学步园

   1:  import java.util.ArrayList;

   2:  import java.util.List;

   3:   

   4:  public class Tools {

   5:      

   6:      /**得到参数的不重复字符串序列返回

   7:       * @param origin

   8:       * @return

   9:       */

  10:      public static List getUniversal(List origin) {

  11:          List val = new ArrayList();

  12:          val.add(origin.get(0));

  13:          for (String str : origin) {

  14:              int i;

  15:              for (i = 0; i 
  16:                  if (str.equals(val.get(i))) {
  17:                      break;
  18:                  }
  19:              }
  20:              if (i > val.size() - 1) {
  21:                  val.add(str);
  22:              }
  23:          }
  24:          return val;
  25:      }
  26:   
  27:      public static void main(String[] args) {
  28:          List a = new ArrayList();
  29:          a.add("String");
  30:          a.add("Integer");
  31:          a.add("String");
  32:          a.add("Long");
  33:          a.add("String");
  34:          a.add("Long");
  35:          a.add("String");
  36:          a.add("Double");
  37:          a.add("String");
  38:          a.add("String");
  39:          System.out.println(Tools.getUniversal(a));
  40:      }
  41:  }

花了些时间写了上面的这些代码，在解决如题的问题时，写过如下代码：

   1:  public static List getUniversal(List origin){

   2:      List val = new ArrayList();

   3:      int i = 0;

   4:      val.add(origin.get(i++));

   5:      for(String s:val){

   6:          String str = origin.get(i++); 

   7:          if(!str.equals(s)){

   8:              val.add(str);

   9:          }

  10:      }

  11:      return val;

  12:  }

运行后得到的一个 ConcurrentModificationException ，查了一下 API，得知这个是 Exception 的大致意思是不能对正在迭代的元素进行修改。可见 foreach 循环是通过 iterator 实现的。 API 如下：

public class ConcurrentModificationException

extends RuntimeException

This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.

For example, it is not generally permissible for one thread to modify a Collection while another thread is iterating over it. In general, the results of the iteration are undefined under these circumstances. Some Iterator implementations (including those of all the general purpose collection implementations provided by the JRE) may choose to throw this exception if this behavior is detected. Iterators that do this are known as fail-fast iterators, as they fail quickly and cleanly, rather that risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception.

Note that fail-fast behavior cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast operations throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: ConcurrentModificationException should be used only to detect bugs.

Since:

1.2

See Also:

Collection, Iterator, ListIterator, Vector, LinkedList, HashSet, Hashtable, TreeMap, AbstractList, Serialized Form

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自我感觉这段代码挺重要，留下笔记。