Guava中针对集合的 filter和过滤功能

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));
}

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