不可修改的Map仍然接受添加条目而未返回UnsupportedOperationException

Question

Am trying out adding entry to unmodifiable Map, JVM catch UnsupportedOperationException when adding entry to a subview of original Map, but JVM does not care about direct adding to the original view, code speaks :

ConcurrentMap<String, Integer> origView= new ConcurrentHashMap<String, Integer>();
                Map<String,Integer> subView = Collections.unmodifiableMap(origView);
                origView.put("s", 44); // ok
                subView.put("p", 77); // java.lang.UnsupportedOperationException

However Documentation of Collections.unmodifiableSortedMap() says :

Attempts to modify the returned sorted map, whether direct, via its collection views, or via its subMap, headMap, or tailMap views, result in an UnsupportedOperationException.

please lend hand, thanks .

Answer 1

"Attempts to modify the returned sorted map. ..."

It will only throw an UnsupportedOperationException on the Map returned from the unmodifiableMap(...) call, which does not include the original Map .

The Map that gets returned from unmodifiableMap(...) is not the same as the original Map .

Answer 2

Collections.unmodifiableMap() returns an unmodifiable view of the original map; it does not change the original map in any way . That means that the original map is still modifiable. You will only get an exception if you try to modify the unmodifiable view.