.base
Package java.util

Interface Comparator<T>

Type Parameters:
T - the type of objects that may be compared by this comparator
All Known Implementing Classes:
Collator,RuleBasedCollator
Functional Interface:
This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
@FunctionalInterfacepublic interfaceComparator<T>
A comparison function, which imposes atotal ordering on some collection of objects. Comparators can be passed to a sort method (such asCollections.sort orArrays.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such assorted sets orsorted maps), or to provide an ordering for collections of objects that don't have anatural ordering.

The ordering imposed by a comparatorc on a set of elementsS is said to beconsistent with equals if and only ifc.compare(e1, e2)==0 has the same boolean value ase1.equals(e2) for everye1 ande2 inS.

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 comparatorc is used with elements (or keys) drawn from a setS. If the ordering imposed byc onS 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 ofequals.

For example, suppose one adds two elementsa andb such that(a.equals(b) && c.compare(a, b) != 0) to an emptyTreeSet with comparatorc. The secondadd operation will return true (and the size of the tree set will increase) becausea andb are not equivalent from the tree set's perspective, even though this is contrary to the specification of theSet.add method.

Note: It is generally a good idea for comparators to also implementjava.io.Serializable, as they may be used as ordering methods in serializable data structures (likeTreeSet,TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implementSerializable.

For the mathematically inclined, therelation that defines theimposed ordering that a given comparatorc imposes on a given set of objectsS is:

       {(x, y) such that c.compare(x, y) <= 0}.
Thequotient for this total order is:
       {(x, y) such that c.compare(x, y) == 0}.
It follows immediately from the contract forcompare that the quotient is anequivalence relation onS, and that the imposed ordering is atotal order onS. When we say that the ordering imposed byc onS isconsistent 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)}.
In other words, when the imposed ordering is consistent with equals, the equivalence classes defined by the equivalence relation of theequals method and the equivalence classes defined by the quotient of thecompare method are the same.

UnlikeComparable, 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.

Since:
1.2
See Also:

  • Method Details

    • compare

      int compare(T o1,T o2)
      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 thatsignum(compare(x, y)) == -signum(compare(y, x)) for allx andy. (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)) impliescompare(x, z)>0.

      Finally, the implementor must ensure thatcompare(x, y)==0 implies thatsignum(compare(x, z))==signum(compare(y, z)) for allz.

      API Note:
      It is generally the case, butnot 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."
      Parameters:
      o1 - the first object to be compared.
      o2 - the second object to be compared.
      Returns:
      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
      ClassCastException - if the arguments' types prevent them from being compared by this comparator.

    • equals

      boolean equals(Object obj)
      Indicates whether some other object is "equal to" this comparator. This method must obey the general contract ofObject.equals(Object). Additionally, this method can returntrueonly if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus,comp1.equals(comp2) implies thatsignum(comp1.compare(o1, o2))==signum(comp2.compare(o1, o2)) for every object referenceo1 ando2.

      Note that it isalways safenot to overrideObject.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.

      Overrides:
      equals in class Object
      Parameters:
      obj - the reference object with which to compare.
      Returns:
      true only if the specified object is also a comparator and it imposes the same ordering as this comparator.
      See Also:

    • reversed

      default Comparator<T> reversed()
      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

    • thenComparing

      default Comparator<T> thenComparing(Comparator<? superT> other)
      Returns a lexicographic-order comparator with another comparator. If thisComparator 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.

