java - How to add value into collection -

i have hashtable this:

hashtable<string, string> ht = new hashtable<string, string>();  ht.put("a", "one");  ht.put("b", "two"); 

then called values() methods values , stored in collection object this:

collection<string> col = ht.values(); 

now collection object has these values:

 one, two. 

then called col.add(" three"); time got error:

exception in thread "main" java.lang.unsupportedoperationexception. 

i checked api:

if collection refuses add particular element reason other contains element, must throw exception (rather returning false) 

the value added("three") collection unique,not duplicate.but can other operations remove() , clear() operations on it.

not able call add().why not allowing add?

the collection returned values() method in hashtable not support adding new elements.

from javadocs:

returns collection view of values contained in hashtable. collection backed hashtable, changes hashtable reflected in collection, , vice-versa. collection supports element removal (which removes corresponding entry hashtable), but not element addition.