问题描述
我是新来的Java，我试图理解为什么第一个code片段不会引起此异常，但第二个呢。因为一个字符串数组传递到Arrays.asList在这两种情况下，不应同时片段产生一个异常或没有产生异常？
I'm new to Java and am trying to understand why the first code snippet doesn't cause this exception but the second one does. Since a string array is passed into Arrays.asList in both cases, shouldn't both snippets produce an exception or not produce an exception?
Exception in thread "main" java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList
首先片段（导致没有例外）：
First snippet (causes no exception):
ArrayList<ArrayList<String>> stuff = new ArrayList<ArrayList<String>>();
String line = "a,b,cdef,g";
String delim = ",";
String[] pieces = line.split(delim);
stuff.add((ArrayList<String>) Arrays.asList(pieces));
第二个片段（原因上面的除外）：
Second snippet (causes above exception):
ArrayList<ArrayList<String>> stuff = new ArrayList<ArrayList<String>>();
String[] titles = {"ticker", "grade", "score"};
stuff.add((ArrayList<String>) Arrays.asList(titles));
如果相关的，我使用的JavaSE 1.6在Eclipse赫利俄斯。
If relevant, I'm using JavaSE 1.6 in Eclipse Helios.
推荐答案
对于我来说（使用Java 1.6.0_26），第一个片断给出了同样的异常作为第二个。其原因是，在<$c$c>Arrays.asList(..)方法并只返回一个列表，不一定是的ArrayList 。因为你真的不知道什么样的（或实施的）列表该方法的回报，你投以的ArrayList＆LT ;弦乐＆GT; 是不是安全。其结果是，它可能或可能无法正常工作。从编码风格的角度来看，良好的修复程序，这将是改变你的的东西声明：
For me (using Java 1.6.0_26), the first snippet gives the same exception as the second one. The reason is that the Arrays.asList(..) method does only return a List, not necessarily an ArrayList. Because you don't really know what kind (or implementation of) of List that method returns, your cast to ArrayList<String> is not safe. The result is that it may or may not work as expected. From a coding style perspective, a good fix for this would be to change your stuff declaration to:
List<List<String>> stuff = new ArrayList<List<String>>();
这将允许添加任何散发出来的 Arrays.asList（..）方法。
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