OrderingExplained

cgdecker edited this page Nov 13, 2017 · 6 revisions

Ordering

Example

assertTrue(byLengthOrdering.reverse().isOrdered(list));

Overview

Ordering is Guava's "fluent" Comparator class, which can be used to build complex comparators and apply them to collections of objects.

At its core, an Ordering instance is nothing more than a special Comparator instance. Ordering simply takes the methods that rely on a Comparator (for example, Collections.max) and makes them available as instance methods. For additional power, Ordering class provides chaining methods to tweak and enhance existing comparators.

Creation

Common orderings are provided by static methods:

Method Description
natural() Uses the natural ordering on Comparable types.
usingToString() Compares objects by the lexicographical ordering of their string representations, as returned by toString().

Making a preexisting Comparator into an Ordering is as simple as using Ordering.from(Comparator).

But the more common way to create a custom Ordering is to skip the Comparator entirely in favor of extending the Ordering abstract class directly:

Ordering<String> byLengthOrdering = new Ordering<String>() {
  public int compare(String left, String right) {
    return Ints.compare(left.length(), right.length());
  }
};

Chaining

A given Ordering can be wrapped to obtain derived orderings. Some of the most commonly used variations include:

Method Description
reverse() Returns the reverse ordering.
nullsFirst() Returns an Ordering that orders nulls before non-null elements, and otherwise behaves the same as the original Ordering. See also nullsLast()
compound(Comparator) Returns an Ordering which uses the specified Comparator to "break ties."
lexicographical() Returns an Ordering that orders iterables lexicographically by their elements.
onResultOf(Function) Returns an Ordering which orders values by applying the function to them and then comparing the results using the original Ordering.

For example, let's say you want a comparator for the class...

class Foo {
  @Nullable String sortedBy;
  int notSortedBy;
}

...that can deal with null values of sortedBy. Here is a solution built atop the chaining methods:

Ordering<Foo> ordering = Ordering.natural().nullsFirst().onResultOf(new Function<Foo, String>() {
  public String apply(Foo foo) {
    return foo.sortedBy;
  }
});

When reading a chain of Ordering calls, work "backward" from right to left. The example above orders Foo instances by looking up their sortedBy field values, first moving any null sortedBy values to the top and then sorting the remaining values by natural string ordering. This backward order arises because each chaining call is "wrapping" the previous Ordering into a new one.

(Exception to the "backwards" rule: For chains of calls to compound, read from left to right. To avoid confusion, avoid intermixing compound calls with other chained calls.)

Chains longer than a few calls can be difficult to understand. We recommend limiting chaining to about three calls as in the example above. Even then, you may wish to simplify the code by separating out intermediate objects such as Function instances:

Ordering<Foo> ordering = Ordering.natural().nullsFirst().onResultOf(sortKeyFunction);

Application

Guava provides a number of methods to manipulate or examine values or collections using the ordering. We list some of the most popular here.

Method Description See also
greatestOf(Iterable iterable, int k) Returns the k greatest elements of the specified iterable, according to this ordering, in order from greatest to least. Not necessarily stable. leastOf
isOrdered(Iterable) Tests if the specified Iterable is in nondecreasing order according to this ordering. isStrictlyOrdered
sortedCopy(Iterable) Returns a sorted copy of the specified elements as a List. immutableSortedCopy
min(E, E) Returns the minimum of its two arguments according to this ordering. If the values compare as equal, the first argument is returned. max(E, E)
min(E, E, E, E...) Returns the minimum of its arguments according to this ordering. If there are multiple least values, the first is returned. max(E, E, E, E...)
min(Iterable) Returns the minimum element of the specified Iterable. Throws a NoSuchElementException if the Iterable is empty. max(Iterable), min(Iterator), max(Iterator)
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