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- Guava中针对集合的 filter和过滤功能
Guava中针对集合的 filter和过滤功能
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JAVA相关
在guava库中，自带了过滤器(filter)的功能，可以用来对collection 进行过滤，先看例子：
Java代码
@Test public void whenFilterWithIterables_thenFiltered() { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Iterable<String> result = Iterables.filter(names, Predicates.containsPattern("a")); assertThat(result, containsInAnyOrder("Jane", "Adam")); }
在这个例子中，给出一个list，过滤出含有字母a的元素
此外，可以使用Collections2.filter() 去进行过滤
Java代码
@Test public void whenFilterWithCollections2_thenFiltered() { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Collection<String> result = Collections2.filter(names, Predicates.containsPattern("a")); assertEquals(2, result.size()); assertThat(result, containsInAnyOrder("Jane", "Adam")); result.add("anna"); assertEquals(5, names.size()); }
这里注意的是，Collections2.filter中，当在上面的result中增加了元素后，会直接影响原来的names这个list的，就是names中的集合元素是5了。
再来看下predicates判断语言，
com.google.common.base. Predicate : 根据输入值得到 true 或者 false
拿Collections2中有2个函数式编程的接口：filter , transform ,例如 ：在Collection<Integer>中过滤大于某数的内容：
Java代码
Collection<Integer> filterList = Collections2.filter(collections , new Predicate<Integer>(){ @Override public boolean apply(Integer input) { if(input > 4) return false; else return true; } });
把Lis<Integer>中的Integer类型转换为String , 并添加test作为后缀字符：
Java代码
List<String> c2 = Lists.transform(list, new Function<Integer , String>(){ @Override public String apply(Integer input) { return String.valueOf(input) + "test"; } });
需要说明的是每次调用返回都是新的对象，同时操作过程不是线程安全的。
再来点例子：
Java代码
@Test public void whenFilterCollectionWithCustomPredicate_thenFiltered() { Predicate<String> predicate = new Predicate<String>() { @Override public boolean apply(String input) { return input.startsWith("A") || input.startsWith("J"); } }; List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Collection<String> result = Collections2.filter(names, predicate); assertEquals(3, result.size()); assertThat(result, containsInAnyOrder("John", "Jane", "Adam")); }
将多个prdicate进行组合
Java代码
@Test public void whenFilterUsingMultiplePredicates_thenFiltered() { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Collection<String> result = Collections2.filter(names, Predicates.or(Predicates.containsPattern("J"), Predicates.not(Predicates.containsPattern("a")))); assertEquals(3, result.size()); assertThat(result, containsInAnyOrder("John", "Jane", "Tom")); }
上面的例子中找出包含J字母或不包含a的元素;
再看下如何将集合中的空元素删除：
Java代码
@Test public void whenRemoveNullFromCollection_thenRemoved() { List<String> names = Lists.newArrayList("John", null, "Jane", null, "Adam", "Tom"); Collection<String> result = Collections2.filter(names, Predicates.notNull()); assertEquals(4, result.size()); assertThat(result, containsInAnyOrder("John", "Jane", "Adam", "Tom")); }
检查一个collection中的所有元素是否符合某个条件：
Java代码
@Test ublic void whenCheckingIfAllElementsMatchACondition_thenCorrect() { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); boolean result = Iterables.all(names, Predicates.containsPattern("n|m")); assertTrue(result); result = Iterables.all(names, Predicates.containsPattern("a")); assertFalse(result);
下面看如何把一个list进行转换，
Java代码
@Test public void whenTransformWithIterables_thenTransformed() { Function<String, Integer> function = new Function<String, Integer>() { @Override public Integer apply(String input) { return input.length(); } }; List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Iterable<Integer> result = Iterables.transform(names, function); assertThat(result, contains(4, 4, 4, 3)); }
再看结合transform和predicates结合使用的例子：
Java代码
@Test public void whenCreatingAFunctionFromAPredicate_thenCorrect() { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Collection<Boolean> result = Collections2.transform(names, Functions.forPredicate(Predicates.containsPattern("m"))); assertEquals(4, result.size()); assertThat(result, contains(false, false, true, true)); }
在这个例子中，将一个LIST中的每一个元素进行使用Predicates.containsPattern，判断是否包含m，返回的是boolean，然后再得到的boolean值一起转换为collection
下面是两个function一起结合使用的例子：
Java代码
@Test public void whenTransformingUsingComposedFunction_thenTransformed() { Function<String,Integer> f1 = new Function<String,Integer>(){ @Override public Integer apply(String input) { return input.length(); } }; Function<Integer,Boolean> f2 = new Function<Integer,Boolean>(){ @Override public Boolean apply(Integer input) { return input % 2 == 0; } }; List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Collection<Boolean> result = Collections2.transform(names, Functions.compose(f2, f1)); assertEquals(4, result.size()); assertThat(result, contains(true, true, true, false)); }
在这个例子中，首先应用函数f1，求出每个元素的长度，然后再根据f1函数，分别返回
它们的boolean值，再转换为collection.
最后看下将filter和transform结合使用的例子：
Java代码
@Test public void whenFilteringAndTransformingCollection_thenCorrect() { Predicate<String> predicate = new Predicate<String>() { @Override public boolean apply(String input) { return input.startsWith("A") || input.startsWith("T"); } }; Function<String, Integer> func = new Function<String,Integer>(){ @Override public Integer apply(String input) { return input.length(); } }; List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Collection<Integer> result = FluentIterable.from(names) .filter(predicate) .transform(func) .toList(); assertEquals(2, result.size()); assertThat(result, containsInAnyOrder(4, 3));
