import java.util.*; 
 
public class ForeachTest 
{ 
  public static void main(String[] args) 
  { 
    // 创建集合、添加元素的代码与前一个程序相同 
    Collection books = new HashSet(); 
    books.add(new String("book1")); 
    books.add(new String("book2")); 
    books.add(new String("book3")); 
    for (Object obj : books) 
    { 
      // 此处的book变量也不是集合元素本身 
      String book = (String)obj; 
      System.out.println(book); 
      if (book.equals("book2")) 
      { 
        // 下面代码会引发ConcurrentModificationException异常 
        //books.remove(book);   //① 
      } 
    } 
    System.out.println(books); 
  } 
} 

book2
book1
book3
[book2, book1, book3]