当retainall用于java中的两个键集时，不可修改的集合问题

Question

Map<String,String> map=request.getParameterMap();

^ is the unmodifiable map.

Set s1= map.keySet();
Set s2= map2.keySet();/* another keyset of local map*/

Using s1.retainAll(s2) throws an exception: at java.util.collections$unmodifiablecollection.retainall

Here request.getParameterMap() returns an unmodifiable map.. I tried creating a local map. But the issue stil persists. Suggest some solution.

Answer 1

The Set.retainAll method modifies the set it's being called on. Assuming the keySet method of you unmodifiable map is just a view on to the underlying map, it shouldn't allow modifications. You probably want to create a new (modifiable) set and then remove items from it:

Set s1 = new HashSet(map.keySet());
s1.retainAll(s3);

Answer 2

You are not allowed to modify the keyset of an unmodifiable map as the keyset returned is also unmodifiableSet. You can create a local map from the unmodifiableMap and than use retainAll on local map keyset.

Map map1 = new HashMap();
map1 = Collections.unmodifiableMap(map1);
Map map2 = new HashMap();
Map map3 = new HashMap(map1);
Set s1 = map1.keySet();
Set s2 = map2.keySet();
Set s3 = map3.keySet();
s3.retainAll(s2);