      API Note:
      For example, to sort a collection ofString 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);
      Parameters:
      other - the other comparator to be used when this comparator compares two objects that are equal.
      Returns:
      a lexicographic-order comparator composed of this and then the other comparator
      Throws:
      NullPointerException - if the argument is null.
      Since:
      1.8

    • thenComparing

      default <U> Comparator<T> thenComparing(Function<? superT,? extends U> keyExtractor,Comparator<? super U> keyComparator)
      Returns a lexicographic-order comparator with a function that extracts a key to be compared with the givenComparator.
      Implementation Requirements:
      This default implementation behaves as if thenComparing(comparing(keyExtractor, cmp)).
      Type Parameters:
      U - the type of the sort key
      Parameters:
      keyExtractor - the function used to extract the sort key
      keyComparator - theComparator used to compare the sort key
      Returns:
      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.
      Since:
      1.8
      See Also:

    • thenComparing

      default <U extendsComparable<? super U>>Comparator<T> thenComparing(Function<? superT,? extends U> keyExtractor)
      Returns a lexicographic-order comparator with a function that extracts aComparable sort key.
      Implementation Requirements:
      This default implementation behaves as if thenComparing(comparing(keyExtractor)).
      Type Parameters:
      U - the type of theComparable sort key
      Parameters:
      keyExtractor - the function used to extract theComparable sort key
      Returns:
      a lexicographic-order comparator composed of this and then theComparable sort key.
      Throws:
      NullPointerException - if the argument is null.
      Since:
      1.8
      See Also:

    • thenComparingInt

      default Comparator<T> thenComparingInt(ToIntFunction<? superT> keyExtractor)
      Returns a lexicographic-order comparator with a function that extracts anint sort key.
      Implementation Requirements:
      This default implementation behaves as if thenComparing(comparingInt(keyExtractor)).
      Parameters:
      keyExtractor - the function used to extract the integer sort key
      Returns:
      a lexicographic-order comparator composed of this and then theint sort key
      Throws:
      NullPointerException - if the argument is null.
      Since:
      1.8
      See Also:

    • thenComparingLong

      default Comparator<T> thenComparingLong(ToLongFunction<? superT> keyExtractor)
      Returns a lexicographic-order comparator with a function that extracts along sort key.
      Implementation Requirements:
      This default implementation behaves as if thenComparing(comparingLong(keyExtractor)).
      Parameters:
      keyExtractor - the function used to extract the long sort key
      Returns:
      a lexicographic-order comparator composed of this and then thelong sort key
      Throws:
      NullPointerException - if the argument is null.
      Since:
      1.8
      See Also:

    • thenComparingDouble

      default Comparator<T> thenComparingDouble(ToDoubleFunction<? superT> keyExtractor)
      Returns a lexicographic-order comparator with a function that extracts adouble sort key.
      Implementation Requirements:
      This default implementation behaves as if thenComparing(comparingDouble(keyExtractor)).
      Parameters:
      keyExtractor - the function used to extract the double sort key
      Returns:
      a lexicographic-order comparator composed of this and then thedouble sort key
      Throws:
      NullPointerException - if the argument is null.
      Since:
      1.8
      See Also:

    • reverseOrder

      static <T extendsComparable<? super T>>Comparator<T> reverseOrder()
      Returns a comparator that imposes the reverse of thenatural ordering.

      The returned comparator is serializable and throwsNullPointerException when comparingnull.

      Type Parameters:
      T - theComparable type of element to be compared
      Returns:
      a comparator that imposes the reverse of thenatural ordering onComparable objects.
      Since:
      1.8
      See Also:

    • naturalOrder

      static <T extendsComparable<? super T>>Comparator<T> naturalOrder()
      Returns a comparator that comparesComparable objects in natural order.

      The returned comparator is serializable and throwsNullPointerException when comparingnull.

      Type Parameters:
      T - theComparable type of element to be compared
      Returns:
      a comparator that imposes thenatural ordering on Comparable objects.
      Since:
      1.8
      See Also:

    • nullsFirst

      static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)
      Returns a null-friendly comparator that considersnull to be less than non-null. When both arenull, they are considered equal. If both are non-null, the specifiedComparator is used to determine the order. If the specified comparator isnull, then the returned comparator considers all non-null values to be equal.

      The returned comparator is serializable if the specified comparator is serializable.

