Guava中针对集合的 filter和过滤功能

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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： 

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1. @Test  
2. public void whenFilterWithIterables_thenFiltered() {  
3.     List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");  
4.     Iterable<String> result = Iterables.filter(names, Predicates.containsPattern("a"));  
5.    
6.     assertThat(result, containsInAnyOrder("Jane", "Adam"));  
7. }  

@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 

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1. @Test  
2. public void whenFilterWithCollections2_thenFiltered() {  
3.     List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");  
4.     Collection<String> result = Collections2.filter(names, Predicates.containsPattern("a"));  
5.        
6.     assertEquals(2, result.size());  
7.     assertThat(result, containsInAnyOrder("Jane", "Adam"));  
8.    
9.     result.add("anna");  
10.     assertEquals(5, names.size());  
11. }  

@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： 

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1. Collection<Integer> filterList = Collections2.filter(collections  
2.   
3.      , new Predicate<Integer>(){  
4.   
5.                   @Override  
6.   
7.                   public boolean apply(Integer input) {  
8.   
9.                         if(input > 4)  
10.   
11.                               return false;  
12.   
13.                         else  
14.   
15.                               return true;  
16.   
17.                   }  
18.   
19. });  

Collection<Integer> filterList = Collections2.filter(collections
 , new Predicate&lt;Integer&gt;(){

              @Override

              public boolean apply(Integer input) {

                    if(input &gt; 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： 

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1. List<String> c2 = Lists.transform(list, new Function<Integer , String>(){  
2.   
3.                   @Override  
4.   
5.                   public String apply(Integer input) {  
6.   
7.                         return String.valueOf(input) + "test";  
8.   
9.                   }              
10.   
11. });  

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： 

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1. @Test  
2. public void whenFilterCollectionWithCustomPredicate_thenFiltered() {  
3.     Predicate<String> predicate = new Predicate<String>() {  
4.         @Override  
5.         public boolean apply(String input) {  
6.             return input.startsWith("A") || input.startsWith("J");  
7.         }  
8.     };  
9.    
10.     List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");  
11.     Collection<String> result = Collections2.filter(names, predicate);  
12.    
13.     assertEquals(3, result.size());  
14.     assertThat(result, containsInAnyOrder("John", "Jane", "Adam"));  
15. }  

@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&lt;String&gt; names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
Collection&lt;String&gt; result = Collections2.filter(names, predicate);

assertEquals(3, result.size());
assertThat(result, containsInAnyOrder("John", "Jane", "Adam"));

}

将多个prdicate进行组合 

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1. @Test  
2. public void whenFilterUsingMultiplePredicates_thenFiltered() {  
3.     List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");  
4.     Collection<String> result = Collections2.filter(names,   
5.       Predicates.or(Predicates.containsPattern("J"),   
6.       Predicates.not(Predicates.containsPattern("a"))));  
7.    
8.     assertEquals(3, result.size());  
9.     assertThat(result, containsInAnyOrder("John", "Jane", "Tom"));  
10. }  

@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： 

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1. @Test  
2. public void whenRemoveNullFromCollection_thenRemoved() {  
3.     List<String> names = Lists.newArrayList("John", null, "Jane", null, "Adam", "Tom");  
4.     Collection<String> result = Collections2.filter(names, Predicates.notNull());  
5.    
6.     assertEquals(4, result.size());  
7.     assertThat(result, containsInAnyOrder("John", "Jane", "Adam", "Tom"));  
8. }  

@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： 

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1. @Test  
2. ublic void whenCheckingIfAllElementsMatchACondition_thenCorrect() {  
3.    List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");  
4.   
5.    boolean result = Iterables.all(names, Predicates.containsPattern("n|m"));  
6.    assertTrue(result);  
7.   
8.    result = Iterables.all(names, Predicates.containsPattern("a"));  
9.    assertFalse(result);  

 @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， 

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1. @Test  
2. public void whenTransformWithIterables_thenTransformed() {  
3.     Function<String, Integer> function = new Function<String, Integer>() {  
4.         @Override  
5.         public Integer apply(String input) {  
6.             return input.length();  
7.         }  
8.     };  
9.    
10.     List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");  
11.     Iterable<Integer> result = Iterables.transform(names, function);  
12.    
13.     assertThat(result, contains(4, 4, 4, 3));  
14. }  

@Test
public void whenTransformWithIterables_thenTransformed() {
    Function<String, Integer> function = new Function<String, Integer>() {
        @Override
        public Integer apply(String input) {
            return input.length();
        }
    };
List&lt;String&gt; names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
Iterable&lt;Integer&gt; result = Iterables.transform(names, function);

assertThat(result, contains(4, 4, 4, 3));

}

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： 

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1. @Test  
2. public void whenTransformingUsingComposedFunction_thenTransformed() {  
3.     Function<String,Integer> f1 = new Function<String,Integer>(){  
4.         @Override  
5.         public Integer apply(String input) {  
6.             return input.length();  
7.         }  
8.     };  
9.    
10.     Function<Integer,Boolean> f2 = new Function<Integer,Boolean>(){  
11.         @Override  
12.         public Boolean apply(Integer input) {  
13.             return input % 2 == 0;  
14.         }  
15.     };  
16.    
17.     List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");  
18.     Collection<Boolean> result = Collections2.transform(names, Functions.compose(f2, f1));  
19.    
20.     assertEquals(4, result.size());  
21.     assertThat(result, contains(true, true, true, false));  
22. }  

@Test
public void whenTransformingUsingComposedFunction_thenTransformed() {
    Function<String,Integer> f1 = new Function<String,Integer>(){
        @Override
        public Integer apply(String input) {
            return input.length();
        }
    };
Function&lt;Integer,Boolean&gt; f2 = new Function&lt;Integer,Boolean&gt;(){
    @Override
    public Boolean apply(Integer input) {
        return input % 2 == 0;
    }
};

List&lt;String&gt; names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
Collection&lt;Boolean&gt; 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： 

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1. @Test  
2. public void whenFilteringAndTransformingCollection_thenCorrect() {  
3.     Predicate<String> predicate = new Predicate<String>() {  
4.         @Override  
5.         public boolean apply(String input) {  
6.             return input.startsWith("A") || input.startsWith("T");  
7.         }  
8.     };  
9.    
10.     Function<String, Integer> func = new Function<String,Integer>(){  
11.         @Override  
12.         public Integer apply(String input) {  
13.             return input.length();  
14.         }  
15.     };  
16.    
17.     List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");  
18.     Collection<Integer> result = FluentIterable.from(names)  
19.                                                .filter(predicate)  
20.                                                .transform(func)  
21.                                                .toList();  
22.    
23.     assertEquals(2, result.size());  
24.     assertThat(result, containsInAnyOrder(4, 3));  
25. }  

@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&lt;String, Integer&gt; func = new Function&lt;String,Integer&gt;(){
    @Override
    public Integer apply(String input) {
        return input.length();
    }
};

List&lt;String&gt; names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
Collection&lt;Integer&gt; result = FluentIterable.from(names)
                                           .filter(predicate)
                                           .transform(func)
                                           .toList();

assertEquals(2, result.size());
assertThat(result, containsInAnyOrder(4, 3));

}

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