UnsupportedOperation Exception

[tmiranda]

Gents,

I am getting a Java UnsupportedOperation Exception and I can't figure out why. Details:

I have an instance variable:

private List<String>    allowedUsers = null;

I initialize it in the constructor:

allowedUsers = new ArrayList<String>();

I add several elements by converting an Array to a List:

        String[] StringArray = S.split(D);

        return Arrays.asList(StringArray);

When I try removing an element, I get the exception:

        if (allowedUsers.contains(userID)) {
            allowedUsers.remove(allowedUsers.indexOf(userID));
        }

I also tried:

        if (allowedUsers.contains(userID)) {
            allowedUsers.remove(userID);
        }

I've checked for nulls, empty strings, etc.

I know I'm doing something really silly, but what?

Tom

[stuckless]

While the toList() doesn't explicitly state that the list is readonly (in terms of adding/removing) elements it does say that changes to the list are reflected in the array. Since you can not grow/shrink the array, i'm guessing the you get UnsupportedOperations when you try to add/remove to the list.

To solve...

ArrayList list = new ArrayList(Arrays.asList(YourArray));

That will copy your array into a new list that can be modified.

[GKusnick]

UnsupportedOperation means pretty much what it says. Apparently you're using a nonmutable List implementation that does not support the remove() method. My guess is that it's the List returned from Arrays.asList(), which is not the same as a mutable ArrayList.

The same applies, by the way, to the Lists implemented by my API wrappers. You can index them, search them, and iterate them, but you can't modify them.

[tmiranda]

Thanks Sean and Greg. Yes, it's because I'm using Arrays.asList, I didn't realize the resulting List was nonmutable. My noob lesson of the day

[tmiranda]

Originally Posted by stuckless

While the toList() doesn't explicitly state that the list is readonly (in terms of adding/removing) elements it does say that changes to the list are reflected in the array. Since you can not grow/shrink the array, i'm guessing the you get UnsupportedOperations when you try to add/remove to the list.

To solve...

ArrayList list = new ArrayList(Arrays.asList(YourArray));

That will copy your array into a new list that can be modified.

I changed my method to return this:

return new ArrayList(Arrays.asList(StringArray));

but now I'm getting warnings:

warning: [unchecked] unchecked call to ArrayList(java.util.Collection<? extends E>) as a member of the raw type java.util.ArrayList
warning: [unchecked] unchecked conversion

Is this normal? When I see warnings I assume I am doing something that I should not be doing.

Here is the entire method:

    static List<String> delimitedStringToList(String S, String D) {
        List<String> TheList = new ArrayList<String>();

        if (S == null || D == null || S.isEmpty()) {
            return TheList;
        }

        String[] StringArray = S.split(D);

        if (StringArray == null || StringArray.length==0) {
            return TheList;
        }

        return new ArrayList(Arrays.asList(StringArray));
    }

[GKusnick]

Originally Posted by tmiranda

When I see warnings I assume I am doing something that I should not be doing.

Or not doing something you should be doing. In this case you want to narrow the generic ArrayList constructor to return a List<String>. Try this:

return new ArrayList<String>(Arrays.asList(StringArray));

[tmiranda]

That made it compile cleanly, thanks Greg.