投射Iterator的最佳方法 <Object> 到一套 <String> ， 例如

Question

Casting an Iterator<Object> to a Set<String>

What would be the cleanest/best practice way?

Answer 1

public Set<B> getBs(){
    Iterator<A> iterator = myFunc.iterator();
    Set<B> result = new HashSet<B>();
    while (iterator.hasNext()) {
        result.add((B) iterator.next();
    }
    return result;
}

But of course, it will fail if all the A s returned by the iterator are not B s.

If you want to filter the iterator, then use instanceof:

public Set<B> getBs(){
    Iterator<A> iterator = myFunc.iterator();
    Set<B> result = new HashSet<B>();
    while (iterator.hasNext()) {
        A a = iterator.next();
        if (a instanceof B) {
            result.add((B) iterator.next();
        }
    }
    return result;
}

Using Guava, the above can be reduced to

return Sets.newHashSet(Iterators.filter(myFunc.iterator(), B.class));

Answer 2

If we are talking about iterators and collections that need to use them, and you need the iterater to be generic enough so that it can be used by different collections .
Just use if/else with instanceof keyword as follows:

while(iterator.hasNext()) {
  Object obj = iterator.next();
  if (obj instanceof A) {
    collection.add((A) o);  
  } else if (obj instanceof B) {
    collection.add((B) o);  
  } else if ...etc
}

Answer 3

I'm still not 100% sure what you want, but check this out and see:

public static void main(String[] args) {
  final Iterator<?> it = Arrays.asList(new Object[] {"a", "b", "c"}).iterator();
  System.out.println(setFromIterator(it));
}

public static Set<String> setFromIterator(Iterator<?> it) {
  final Set<String> s = new HashSet<String>();
  while (it.hasNext()) s.add(it.next().toString());
  return s;
}

Answer 4

That's the only way, probably.

while(iterator.hasNext()) {
  Object o = iterator.next();
  if (o instanceof B) {
    collection.add((B) o);  
  }
}

Answer 5

org.apache.commons.collections.IteratorUtils can be used to do this.

Here is an example to convert iterator to set;

Set<String> mySet = new HashSet<String>(IteratorUtils.toList(myIterator))

Answer 6

You can not cast the Iterator to Set directly. Iterator pattern provides the way to access the elements of an aggregate object sequentially without exposing its underlying presentation. The possible solution is to travel each elements sequentially and add each element to the set

while (iterator.hasNext()) {
    Object obj = iterator.next();
    set.add(obj.toString());
}