ArrayList<HashMap<String,String>> keys iteration/output to arrays

Question

If I have a type

ArrayList<HashMap<String,String>> keys = functionWhichReturnsThisType();

How can I iterate through so that I end up with all <1st string of hashmap> in a string array and likewise <2nd string of hashmap> into another string array.

I have tried to use the iterator but the hierarchy in the data type is confusing me.

 appPrefs = new AppPreferences(context.getApplicationContext());
      ArrayList<HashMap<String,String>> keys = appPrefs.getDownloadUrls();
      ArrayList<String> urls = new ArrayList<String>();
      ArrayList<String> filenames = new ArrayList<String>();
      Iterator myIterator = keys.keySet().iterator();
      while(myIterator.hasNext()) {
          urls.add((String)myIterator.next());
          filenames.add((String)keys.get(myIterator.next()));
      }

Answer 1

If the order doesn't matter, you can try

for (HashMap<String, String> map : keys) {
    urls.addAll(map.keys());
    filenames.addAll(map.values());
}

If you want to keep the order, you can try

for (HashMap<String, String> map : keys) {
    for (Map.Entry<String, String> entry : map.entrySet()) {
        urls.add(entry.getKey());
        filenames.add(entry.getValue());
    }
}

Answer 2

OK, I'll walk through your sample code and show where you're running into issues, and suggest how you can get it to work.

  ArrayList<HashMap<String,String>> keys = appPrefs.getDownloadUrls();

This (above) is fine - but remember keys is an ArrayList . It's a list of HashMap objects, but it's still a list

  ArrayList<String> urls = new ArrayList<String>();
  ArrayList<String> filenames = new ArrayList<String>();

These are good, but in typical Java, it would be better to have List<String> urls = new ArrayList<String>(); to try and keep your variables using interfaces instead of concrete implementations.

  Iterator myIterator = keys.keySet().iterator();
  while(myIterator.hasNext()) {

This won't work, because keys is an ArrayList , and a list does not have a keySet() you want to do:

 Iterator<HashMap<String,String> listIterator = keys.iterator();
 while(listIterator.hasNext()) {
     HashMap<String,String> map = listIterator.next();
     Iterator<String> myIterator = map.keySet().iterator();
     while(myIterator.hasNext()) {

Or, even better would be to use the Java 1.5 for(each) loop:

 for( Map<String,String> map : keys ) {
    for( String url : map.keySet() ) {

--

      urls.add((String)myIterator.next());

The above would work, once you get myIterator to be an iterator over the map, rather than the list.

      filenames.add((String)keys.get(myIterator.next()));

But this won't for 2 reasons

1. Because keys is still a list.
2. If you call next on an iterator twice then you get 2 different objects.

You need to have:

      String url = myIterator.next();
      urls.add(url);
      filenames.add(map.get(url));

Or, if you use the for(each) loop I suggested above, then you can skip that first line.

Hope that helps - if something's unclear please add a comment.

Note: solilo 's solution is a lot simpler and is a good way to do it, my answer is here to help you see where you were running into trouble.

Answer 3

This method will work for you extract first and second strings

private void getFirstAndSecondStrings(ArrayList<HashMap<String,String>> keys){
    ArrayList<String> firstStrings = new ArrayList<String>();
    ArrayList<String> secondStrings = new ArrayList<String>();
    for (HashMap<String, String> map : keys) {
        Iterator iterator = map.entrySet().iterator();

        while (iterator.hasNext()) {
            Map.Entry pairs = (Map.Entry)iterator.next();
            firstStrings.add((String)pairs.getValue());
            secondStrings.add((String)pairs.getKey());
        }
    }
}