      Type Parameters:
      T - the type of the elements to be compared
      Parameters:
      comparator - aComparator for comparing non-null values
      Returns:
      a comparator that considersnull to be less than non-null, and compares non-null objects with the suppliedComparator.
      Since:
      1.8

    • nullsLast

      static <T> Comparator<T> nullsLast(Comparator<? super T> comparator)
      Returns a null-friendly comparator that considersnull to be greater than non-null. When both arenull, they are considered equal. If both are non-null, the specifiedComparator is used to determine the order. If the specified comparator isnull, then the returned comparator considers all non-null values to be equal.

      The returned comparator is serializable if the specified comparator is serializable.

      Type Parameters:
      T - the type of the elements to be compared
      Parameters:
      comparator - aComparator for comparing non-null values
      Returns:
      a comparator that considersnull to be greater than non-null, and compares non-null objects with the suppliedComparator.
      Since:
      1.8

    • comparing

      static <T,U> Comparator<T> comparing(Function<? super T,? extends U> keyExtractor,Comparator<? super U> keyComparator)
      Accepts a function that extracts a sort key from a typeT, and returns aComparator<T> that compares by that sort key using the specifiedComparator.

      The returned comparator is serializable if the specified function and comparator are both serializable.

      API Note:
      For example, to obtain aComparator that compares Person objects by their last name ignoring case differences,
           Comparator<Person> cmp = Comparator.comparing(             Person::getLastName,             String.CASE_INSENSITIVE_ORDER);
      Type Parameters:
      T - the type of element to be compared
      U - the type of the sort key
      Parameters:
      keyExtractor - the function used to extract the sort key
      keyComparator - theComparator used to compare the sort key
      Returns:
      a comparator that compares by an extracted key using the specifiedComparator
      Throws:
      NullPointerException - if either argument is null
      Since:
      1.8

    • comparing

      static <T,U extendsComparable<? super U>>Comparator<T> comparing(Function<? super T,? extends U> keyExtractor)
      Accepts a function that extracts aComparable sort key from a typeT, and returns a Comparator<T> that compares by that sort key.

      The returned comparator is serializable if the specified function is also serializable.

      API Note:
      For example, to obtain aComparator that compares Person objects by their last name,
           Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
      Type Parameters:
      T - the type of element to be compared
      U - the type of theComparable sort key
      Parameters:
      keyExtractor - the function used to extract theComparable sort key
      Returns:
      a comparator that compares by an extracted key
      Throws:
      NullPointerException - if the argument is null
      Since:
      1.8

    • comparingInt

      static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor)
      Accepts a function that extracts anint sort key from a typeT, and returns aComparator<T> that compares by that sort key.

      The returned comparator is serializable if the specified function is also serializable.

      Type Parameters:
      T - the type of element to be compared
      Parameters:
      keyExtractor - the function used to extract the integer sort key
      Returns:
      a comparator that compares by an extracted key
      Throws:
      NullPointerException - if the argument is null
      Since:
      1.8
      See Also:

    • comparingLong

      static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor)
      Accepts a function that extracts along sort key from a typeT, and returns aComparator<T> that compares by that sort key.

      The returned comparator is serializable if the specified function is also serializable.

      Type Parameters:
      T - the type of element to be compared
      Parameters:
      keyExtractor - the function used to extract the long sort key
      Returns:
      a comparator that compares by an extracted key
      Throws:
      NullPointerException - if the argument is null
      Since:
      1.8
      See Also:

    • comparingDouble

      static <T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor)
      Accepts a function that extracts adouble sort key from a typeT, and returns aComparator<T> that compares by that sort key.

      The returned comparator is serializable if the specified function is also serializable.

      Type Parameters:
      T - the type of element to be compared
      Parameters:
      keyExtractor - the function used to extract the double sort key
      Returns:
      a comparator that compares by an extracted key
      Throws:
      NullPointerException - if the argument is null
      Since:
      1.8
      See Also: