如何使方法不返回List并争取不变性？

Question

If we have a method like this:

List<TypeA> retrieve(List<TypeB> sTypes) { 
   List<TypeA> aList = null;
   if(sTypes != null)
   for(TypeB type : sTypes){ 
     TypeA a = type.getA();
     aList.add(a);
   }
   return aList;
}

Instead of returning this list, is there a way we can make it immutable not using final keyword? If we return the list there might be chance that someone can add to the list and hence not immutable anymore.

Answer 1

Collections.unmodifiableList() may help you.This method can convert a list to immutable list.

 List retrieve(List sTypes) {
        List aList = null;
        if (sTypes != null)
            for (TypeB type : sTypes) {
                TypeA a = type.getA();
                aList.add(a);

            }
        return **Collections.unmodifiableList(aList);**
    }