Guava：Ordering 排序工具

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简介

排序器 Ordering 是 Guava流畅风格比较器 Comparator 的实现，它可以用来为构建复杂的比较器，以完成集合排序的功能。

从实现上说，Ordering 实例就是一个特殊的 Comparator 实例。Ordering 把很多基于 Comparator 的静态方法（如 Collections.max）包装为自己的实例方法（非静态方法），并且提供了链式调用方法，来定制和增强现有的比较器。

类方法说明

官方文档：Source code

方法名称	方法描述
allEqual()	返回一个Ordering，所有值的排序地位都是平等的，表明无排序。 将此排序传递给任何稳定排序算法都不会导致元素顺序发生变化。
arbitrary()	返回所有对象的任意顺序，即compare(a, b) == 0 就是 a == b (identity equality)。 本身的排序是没有任何含义，但是在VM的生命周期是一个常量。
binarySearch(List<? extends T> sortedList, T key)	已过时，请使用 Collections.binarySearch(List, Object, Comparator).
compare(T left, T right)	比较两个参数的顺序。
compound(Comparator<? super U> secondaryComparator)	返回一个Ordering，传入比较器作为第二排序元素。
compound(Iterable<? extends Comparator<? super T>> comparators)	返回一个Ordering，会根据传入比较器集合一次比较，直到找到一个非零的结果。
explicit(List<T> valuesInOrder)	返回一个Ordering，根据他们出现在给定的列表的顺序比较对象
explicit(T leastValue, T... remainingValuesInOrder)	返回一个Ordering，比较对象根据它们的传入的顺序。
from(Comparator<T> comparator)	返回一个传入comparator实例的Ordering。
from(Ordering<T> ordering)	已过时。 不需要使用它
greatestOf(Iterable<E> iterable, int k)	根据Ordering对传入iterable由大到小排序，返回前K位的集合。
greatestOf(Iterator<E> iterator, int k)	根据Ordering对传入iterable由大到小排序，返回前K位的集合。
immutableSortedCopy(Iterable<E> elements)	返回一个不可变的集合，包含根据Ordering对传入元素排序后的所有元素。
isOrdered(Iterable<? extends T> iterable)	根据Ordering对传入iterable元素迭代，如果下一个元素大于或等于上一个元素，返回true。
isStrictlyOrdered(Iterable<? extends T> iterable)	根据Ordering对传入iterable元素迭代，如果下一个元素严格大于上一个元素，返回true。
leastOf(Iterable<E> iterable, int k)	根据Ordering对传入iterable由小到大排序，返回前K位的集合。
leastOf(Iterator<E> iterator, int k)	根据Ordering对传入iterable由小到大排序，返回前K位的集合。
lexicographical()	返回一个新的Ordering，通过相应的元素两两迭代，直到找到一个非零的结果。强加“字典顺序”。
max(E a, E b)	根据Ordering返回传入参数的最大值。
max(E a, E b, E c, E... rest)	根据Ordering返回传入参数的最大值。
max(Iterable<E> iterable)	根据Ordering返回传入参数的最大值。
max(Iterator<E> iterator)	根据Ordering返回传入参数的最大值。
min(E a, E b)	根据Ordering返回传入参数的最小值。
min(E a, E b, E c, E... rest)	根据Ordering返回传入参数的最小值。
min(Iterable<E> iterable)	根据Ordering返回传入参数的最小值。
min(Iterator<E> iterator)	根据Ordering返回传入参数的最小值。
natural()	对可排序类型做自然排序，如数字按大小，日期按先后排序
nullsFirst()	根据Ordering排序，null值放在最前面，并使用它来比较非空值。
nullsLast()	根据Ordering排序，null值放在最后面，并使用此排序来比较非空值。
onResultOf(Function<F,? extends T> function)	将传入function应用到每个元素上面，再通过Ordering进行排序。
<S extends T>Ordering<S>	reverse()  返回与当前Ordering相反的排序。相当于 Collections.reverseOrder(Comparator)。
<E extends T> List<E>	sortedCopy(Iterable<E> elements)  返回一个可变的集合，包含根据Ordering对传入元素排序后的所有元素。
usingToString()	根据toString返回的字符串按照字典顺序排序。

