Question

I’m looking to make a recursive method iterative.

I have a list of Objects I want to iterate over, and then check their subobjects.

Recursive:

doFunction(Object) while(iterator.hasNext()) {    //doStuff    doFunction(Object.subObjects); } 

I want to change it to something like this

doFunction(Object) iIterator = hashSet.iterator(); while(Iterator.hasNext() {    //doStuff    hashSet.addAll(Object.subObjects); } 

Sorry for the poor psuedo code, but basically I want to iterate over subobjects while appending new objects to the end of the list to check.

I could do this using a list, and do something like

while(list.size() > 0) {    //doStuff    list.addAll(Object.subObjects); } 

But I would really like to not add duplicate subObjects. Of course I could just check whether list.contains(each subObject) before I added It.

But I would love to use a Set to accomplish that cleaner.

So Basically is there anyway to append to a set while Iterating over it, or is there an easier way to make a List act like a set rather than manually checking .contains()?

Any comments are appreciated.

Thanks

Answer 1

I would use two data structures — a queue (e.g. ArrayDeque) for storing objects whose subobjects are to be visited, and a set (e.g. HashSet) for storing all visited objects without duplication.

Set visited = new HashSet();   // all visited objects Queue next = new ArrayDeque(); // objects whose subobjects are to be visited  // NOTE: At all times, the objects in 'next' are contained in 'visited'  // add the first object visited.add(obj);  Object nextObject = obj;  while (nextObject != null) {     // do stuff to nextObject      for (Object o : nextObject.subobjects)     {         boolean fresh = visited.add(o);          if (fresh)         {             next.add(o);         }     }      nextObject = next.poll(); // removes the next object to visit, null if empty }  // Now, 'visited' contains all the visited objects 

NOTES:

• ArrayDeque is a space-efficient queue. It is implemented as a cyclic array, which means you use less space than a List that keeps growing when you add elements.
• ‘boolean fresh = visited.add(o)‘ combines ‘boolean fresh = !visited.contains(o)‘ and ‘if (fresh) visited.add(o)‘.