Comparator

  • The Comparator interface defines a custom comparison method for ordering objects, particularly useful when natural ordering is absent or needs to be overridden.

  • Comparators are used with sorting methods and sorted data structures to specify how elements should be ordered, and they can be made serializable for use in TreeSet and TreeMap.

  • A comparator's ordering should ideally be consistent with the equals method to avoid unexpected behavior in sorted collections, and also must be transitive.

  • The interface provides static methods like naturalOrder(), nullsFirst(), and nullsLast() to handle common comparison scenarios and instance methods for creating lexicographic orderings and reversing the order.

  • compare(T o1, T o2) is the core method for comparing objects, returning negative, zero, or positive values to indicate the relative order of o1 and o2.
public interface Comparator
Known Indirect Subclasses

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.

See Also

Public Method Summary

abstract int
compare(T o1, T o2)
Compares its two arguments for order.
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 type T, and returns a Comparator<T> that compares by that sort key using the specified Comparator.
static <T, U extends Comparable<? super U>> Comparator<T>
comparing(Function<? super T, ? extends U> keyExtractor)
Accepts a function that extracts a Comparable sort key from a type T, and returns a Comparator<T> that compares by that sort key.
static <T> Comparator<T>
comparingDouble(ToDoubleFunction<? super T> keyExtractor)
Accepts a function that extracts a double sort key from a type T, and returns a Comparator<T> that compares by that sort key.
static <T> Comparator<T>
comparingInt(ToIntFunction<? super T> keyExtractor)
Accepts a function that extracts an int sort key from a type T, and returns a Comparator<T> that compares by that sort key.
static <T> Comparator<T>
comparingLong(ToLongFunction<? super T> keyExtractor)
Accepts a function that extracts a long sort key from a type T, and returns a Comparator<T> that compares by that sort key.
abstract boolean
equals(Object obj)
Indicates whether some other object is "equal to" this comparator.
static <T extends Comparable<? super T>> Comparator<T>
naturalOrder()
Returns a comparator that compares Comparable objects in natural order.
static <T> Comparator<T>
nullsFirst(Comparator<? super T> comparator)
Returns a null-friendly comparator that considers null to be less than non-null.
static <T> Comparator<T>
nullsLast(Comparator<? super T> comparator)
Returns a null-friendly comparator that considers null to be greater than non-null.
static <T extends Comparable<? super T>> Comparator<T>
reverseOrder()
Returns a comparator that imposes the reverse of the natural ordering.
Comparator<T>
reversed()
Returns a comparator that imposes the reverse ordering of this comparator.
<U extends Comparable<? super U>> Comparator<T>
thenComparing(Function<? super T, ? extends U> keyExtractor)
Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.
<U> Comparator<T>
thenComparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator)
Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.
Comparator<T>
thenComparing(Comparator<? super T> other)
Returns a lexicographic-order comparator with another comparator.
Comparator<T>
thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
Returns a lexicographic-order comparator with a function that extracts a double sort key.
Comparator<T>
thenComparingInt(ToIntFunction<? super T> keyExtractor)
Returns a lexicographic-order comparator with a function that extracts a int sort key.
Comparator<T>
thenComparingLong(ToLongFunction<? super T> keyExtractor)
Returns a lexicographic-order comparator with a function that extracts a long sort key.

Public Methods

public abstract 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.

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.

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."

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.

public static Comparator<T> comparing (Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator)

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.

Parameters
keyExtractor the function used to extract the sort key
keyComparator the Comparator used to compare the sort key
Returns
  • a comparator that compares by an extracted key using the specified Comparator
Throws
NullPointerException if either argument is null

public static Comparator<T> comparing (Function<? super T, ? extends U> keyExtractor)

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.

Parameters
keyExtractor the function used to extract the Comparable sort key
Returns
  • a comparator that compares by an extracted key
Throws
NullPointerException if the argument is null

public static Comparator<T> comparingDouble (ToDoubleFunction<? super T> keyExtractor)

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.

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
See Also

public static Comparator<T> comparingInt (ToIntFunction<? super T> keyExtractor)

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.

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
See Also

public static Comparator<T> comparingLong (ToLongFunction<? super T> keyExtractor)

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.

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
See Also

public abstract boolean equals (Object obj)

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.

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

public static Comparator<T> naturalOrder ()

Returns a comparator that compares Comparable objects in natural order.

The returned comparator is serializable and throws NullPointerException when comparing null.

Returns
  • a comparator that imposes the natural ordering on Comparable objects.
See Also

public static Comparator<T> nullsFirst (Comparator<? super T> comparator)

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.

Parameters
comparator a Comparator for comparing non-null values
Returns
  • a comparator that considers null to be less than non-null, and compares non-null objects with the supplied Comparator.

public static Comparator<T> nullsLast (Comparator<? super T> 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.

Parameters
comparator a Comparator for comparing non-null values
Returns
  • a comparator that considers null to be greater than non-null, and compares non-null objects with the supplied Comparator.

public static Comparator<T> reverseOrder ()

Returns a comparator that imposes the reverse of the natural ordering.

The returned comparator is serializable and throws NullPointerException when comparing null.

Returns
  • a comparator that imposes the reverse of the natural ordering on Comparable objects.
See Also

public 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.

public Comparator<T> thenComparing (Function<? super T, ? extends U> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.

Parameters
keyExtractor the function used to extract the Comparable sort key
Returns
  • a lexicographic-order comparator composed of this and then the Comparable sort key.
Throws
NullPointerException if the argument is null.
See Also

public Comparator<T> thenComparing (Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator)

Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.

Parameters
keyExtractor the function used to extract the sort key
keyComparator the Comparator 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.
See Also

public Comparator<T> thenComparing (Comparator<? super T> other)

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.

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.

public Comparator<T> thenComparingDouble (ToDoubleFunction<? super T> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a double sort key.

Parameters
keyExtractor the function used to extract the double sort key
Returns
  • a lexicographic-order comparator composed of this and then the double sort key
Throws
NullPointerException if the argument is null.
See Also

public Comparator<T> thenComparingInt (ToIntFunction<? super T> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a int sort key.

Parameters
keyExtractor the function used to extract the integer sort key
Returns
  • a lexicographic-order comparator composed of this and then the int sort key
Throws
NullPointerException if the argument is null.
See Also

public Comparator<T> thenComparingLong (ToLongFunction<? super T> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a long sort key.

Parameters
keyExtractor the function used to extract the long sort key
Returns
  • a lexicographic-order comparator composed of this and then the long sort key
Throws
NullPointerException if the argument is null.
See Also