 使用Demo

import com.google.common.base.Function;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterators;
import com.google.common.collect.Lists;
import com.google.common.collect.Ordering;
import com.google.common.primitives.Doubles;
import junit.framework.TestCase;
import org.checkerframework.checker.nullness.qual.Nullable;
import java.util.*;
import static java.util.Arrays.asList;/*** 排序*/
public class OrderingTests extends TestCase {/*** 1、allEqual 允许有null*/public void testAllEqual() {Ordering<Object> comparator = Ordering.allEqual();System.out.println(comparator.equals(comparator.reverse()));assertEquals(0, comparator.compare(null, null));assertEquals(0, comparator.compare(new Object(), new Object()));assertEquals(0, comparator.compare("apples", "oranges"));assertEquals("Ordering.allEqual()", comparator.toString());List<String> strings = ImmutableList.of("b", "a", "d", "c");List<String> sortedCopy = comparator.sortedCopy(strings);assertEquals(strings, sortedCopy);ImmutableList<String> immutableSortedCopy = comparator.immutableSortedCopy(strings);assertEquals(strings, immutableSortedCopy);}/*** 2、natural 不能有null*/public void testNatural() {Ordering<Integer> comparator = Ordering.natural();try {comparator.compare(1, null);fail();} catch (NullPointerException expected) {}try {comparator.compare(null, 2);fail();} catch (NullPointerException expected) {}try {comparator.compare(null, null);fail();} catch (NullPointerException expected) {}assertEquals("Ordering.natural()", comparator.toString());}/*** 3、List集合 复杂排序示例*/public void testComplicatedOrderingExample() {Ordering<Iterable<Integer>> example = Ordering.<Integer>natural().nullsFirst().reverse().lexicographical().reverse().nullsLast();List<Integer> list1 = Lists.newArrayList();List<Integer> list2 = Lists.newArrayList(1);List<Integer> list3 = Lists.newArrayList(1, 1);List<Integer> list4 = Lists.newArrayList(1, 2);List<Integer> list5 = Lists.newArrayList(1, null, 2);List<Integer> list6 = Lists.newArrayList(2);Integer nullInt = null;List<Integer> list7 = Lists.newArrayList(nullInt);List<Integer> list8 = Lists.newArrayList(nullInt, nullInt);List<List<Integer>> list = Lists.newArrayList(list1, list2, list3, list4, list5, list6, list7, list8, null);List<List<Integer>> sorted = example.sortedCopy(list);/*** [null, null]* [null]* [1, null, 2]* [1, 1]* [1, 2]* [1]* [2]* []* null*/sorted.forEach(System.out::println);}/*** 4、from 把给定的 Comparator 转化为排序器*/public void testFrom() {// String.CASE_INSENSITIVE_ORDER 按照 ASCII 排序Ordering<String> caseInsensitiveOrdering = Ordering.from(String.CASE_INSENSITIVE_ORDER);assertTrue(caseInsensitiveOrdering.compare("A", "a") == 0);assertTrue(caseInsensitiveOrdering.compare("a", "B") < 0);assertTrue(caseInsensitiveOrdering.compare("B", "a") > 0);ArrayList<String> list = Lists.newArrayList("dehua", "abcdef", "ABCDEF", "rapido", "lbsse","lasse");List<String> sortedCopy = caseInsensitiveOrdering.sortedCopy(list);sortedCopy.forEach(System.out::println);}/** 5、explicit（ExplicitOrdering）返回一个Ordering，根据它们的传入的顺序比较对象。只能比较参数列表中存在的对象*/public void testExplicit_none() {Comparator<Integer> c = Ordering.explicit(Collections.emptyList());try {c.compare(0, 0);} catch (Exception e) {
//            e.printStackTrace();}assertEquals("Ordering.explicit([])", c.toString());}public void testExplicit_one() {Comparator<Integer> c = Ordering.explicit(0);assertEquals(0, c.compare(0, 0));try {c.compare(0, 1);fail();} catch (Exception e) {
//            e.printStackTrace();}assertEquals("Ordering.explicit([0])", c.toString());}public void testExplicit_two() {
//        Comparator<Integer> c = Ordering.explicit(42, 5);
//        assertEquals(0, c.compare(5, 5));
//        assertTrue(c.compare(5, 42) > 0);
