UnsupportedOperationException on Collection

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

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 2

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.

Answer 3

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.