Useful factory for defensive copying (short-circuit with new immutable collections)

[Daniel Latrémolière]

In a library, when you receive externally an object at construction of 
one of your class and mandate it to be immutable, you need to make a 
defensive copy. Given new immutable collections, it would be useful to 
have a factory for defensive copy, doing nothing if object to be copied 
is already an immutable collection (List, Set, Map), e.g. with a Map:

public static <K,V> Map<K,V> ofEntries(Map<K,V> map) {
   // short-circuit if already an immutable Map
   if (map.getClass().getDeclaringClass() == java.util.ImmutableCollections.class) {
    return map;
   }
   // if mutable, then make an immutable copy
   Map.Entry[] entries = new Map.Entry[map.size()];
   int i = 0;
   for(Map.Entry<K,V> entry : map.entrySet()) {
     entries[i] = Map.entry(entry.getKey(), entry.getValue());
     i += 1;
  }
  return Map.ofEntries(entries);
}


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Thanks,
Daniel.