//        assertTrue(c.compare(42, 5) < 0);Comparator<Integer> c = Ordering.explicit(5, 10);assertEquals(0, c.compare(5, 5));assertTrue(c.compare(5, 10) < 0);assertTrue(c.compare(10, 5) > 0);try {c.compare(5, 9);fail();} catch (Exception e) {System.out.println(e.getMessage());}}public void testExplicit_three() {// explicit:根据传入对象的顺序排序Double first = 0.2;Double[] second = {0.1, 0.3, 0.5};List<Double> numbers = Lists.asList(first, second);//排序比较器：根据原始的大小排序Ordering<Double> peopleOrdering = new Ordering<Double>() {@Overridepublic int compare(Double left, Double right) {return Doubles.compare(left, right);}};peopleOrdering.reverse().explicit(numbers).sortedCopy(numbers).forEach(System.out::println);//[ 0.2,0.1, 0.3, 0.5]}public void testExplicit_sortingExample() {Comparator<Integer> c = Ordering.explicit(2, 8, 6, 1, 7, 5, 3, 4, 0, 9);List<Integer> list = Arrays.asList(0, 3, 5, 6, 7, 8, 9);Collections.sort(list, c);// 8, 6, 7, 5, 3, 0, 9list.forEach(System.out::println);}/*** key重复异常*/public void testExplicit_withDuplicates() {try {Ordering.explicit(1, 2, 3, 4, 2);fail();} catch (IllegalArgumentException expected) {expected.printStackTrace();}}/*** 6、arbitrary 返回所有对象的任意顺序*/public void testArbitrary_withoutCollisions() {List<Integer> list = Lists.newArrayList();for (int i = 0; i < 50; i++) {list.add(i);}Ordering<Object> arbitrary = Ordering.arbitrary();Collections.sort(list, arbitrary);list.forEach(System.out::println);assertEquals("Ordering.arbitrary()", arbitrary.toString());}/*** 7、usingToString* 按对象的字符串形式做字典排序 [lexicographical ordering]*/public void testUsingToString() {Ordering<Object> ordering = Ordering.usingToString();assertEquals("Ordering.usingToString()", ordering.toString());List<String> list = Lists.newArrayList("lasse", "jerry", "harry", "eva", "jhon", "neron");System.out.println("list:" + list);// 使用Comparable类型的自然顺序， 例如：整数从小到大，字符串是按字典顺序;Ordering<String> naturalOrdering = Ordering.natural();// 使用toString()返回的字符串按字典顺序进行排序；Ordering<Object> usingToStringOrdering = Ordering.usingToString();// 返回一个所有对象的任意顺序Ordering<Object> arbitraryOrdering = Ordering.arbitrary();System.out.println("naturalOrdering:" + naturalOrdering.sortedCopy(list));System.out.println("usingToStringOrdering:" + usingToStringOrdering.sortedCopy(list));System.out.println("arbitraryOrdering:" + arbitraryOrdering.sortedCopy(list));}/*** 8、reverse 取返*/public void testReverse() {List<String> list = Lists.newArrayList("lasse", "jerry", "harry", "eva", "jhon", "neron");Collections.sort(list, Ordering.natural().reverse());list.forEach(System.out::println);}private enum StringLengthFunction implements Function<String, Integer> {StringLength;@Overridepublic Integer apply(String string) {return string.length();}}/*** 9、onResultOf 将传入function应用到每个元素上面，再通过Ordering进行排序。*/public void testOnResultOf_1() {// 外部枚举函数Ordering<String> ordering = Ordering.natural().onResultOf(StringLengthFunction.StringLength);assertTrue(ordering.compare("to", "be") == 0);assertTrue(ordering.compare("or", "not") < 0);assertTrue(ordering.compare("that", "to") > 0);assertEquals("Ordering.natural().onResultOf(StringLength)", ordering.toString());ArrayList<String> list = Lists.newArrayList("lasse", "abcds", "ABCDEF", "rapido", "chengxumiao");ordering.sortedCopy(list).forEach(System.out::println);}public void testOnResultOf_2() {// 匿名内部类函数Ordering<String> ordering = Ordering.natural().onResultOf(new Function<String, Comparable>() {@Overridepublic Comparable apply(@Nullable String input) {return input.length();}});ArrayList<String> list = Lists.newArrayList("lasse", "abcds", "ABCDEF", "rapido", "chengxumiao");ordering.sortedCopy(list).forEach(System.out::println);}public void testOnResultOf_3() {// lambda 表达式函数Ordering<String> ordering = Ordering.natural().reverse().onResultOf(str -> str.length());ArrayList<String> list = Lists.newArrayList("lasse", "abcds", "ABCDEF", "rapido", "chengxumiao");ordering.sortedCopy(list).forEach(System.out::println);}/*** 