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：

  

@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

@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：

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：

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：
  

@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

@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：
  

@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：
  

 @Test
public 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，

@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：
  

@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：

 

@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：
  

@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));
}