why need to convert type to Object array in ArrayList's Construction?

Question

  public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
    elementData = Arrays.copyOf(elementData, size, Object[].class);
}

the code is the construction of java.util.ArrayList. you can check the details of bug 6260652, here https://bugs.java.com/bugdatabase/view_bug?bug_id=6260652

my question is if I don't convert the type, what problem will happen? Because I think it's totally OK if elementData refer to subType array.

axtavt · Accepted Answer · 2011-12-15 14:21:17Z

12

Here is an example of what may go wrong without the conversion in question:

List<Object> l = new ArrayList<Object>(Arrays.asList("foo", "bar"));

// Arrays.asList("foo", "bar").toArray() produces String[], 
// and l is backed by that array

l.set(0, new Object()); // Causes ArrayStoreException, because you cannot put
                        // arbitrary Object into String[]

answered Dec 15, 2011 at 14:21

axtavt

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1 Comment

nanosoft Over a year ago

That's wrong. Tested with java 8 and it runs without any error/Exception. And if you print the list, it gets printed as well without any problem.

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