Why does collections.sort throw unsupported operation exception while sorting by comparator in Java?

Question

Following is my code used to sort a list with predefined order. Defined order is mentioned in itemsSorted list.

final List<String> itemsSorted = myMethod.getSortedItems();  List<String> plainItemList = myMethod2.getAllItems();  final Comparator<String> comparator = new Comparator<String>() {              public int compare(String str1, String str2) {         return orderOf(str1) - orderOf(str2);     }      private int orderOf(String name) {                   return ((itemsSorted)).indexOf(name);     }  };  Collections.sort(plainItemList, comparator);  return plainItemList; 

The above code throws

Caused by: java.lang.UnsupportedOperationException     at java.util.Collections$UnmodifiableList$1.set(Collections.java:1244)     at java.util.Collections.sort(Collections.java:221) 

I'm not sure why the list is unmodifiable. Please help me on this.

Answer 1

The list is not modifiable, obviously your client method is creating an unmodifiable list (using e.g. Collections#unmodifiableList etc.). Simply create a modifiable list before sorting:

List<String> modifiableList = new ArrayList<String>(unmodifiableList); Collections.sort(modifiableList, comparator);