casting Arrays.asList causing exception: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList

Question

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 

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)); 

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)); 

If relevant, I'm using JavaSE 1.6 in Eclipse Helios.

Answer 1

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>>(); 

which will allow to add whatever comes out of the Arrays.asList(..) method.

Answer 2

If you do this, you won't get any CCE:

ArrayList<ArrayList<String>> stuff = new ArrayList<ArrayList<String>>(); String[] titles = {"ticker", "grade", "score"}; stuff.add(new ArrayList<String>(Arrays.asList(titles))); 

As the error clearly states, the class java.util.ArrayList isn't the same as nested static class java.util.Arrays.ArrayList. Hence the exception. We overcome this by wrapping the returned list using a java.util.ArrayList.