注：以下分析均基于Guava18

背景

除了我们会如此命名XXXService2 原来Google也会这样命名O(∩_∩)O哈！

由于jdk已经占据了Collections类名 因此Google在Collection基础上创建了

Collections2

基本知识

Google的工具类通常以s结尾 比如Lists Sets Collections2 Ints Strings 等等

进阶

filter

相对于Lists来说filter可以对易到transform方法，提供了一个过滤集合元素的方法

这也是开发过程中常见的用法，给定一个集合 通过某种方式过滤掉一些元素

/**  * Returns the elements of {@code unfiltered} that satisfy a predicate. The  * returned collection is a live view of {@code unfiltered}; changes to one  * affect the other.  *  * <p>The resulting collection's iterator does not support {@code remove()},  * but all other collection methods are supported. When given an element that  * doesn't satisfy the predicate, the collection's {@code add()} and {@code  * addAll()} methods throw an {@link IllegalArgumentException}. When methods  * such as {@code removeAll()} and {@code clear()} are called on the filtered  * collection, only elements that satisfy the filter will be removed from the  * underlying collection.  *  * <p>The returned collection isn't threadsafe or serializable, even if  * {@code unfiltered} is.  *  * <p>Many of the filtered collection's methods, such as {@code size()},  * iterate across every element in the underlying collection and determine  * which elements satisfy the filter. When a live view is <i>not</i> needed,  * it may be faster to copy {@code Iterables.filter(unfiltered, predicate)}  * and use the copy.  *  * <p><b>Warning:</b> {@code predicate} must be <i>consistent with equals</i>,  * as documented at {@link Predicate#apply}. Do not provide a predicate such  * as {@code Predicates.instanceOf(ArrayList.class)}, which is inconsistent  * with equals. (See {@link Iterables#filter(Iterable, Class)} for related  * functionality.)  */ // TODO(kevinb): how can we omit that Iterables link when building gwt // javadoc? public static <E> Collection<E> filter(     Collection<E> unfiltered, Predicate<? super E> predicate) {   if (unfiltered instanceof FilteredCollection) {     // Support clear(), removeAll(), and retainAll() when filtering a filtered     // collection.     return ((FilteredCollection<E>) unfiltered).createCombined(predicate);   }     return new FilteredCollection<E>(       checkNotNull(unfiltered), checkNotNull(predicate)); }

和之前Lists的相关方法类似 Guava的处理都是通过抽象出一个函数 将函数传入，在对应多个方法之前进行处理

那么首先看一下Predicate接口

/**  * Determines a true or false value for a given input.  *  * <p>The {@link Predicates} class provides common predicates and related utilities.  *  * <p>See the Guava User Guide article on <a href=  * "http://code.google.com/p/guava-libraries/wiki/FunctionalExplained">the use of {@code  * Predicate}</a>.  *  * @author Kevin Bourrillion  * @since 2.0 (imported from Google Collections Library)  */ @GwtCompatible public interface Predicate<T> {   /**    * Returns the result of applying this predicate to {@code input}. This method is <i>generally    * expected</i>, but not absolutely required, to have the following properties:    *    * <ul>    * <li>Its execution does not cause any observable side effects.    * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal    *     Objects.equal}{@code (a, b)} implies that {@code predicate.apply(a) ==    *     predicate.apply(b))}.    * </ul>    *    * @throws NullPointerException if {@code input} is null and this predicate does not accept null    *     arguments    */   boolean apply(@Nullable T input);     /**    * Indicates whether another object is equal to this predicate.    *    * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.    * However, an implementation may also choose to return {@code true} whenever {@code object} is a    * {@link Predicate} that it considers <i>interchangeable</i> with this one. "Interchangeable"    * <i>typically</i> means that {@code this.apply(t) == that.apply(t)} for all {@code t} of type    * {@code T}). Note that a {@code false} result from this method does not imply that the    * predicates are known <i>not</i> to be interchangeable.    */   @Override   boolean equals(@Nullable Object object); }

