Class Comparators

java.lang.Object
com.google.common.collect.Comparators
@GwtCompatible public final class Comparators extends Object
Provides static methods for working with Comparator instances. For many other helpful comparator utilities, see either Comparator itself (for Java 8+), or com.google.common.collect.Ordering (otherwise).

Relationship to Ordering

In light of the significant enhancements to Comparator in Java 8, the overwhelming majority of usages of Ordering can be written using only built-in JDK APIs. This class is intended to "fill the gap" and provide those features of Ordering not already provided by the JDK.

Since:
21.0
Author:
Louis Wasserman

  • Method Details

    • lexicographical

      public static <T extends @Nullable Object, S extends T> Comparator<Iterable<S>> lexicographical(Comparator<T> comparator)
      Returns a new comparator which sorts iterables by comparing corresponding elements pairwise until a nonzero result is found; imposes "dictionary order." If the end of one iterable is reached, but not the other, the shorter iterable is considered to be less than the longer one. For example, a lexicographical natural ordering over integers considers [] < [1] < [1, 1] < [1, 2] < [2].

      Note that Collections.reverseOrder(lexicographical(comparator)) is not equivalent to lexicographical(Collections.reverseOrder(comparator)) (consider how each would order [1] and [1, 1]).

    • isInOrder

      public static <T extends @Nullable Object> boolean isInOrder(Iterable<? extends T> iterable, Comparator<T> comparator)
      Returns true if each element in iterable after the first is greater than or equal to the element that preceded it, according to the specified comparator. Note that this is always true when the iterable has fewer than two elements.

    • isInStrictOrder

      public static <T extends @Nullable Object> boolean isInStrictOrder(Iterable<? extends T> iterable, Comparator<T> comparator)
      Returns true if each element in iterable after the first is strictly greater than the element that preceded it, according to the specified comparator. Note that this is always true when the iterable has fewer than two elements.

    • least

      public static <T extends @Nullable Object> Collector<T,?,List<T>> least(int k, Comparator<? super T> comparator)
      Returns a Collector that returns the k smallest (relative to the specified Comparator) input elements, in ascending order, as an unmodifiable List. Ties are broken arbitrarily.

      For example: 

      Stream.of("foo", "quux", "banana", "elephant")
    .collect(least(2, comparingInt(String::length)))
// returns {"foo", "quux"}

      This Collector uses O(k) memory and takes expected time O(n) (worst-case O(n log k)), as opposed to e.g. Stream.sorted(comparator).limit(k), which currently takes O(n log n) time and O(n) space.

      Throws:
      IllegalArgumentException - if k < 0
      Since:
      22.0

    • greatest

      public static <T extends @Nullable Object> Collector<T,?,List<T>> greatest(int k, Comparator<? super T> comparator)
      Returns a Collector that returns the k greatest (relative to the specified Comparator) input elements, in descending order, as an unmodifiable List. Ties are broken arbitrarily.

      For example: 

      Stream.of("foo", "quux", "banana", "elephant")
    .collect(greatest(2, comparingInt(String::length)))
// returns {"elephant", "banana"}

      This Collector uses O(k) memory and takes expected time O(n) (worst-case O(n log k)), as opposed to e.g. Stream.sorted(comparator.reversed()).limit(k), which currently takes O(n log n) time and O(n) space.

      Throws:
      IllegalArgumentException - if k < 0
      Since:
      22.0

    • emptiesFirst

      public static <T> Comparator<Optional<T>> emptiesFirst(Comparator<? super T> valueComparator)
      Returns a comparator of Optional values which treats Optional.empty() as less than all other values, and orders the rest using valueComparator on the contained value.
      Since:
      22.0 (but only since 33.4.0 in the Android flavor)

    • emptiesLast

      public static <T> Comparator<Optional<T>> emptiesLast(Comparator<? super T> valueComparator)
      Returns a comparator of Optional values which treats Optional.empty() as greater than all other values, and orders the rest using valueComparator on the contained value.
      Since:
      22.0 (but only since 33.4.0 in the Android flavor)

    • min

      public static <T extends Comparable<? super T>> T min(T a, T b)
      Returns the minimum of the two values. If the values compare as 0, the first is returned.

      The recommended solution for finding the minimum of some values depends on the type of your data and the number of elements you have. Read more in the Guava User Guide article on Comparators.

      Parameters:
      a - first value to compare, returned if less than or equal to b.
      b - second value to compare.
      Throws:
      ClassCastException - if the parameters are not mutually comparable.
      Since:
      30.0

    • min

      public static <T extends @Nullable Object> T min(T a, T b, Comparator<? super T> comparator)
      Returns the minimum of the two values, according to the given comparator. If the values compare as equal, the first is returned.

      The recommended solution for finding the minimum of some values depends on the type of your data and the number of elements you have. Read more in the Guava User Guide article on Comparators.

      Parameters:
      a - first value to compare, returned if less than or equal to b
      b - second value to compare.
      Throws:
      ClassCastException - if the parameters are not mutually comparable using the given comparator.
      Since:
      30.0

    • max

      public static <T extends Comparable<? super T>> T max(T a, T b)
      Returns the maximum of the two values. If the values compare as 0, the first is returned.

      The recommended solution for finding the maximum of some values depends on the type of your data and the number of elements you have. Read more in the Guava User Guide article on Comparators.

      Parameters:
      a - first value to compare, returned if greater than or equal to b.
      b - second value to compare.
      Throws:
      ClassCastException - if the parameters are not mutually comparable.
      Since:
      30.0

    • max

      public static <T extends @Nullable Object> T max(T a, T b, Comparator<? super T> comparator)
      Returns the maximum of the two values, according to the given comparator. If the values compare as equal, the first is returned.

      The recommended solution for finding the maximum of some values depends on the type of your data and the number of elements you have. Read more in the Guava User Guide article on Comparators.

      Parameters:
      a - first value to compare, returned if greater than or equal to b.
      b - second value to compare.
      Throws:
      ClassCastException - if the parameters are not mutually comparable using the given comparator.
      Since:
      30.0