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.

So... what does myMethod2.getAllItems() return? You need to provide enough information to help. We cannot see your code. — Dave Newton
– Dave Newton, Commented Feb 18, 2014 at 12:50
It depends on how the list is created. Show us the code of myMethod2.getAllItems() — Marco Bolis
– Marco Bolis, Commented Feb 18, 2014 at 12:50
Doesn't this Collections$ UnmodifiableList $1 ring a bell? — m0skit0
– m0skit0, Commented Feb 18, 2014 at 12:53
Why immutable list? stackoverflow.com/questions/16891659/… — m0skit0
– m0skit0, Commented Feb 18, 2014 at 12:55

qqilihq · Accepted Answer · 2014-02-18 13:15:28Z

110

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);

edited Feb 18, 2014 at 13:15

answered Feb 18, 2014 at 12:52

qqilihq

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4 Comments

Kumar Abhinav Over a year ago

Arrays.asList is always modifiable.It is only not resizable

Varvara Kalinina Over a year ago

@qqilihq I'm just curious about your opinion - isn't this UnsupportedOperationException an example of bad design of Collections and breaking SOLID principles (namely, Liskov substitaution principle). Nothing breaks until runtime. Collections.sort expects just List interface, while it breaks on certain implementations. Isn't this sort-in-place thing an overall bad idea and a cleaner approach would be returning sorted Iterable, do you think?

qqilihq Over a year ago

@VarvaraKalinina I tend to agree. E.g. C# (afaik?) has an explicit ImmutableList type. I think similar constructs are available via 3rd party libs in Java, but interoperability with existing APIs will of course be bad. Imho an Iterable would make things only a little better, as Iterator again has a remove method, which would allow modifications (which means, at the end this method will have to throw an UnsupportedOperationException again to signal that it doesn't allow modifications).

Varvara Kalinina Over a year ago

@qqilihq I have come from some C# background to Java, and comparing to C#'s LINQ and basic collections with very clean and concise interfaces, Java's collection interfaces sometimes feel like written by a drunk person. Why indeed Iterator has remove method? C#'s Enumerator does not.

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