方法apply需要一个返回值 true表示该元素需要保留 false则需要在新的集合不保留

正如上文所说Guava的工具类通常以s结尾

因此Guava也提供了一些常见的Predicate 可以通过Predicates获取

一般来说我们会使用notNull等 其也提供了 and or等语法 甚至正则等等 十分强大。即使仍然不然满足条件还是可以自定义

可以看到了这边存在一个小问题 Lists没有filter方法 因此想要获得list还得需要Lists.newArrayList

考虑一下为何Lists没有对应的filter方法 可以参考 guava-why-is-there-no-lists-filter-function

It wasn't implemented because it would expose a perilous large number of slow methods, such as #get(index) on the returned List view (inviting performance bugs). And ListIterator would be a pain to implement as well (though I submitted a patch years ago to cover that).

Since indexed methods can't be efficient in the filtered List view, it's better to just go with a filtered Iterable, which doesn't have them

orderedPermutations

排列也是我们常使用的功能

/**  * Returns a {@link Collection} of all the permutations of the specified  * {@link Iterable}.  *  * <p><i>Notes:</i> This is an implementation of the algorithm for  * Lexicographical Permutations Generation, described in Knuth's "The Art of  * Computer Programming", Volume 4, Chapter 7, Section 7.2.1.2. The  * iteration order follows the lexicographical order. This means that  * the first permutation will be in ascending order, and the last will be in  * descending order.  *  * <p>Duplicate elements are considered equal. For example, the list [1, 1]  * will have only one permutation, instead of two. This is why the elements  * have to implement {@link Comparable}.  *  * <p>An empty iterable has only one permutation, which is an empty list.  *  * <p>This method is equivalent to  * {@code Collections2.orderedPermutations(list, Ordering.natural())}.  *  * @param elements the original iterable whose elements have to be permuted.  * @return an immutable {@link Collection} containing all the different  *     permutations of the original iterable.  * @throws NullPointerException if the specified iterable is null or has any  *     null elements.  * @since 12.0  */ @Beta public static <E extends Comparable<? super E>>     Collection<List<E>> orderedPermutations(Iterable<E> elements) {   return orderedPermutations(elements, Ordering.natural()); }   /**  * Returns a {@link Collection} of all the permutations of the specified  * {@link Iterable} using the specified {@link Comparator} for establishing  * the lexicographical ordering.  *  * <p>Examples: <pre>   {@code  *  *   for (List<String> perm : orderedPermutations(asList("b", "c", "a"))) {  *     println(perm);  *   }  *   // -> ["a", "b", "c"]  *   // -> ["a", "c", "b"]  *   // -> ["b", "a", "c"]  *   // -> ["b", "c", "a"]  *   // -> ["c", "a", "b"]  *   // -> ["c", "b", "a"]  *  *   for (List<Integer> perm : orderedPermutations(asList(1, 2, 2, 1))) {  *     println(perm);  *   }  *   // -> [1, 1, 2, 2]  *   // -> [1, 2, 1, 2]  *   // -> [1, 2, 2, 1]  *   // -> [2, 1, 1, 2]  *   // -> [2, 1, 2, 1]  *   // -> [2, 2, 1, 1]}</pre>  *  * <p><i>Notes:</i> This is an implementation of the algorithm for  * Lexicographical Permutations Generation, described in Knuth's "The Art of  * Computer Programming", Volume 4, Chapter 7, Section 7.2.1.2. The  * iteration order follows the lexicographical order. This means that  * the first permutation will be in ascending order, and the last will be in  * descending order.  *  * <p>Elements that compare equal are considered equal and no new permutations  * are created by swapping them.  *  * <p>An empty iterable has only one permutation, which is an empty list.  *  * @param elements the original iterable whose elements have to be permuted.  * @param comparator a comparator for the iterable's elements.  * @return an immutable {@link Collection} containing all the different  *     permutations of the original iterable.  * @throws NullPointerException If the specified iterable is null, has any  *     null elements, or if the specified comparator is null.  * @since 12.0  */ @Beta public static <E> Collection<List<E>> orderedPermutations(     Iterable<E> elements, Comparator<? super E> comparator) {   return new OrderedPermutationCollection<E>(elements, comparator); }

当只传一个参数 默认是自然序 注释也比较明确了

当然Collections2也有transform方法 和Lists类似