Ordering是Guava類庫提供的一個犀利強大的比較器工具，Guava的Ordering和JDK Comparator相比功能更強。它非常容易擴展，可以輕鬆建構複雜的comparator，然後用在容器的比較、排序等操作。
本質上來說，Ordering 實例無非就是一個特殊的Comparator 實例。 Ordering只是需要依賴一個比較器（例如，Collections.max）的方法，並使其可作為實例方法。另外，Ordering提供了鍊式方法呼叫和加強現有的比較器。
下面我們來看看Ordering中一些具體的方法和簡單的使用實例。
natural()：使用Comparable類型的自然順序， 例如：整數從小到大，字串是依字典順序;
usingToString() 傳回字典的使用字串(toString()順序進行排序；
arbitrary() ：傳回一個所有物件的任意順序， 即compare(a, b) == 0 就是a == b (identity equality)。 本身的排序是沒有任何意義， 但是在VM的生命週期是一個常數。
[code]import java.util.List;
import org.junit.Test;
import com.google.common.collect.Lists;
import com.google.common.collect.Ordering;
public class OrderingTest {
@Test
public void testStaticOrdering(){
List<String> list = Lists.newArrayList();
list.add("peida");
list.add("jerry");
list.add("harry");
list.add("eva");
list.add("jhon");
list.add("neron");
System.out.println("list:"+ list);
Ordering<String> naturalOrdering = Ordering.natural();
Ordering<Object> usingToStringOrdering = Ordering.usingToString();
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));
}
}[code]list:[peida, jerry, harry, eva, jhon, neron] naturalOrdering:[eva, harry, jerry, jhon, neron, peida] usingToStringOrdering:[eva, harry, jerry, jhon, neron, peida] arbitraryOrdering:[neron, harry, eva, jerry, peida, jhon]
compound(Comparator)：返回一個使用Comparator的Ordering，Comparator作為第二排序元素，例如對bug列表進行排序，先對bug列表進行排序，先依照bug的級別，再依照優先順序排序；
lexicographical()：傳回一個依照字典元素迭代的Ordering；
onResultOf(Function)：將function應用在各元素上進行排序；
greatestOf(Iterable iterable, int k)：傳回指定的第k個可迭代的最大的元素，按照這個從最大到最小的順序。是不穩定的。
leastOf(Iterable iterable,int k)：傳回指定的第k個可迭代的最小的元素，按照這個從最小到最大的順序。是不穩定的。
isOrdered(Iterable)：是否有序，Iterable不能少於2個元素。
isStrictlyOrdered(Iterable)：是否嚴格有序。請注意，Iterable不能少於兩個元素。
sortedCopy(Iterable)：傳回指定的元素作為一個清單的排序副本。
[code]import java.util.List;
import org.junit.Test;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;
import com.google.common.collect.Ordering;
public class OrderingTest {
@Test
public void testOrdering(){
List<String> list = Lists.newArrayList();
list.add("peida");
list.add("jerry");
list.add("harry");
list.add("eva");
list.add("jhon");
list.add("neron");
System.out.println("list:"+ list);
Ordering<String> naturalOrdering = Ordering.natural();
System.out.println("naturalOrdering:"+ naturalOrdering.sortedCopy(list));
List<Integer> listReduce= Lists.newArrayList();
for(int i=9;i>0;i--){
listReduce.add(i);
}
List<Integer> listtest= Lists.newArrayList();
listtest.add(1);
listtest.add(1);
listtest.add(1);
listtest.add(2);
Ordering<Integer> naturalIntReduceOrdering = Ordering.natural();
System.out.println("listtest:"+ listtest);
System.out.println(naturalIntReduceOrdering.isOrdered(listtest));
System.out.println(naturalIntReduceOrdering.isStrictlyOrdered(listtest));
System.out.println("naturalIntReduceOrdering:"+ naturalIntReduceOrdering.sortedCopy(listReduce));
System.out.println("listReduce:"+ listReduce);
System.out.println(naturalIntReduceOrdering.isOrdered(naturalIntReduceOrdering.sortedCopy(listReduce)));
System.out.println(naturalIntReduceOrdering.isStrictlyOrdered(naturalIntReduceOrdering.sortedCopy(listReduce)));
Ordering<String> natural = Ordering.natural();
List<String> abc = ImmutableList.of("a", "b", "c");
System.out.println(natural.isOrdered(abc));
System.out.println(natural.isStrictlyOrdered(abc));
System.out.println("isOrdered reverse :"+ natural.reverse().isOrdered(abc));
List<String> cba = ImmutableList.of("c", "b", "a");
System.out.println(natural.isOrdered(cba));
System.out.println(natural.isStrictlyOrdered(cba));
System.out.println(cba = natural.sortedCopy(cba));
System.out.println("max:"+natural.max(cba));
System.out.println("min:"+natural.min(cba));
System.out.println("leastOf:"+natural.leastOf(cba, 2));
System.out.println("naturalOrdering:"+ naturalOrdering.sortedCopy(list));
System.out.println("leastOf list:"+naturalOrdering.leastOf(list, 3));
System.out.println("greatestOf:"+naturalOrdering.greatestOf(list, 3));
System.out.println("reverse list :"+ naturalOrdering.reverse().sortedCopy(list));
System.out.println("isOrdered list :"+ naturalOrdering.isOrdered(list));
System.out.println("isOrdered list :"+ naturalOrdering.reverse().isOrdered(list));
list.add(null);
System.out.println(" add null list:"+list);
System.out.println("nullsFirst list :"+ naturalOrdering.nullsFirst().sortedCopy(list));
System.out.println("nullsLast list :"+ naturalOrdering.nullsLast().sortedCopy(list));
}
}
//============输出==============
list:[peida, jerry, harry, eva, jhon, neron]
naturalOrdering:[eva, harry, jerry, jhon, neron, peida]
listtest:[1, 1, 1, 2]
true
false
naturalIntReduceOrdering:[1, 2, 3, 4, 5, 6, 7, 8, 9]
listReduce:[9, 8, 7, 6, 5, 4, 3, 2, 1]
true
true
true
true
isOrdered reverse :false
false
false
[a, b, c]
max:c
min:a
leastOf:[a, b]
naturalOrdering:[eva, harry, jerry, jhon, neron, peida]
leastOf list:[eva, harry, jerry]
greatestOf:[peida, neron, jhon]
reverse list :[peida, neron, jhon, jerry, harry, eva]
isOrdered list :false
isOrdered list :false
add null list:[peida, jerry, harry, eva, jhon, neron, null]
nullsFirst list :[null, eva, harry, jerry, jhon, neron, peida]
nullsLast list :[eva, harry, jerry, jhon, neron, peida, null]
