1.4-排序: Guava强大的”流畅风格比较器” · guava

# 1.4-排序: Guava强大的”流畅风格比较器” # 1.4-排序: Guava强大的”流畅风格比较器” [原文链接](http://code.google.com/p/guava-libraries/wiki/OrderingExplained) 译者: 沈义扬 [排序器\[Ordering\]](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html)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。 **创建排序器**：常见的排序器可以由下面的静态方法创建 **方法****描述**[`natural()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#natural%28%29)对可排序类型做自然排序，如数字按大小，日期按先后排序[`usingToString()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#usingToString%28%29)按对象的字符串形式做字典排序\[lexicographical ordering\][`from(Comparator)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#from%28java.util.Comparator%29)把给定的Comparator转化为排序器实现自定义的排序器时，除了用上面的from方法，也可以跳过实现Comparator，而直接继承Ordering： ``` <pre class="calibre11">``` Ordering<String> byLengthOrdering = new Ordering<String>() { public int compare(String left, String right) { return Ints.compare(left.length(), right.length()); } }; ``` ``` **链式调用方法**：通过链式调用，可以由给定的排序器衍生出其它排序器 **方法****描述**[`reverse()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#reverse%28%29)获取语义相反的排序器[`nullsFirst()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#nullsFirst%28%29)使用当前排序器，但额外把null值排到最前面。[`nullsLast()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#nullsLast%28%29)使用当前排序器，但额外把null值排到最后面。[`compound(Comparator)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#compound%28java.util.Comparator%29)合成另一个比较器，以处理当前排序器中的相等情况。[`lexicographical()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#lexicographical%28%29)基于处理类型T的排序器，返回该类型的可迭代对象Iterable<T>的排序器。[`onResultOf(Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6le/common/collect/Ordering.html#onResultOf%28com.google.common.base.Function%29)对集合中元素调用Function，再按返回值用当前排序器排序。例如，你需要下面这个类的排序器。 ``` <pre class="calibre11">``` class Foo { @Nullable String sortedBy; int notSortedBy; } ``` ``` 考虑到排序器应该能处理sortedBy为null的情况，我们可以使用下面的链式调用来合成排序器： ``` <pre class="calibre11">``` Ordering<Foo> ordering = Ordering.natural().nullsFirst().onResultOf(new Function<Foo, String>() { public String apply(Foo foo) { return foo.sortedBy; } }); ``` ``` 当阅读链式调用产生的排序器时，应该从后往前读。上面的例子中，排序器首先调用apply方法获取sortedBy值，并把sortedBy为null的元素都放到最前面，然后把剩下的元素按sortedBy进行自然排序。之所以要从后往前读，是因为每次链式调用都是用后面的方法包装了前面的排序器。 *注：用compound方法包装排序器时，就不应遵循从后往前读的原则。为了避免理解上的混乱，请不要把compound写在一长串链式调用的中间，你可以另起一行，在链中最先或最后调用compound。* 超过一定长度的链式调用，也可能会带来阅读和理解上的难度。我们建议按下面的代码这样，在一个链中最多使用三个方法。此外，你也可以把Function分离成中间对象，让链式调用更简洁紧凑。 ``` <pre class="calibre11">``` Ordering<Foo> ordering = Ordering.natural().nullsFirst().onResultOf(sortKeyFunction) ``` ``` **运用排序器：**Guava的排序器实现有若干操纵集合或元素值的方法 **方法****描述****另请参见**[`greatestOf(Iterable iterable, int k)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#greatestOf%28java.lang.Iterable,%20int%29)获取可迭代对象中最大的k个元素。[`leastOf`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#leastOf%28java.lang.Iterable,%20int%29)[`isOrdered(Iterable)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#isOrdered%28java.lang.Iterable%29)判断可迭代对象是否已按排序器排序：允许有排序值相等的元素。[`isStrictlyOrdered`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#isStrictlyOrdered%28java.lang.Iterable%29)[`sortedCopy(Iterable)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#sortedCopy%28java.lang.Iterable%29)判断可迭代对象是否已严格按排序器排序：不允许排序值相等的元素。[`immutableSortedCopy`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#immutableSortedCopy%28java.lang.Iterable%29)[`min(E, E)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#min%28E,%20E%29)返回两个参数中最小的那个。如果相等，则返回第一个参数。[`max(E, E)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#max%28E,%20E%29)[`min(E, E, E, E...)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#min%28E,%20E,%20E,%20E...%29)返回多个参数中最小的那个。如果有超过一个参数都最小，则返回第一个最小的参数。[`max(E, E, E, E...)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#max%28E,%20E,%20E,%20E...%29)[`min(Iterable)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#min%28java.lang.Iterable%29)返回迭代器中最小的元素。如果可迭代对象中没有元素，则抛出NoSuchElementException。[`max(Iterable)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#max%28java.lang.Iterable%29), [`min(Iterator)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#min%28java.util.Iterator%29), [`max(Iterator)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#max%28java.util.Iterator%29)