Collection上的UnsupportedOperationException

Question

While studying the Collection API, we find that some methods ( add , remove ,...) may throw a java.lang.UnsupportedOperationException if the current implementation of the Collection does not support those functionalities.

Is there,actually, in the JDK, a concrete Collection that does not support those methods ?

Thanks a lot for your answers.

Answer 1

Apart from the collections returned by the Collections.unmodifiable* methods, there are a couple more of interesting cases where UnsupportedOperationException is actually thrown:

• the collection views of a Map , accessed via entrySet() , keySet() and values() can have elements removed but not added,
• the list view returned by Arrays.asList can have elements neither added nor removed,
• moreover, the objects obtained from the Collections.empty* and Collections.singleton* methods are also marked as "immutable" , so - although it is not explicitly stated in the API docs - I suppose these throw the exception as well on attempts to modify them.

Answer 2

The obvious examples are the implementations returned from, say, Collections.unmodifiableCollection() and other similar methods. Methods that would change the Collection throw this exception.

Answer 3

Normally when you create a list like List<String> sample=Collections.emptyList(); . The List sample will be created as a Collections.unmodifiableCollection() .

• So the list sample does not support dynamic list operations. You can only assign another list to this list using assignment operator. Eg>

   List<String> ls=new ArrayList<String>(); ls.add("one"); ls.add("Three"); ls.add("two"); ls.add("four"); sample = ls; 

• For dynamic list operations you should have a syntax like List<String> sample= new ArrayList<String>(); . In this list you can perform sample.add(), sample.addAll() etc...

Answer 4

Yes. For example when you call Collections.unmodifiableList(list) , the returned list does not support add(..)

These collections, however, are mostly private classes which are not exposed an an API, so you cannot instantiate them.