Why I get UnsupportedOperationException for this?

Question

I am using a List to hold some data obtained by calling Array.asList() method. Then I am trying to remove an element using myList.Remove(int i) method. But while I try to do that I am getting an UnsupportedOperationException. What would be the reason for this? How should I resolve this problem?

Answer 1

the implementation you receive from asList doesnt implement a full List interface.I would transform the list to ArrayList and then do modifications on it.

See http://java.sun.com/j2se/1.4.2/docs/api/java/util/List.html#remove%28int%29

Answer 2

It's not java.util.ArrayList. Arrays.asList() returns its own List implementation and it does not support removal. You can create a real ArrayList from it:

new java.util.ArrayList(Arrays.asList(someArray));

It's very confusing how asList() works, I must admit.

Answer 3

Because you get read-only list. try

List newList = new ArrayList(myList);

Answer 4

Array.asList() wraps an array in the list interface. The list is still backed by the array. Arrays are a fixed size - they don't support adding or removing elements, so the wrapper can't either.

The docs don't make this as clear as they might, but they do say:

Returns a fixed-size list backed by the specified array.

The "fixed-size" bit should be a hint that you can't add or remove elements :)

Although there are other ways around this (other ways to create a new ArrayList from an array) without extra libraries, I'd personally recommend getting hold of the Google Collections Library (or Guava, when it's released). You can then use:

List<Integer> list = Lists.newArrayList(array);

The reason I'm suggesting this is that the GCL is a generally good thing, and well worth using.

Answer 5

Please read the API docs for Arrays.asList():

Returns a fixed-size list backed by the specified array. (Changes to the returned list "write through" to the array.)

Note that Collections.remove(int) is marked in the Javadocs as an "optional operation", meaning not all Collections will support it. "fixed-size list" means you cannot change the list's size, which remove() would do. So it's not supported.

If you want to change the list generated by Arrays.asList(), just copy it, e.g. new ArrayList(Arrays.asList(...)).