类型安全： generics v.s arrays （二）

代码实例: 一个类型安全的过滤器

 

 1 import java.util.ArrayList;
 2 import java.util.Arrays;
 3 import java.util.Collection;
 4 import java.util.List;
 5 
 6 /**
 7  * A sample usage : 
 8  * <code><pre>
 9  *   Filter.Checker&lt;Object&gt; checker = new Filter.Checker&lt;Object&gt;() {
10  *     public boolean check(Object value) {
11  *       return String.valueOf(value).contains("3");
12  *     }
13  *   };
14  * 
15  *   List&lt;String&gt; strWith3s = Filter.filt(
16  *       checker, 
17  *       "1 + 2 = 3", 
18  *       "2 + 3 = 4", 
19  *       "oops, 2 + 3 = 5",
20  *       "3 + 5 = 8", 
21  *       "5 + 8 = 13"
22  *   );
23  *   
24  *   List&lt;Number&gt; numsWith3s = Filter.&lt;Number&gt;filt(
25  *     checker, Arrays.asList(new Integer[]{ 1, 2, 3, 12, 13, 23 })
26  *   );
27  * </pre></code>
28  * 
29  * @author 那十无忧
30  */
31 public class Filter {
32   
33   // strategy interface
34   public interface Checker<V> {
35     
36     public boolean check(V value);
37   }
38   
39   public static <E> List<E> filt(Checker<? super E> checker, 
40                                  Collection<? extends E> values) {
41     assert checker != null;
42     
43     if( values == null ) return null;
44     
45     @SuppressWarnings("unchecked")
46     E[] snapshot = (E[])values.toArray(new Object[values.size()]);
47     
48     List<E> result = new ArrayList<E>(snapshot.length); 
49     
50     for(E e : snapshot)
51       if( checker.check(e) )
52         result.add(e);
53     snapshot = null;
54     return result;
55   }
56   
57   public static <E> List<E> filt(Checker<? super E> checker, E... values) {
58     if( values == null ) return null;
59     
60     return filt(checker, Arrays.asList(values));
61   }
62 }