10、nullsFirst*/public void testNullsFirst_NullsLast() {ArrayList<String> list = Lists.newArrayList("lasse", null, "abcds", "ABCDEF", null, "rapido", "chengxumiao");Collections.sort(list, Ordering.natural().nullsFirst());list.forEach(System.out::println);Ordering.natural().nullsLast().sortedCopy(list).forEach(System.out::println);}/*** 11、NullsLast*/public void testNullsLast() {ArrayList<String> list = Lists.newArrayList("tingfeng", null, "abcds", "ABCDEF", null, "rapido", "chengxumiao");Ordering.natural().nullsLast().sortedCopy(list).forEach(System.out::println);}/*** 12、isOrdered 下一个元素大于或等于上一个元素，返回true*/public void testIsOrdered() {Ordering<Comparable> ordering = Ordering.natural();assertFalse(ordering.isOrdered(asList(5, 3, 0, 9)));assertFalse(ordering.isOrdered(asList(0, 5, 3, 9)));assertTrue(ordering.isOrdered(asList(0, 3, 5, 9)));assertTrue(ordering.isOrdered(asList(0, 0, 3, 3)));assertTrue(ordering.isOrdered(asList(0, 3)));assertTrue(ordering.isOrdered(Collections.singleton(1)));assertTrue(ordering.isOrdered(Collections.<Integer>emptyList()));}/*** 13、isStrictlyOrdered 下一个元素大于上一个元素，返回true*/public void testIsStrictlyOrdered() {Ordering<Comparable> ordering = Ordering.natural();assertFalse(ordering.isStrictlyOrdered(asList(5, 3, 0, 9)));assertFalse(ordering.isStrictlyOrdered(asList(0, 5, 3, 9)));assertFalse(ordering.isStrictlyOrdered(asList(0, 0, 3, 3)));assertTrue(ordering.isStrictlyOrdered(asList(0, 3, 5, 9)));assertTrue(ordering.isStrictlyOrdered(asList(0, 3)));assertTrue(ordering.isStrictlyOrdered(Collections.singleton(1)));assertTrue(ordering.isStrictlyOrdered(Collections.<Integer>emptyList()));}/*** 判断集合是否只读*/private static void assertListImmutable(List<Integer> result) {try {result.set(0, 1);fail();} catch (UnsupportedOperationException expected) {// pass}}/*** 14、leastOf 有点类似截取集合前几位的概念*/public void testLeastOfIterable_simple_1() {List<Integer> result = Ordering.natural().leastOf(Arrays.asList(3, 4, 5, -1), 2);assertTrue(result instanceof RandomAccess);assertListImmutable(result);assertEquals(ImmutableList.of(-1, 3), result);}public void testLeastOfIterator_simple_1() {List<Integer> result = Ordering.natural().leastOf(Iterators.forArray(3, 4, 5, -1), 2);assertTrue(result instanceof RandomAccess);assertListImmutable(result);assertEquals(ImmutableList.of(-1, 3), result);}public void testLeastOfIterable_simple_nMinusOne_withNullElement() {List<Integer> list = Arrays.asList(3, null, 5, -1);List<Integer> result = Ordering.natural().nullsLast().leastOf(list, list.size() - 1);assertTrue(result instanceof RandomAccess);assertListImmutable(result);assertEquals(ImmutableList.of(-1, 3, 5), result);}/*** 15、min 和 max*/public void testIteratorMinAndMax() {Ordering<Comparable> ordering = Ordering.natural();List<Integer> ints = Lists.newArrayList(5, 3, 0, 9);assertEquals(9, (int) ordering.max(ints));assertEquals(0, (int) ordering.min(ints));assertEquals(9, (int) ordering.max(ints.iterator()));assertEquals(0, (int) ordering.min(ints.iterator()));assertEquals(9, (int) ordering.max(ints.listIterator()));assertEquals(0, (int) ordering.min(ints.listIterator()));// 当值相同时，返回第一个参数，此处a正确，b就不正确Integer a = new Integer(4);Integer b = new Integer(4);ints = Lists.newArrayList(a, b, b);assertSame(a, ordering.max(ints.iterator()));assertSame(a, ordering.min(ints.iterator()));}public void testIteratorMinExhaustsIterator() {List<Integer> ints = Lists.newArrayList(9, 0, 3, 5);Iterator<Integer> iterator = ints.iterator();assertEquals(0, (int) Ordering.natural().min(iterator));assertFalse(iterator.hasNext());}public void testIteratorMaxExhaustsIterator() {List<Integer> ints = Lists.newArrayList(9, 0, 3, 5);Iterator<Integer> iterator = ints.iterator();assertEquals(9, (int) Ordering.natural().max(iterator));assertFalse(iterator.hasNext());}}

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