Merge two ArrayLists of Custom Object Type

Question

I want to combine two ArrayList into a single one. These ArrayLists are NOT the type of "String". Object type is a custom class.

private ArrayList<NewsItem> newsList;
private ArrayList<NewsItem> eventList;
private ArrayList<NewsItem> combinedList;

This is how I am trying to combine those two ArrayLists:

private void combineArrays() {

    if(newsList.size() > 0){
        for (int i = 0; i<newsList.size(); i++){

            NewsItem aBean = newsList.get(i);
            combinedList.add(aBean);
        }
    }
    if(eventList.size() > 0){
        for (int i = 0; i<eventList.size(); i++){
            NewsItem aBean = eventList.get(i);
            combinedList.add(aBean);;
        }
    }
}

The app is crashing. What's the wrong with this approach?

you missed the new operator on your lists: newsList = new ArrayList<>(); — Blackbelt
– Blackbelt, Commented Mar 25, 2015 at 9:28
@Blackbelt Why should I use newsList = new ArrayList<>();? newsList and evenList are NOT empty when it comes to private void combineArrays() method — smartsanja
– smartsanja, Commented Mar 25, 2015 at 9:31
I see only the declaration of these objects, never their initialisation. Can you post the stacktrace — Blackbelt
– Blackbelt, Commented Mar 25, 2015 at 9:33
@sajaz It doesnt matter if your existing lists are not empty. What matters is that you want to create a new list to combine the other two lists. This new list must be created before you can use it. Therefore you need a new somewhere. (Unless you would want to append list B to existing list A) — GhostCat
– GhostCat, Commented Mar 25, 2015 at 9:35

Florian Schaetz · Accepted Answer · 2015-03-25 09:33:03Z

4

You can do that simpler...

combinedList = new ArrayList<NewsItem>();
if (newsList != null)
  combinedList.addAll( newsList );
if (eventList!= null)
  combinedList.addAll( eventList);

If you don't want to have duplicates (implies you have implemented equals() correctly) then you could use a HashSet, for example, but that implies you don't need a specific order of elements.

answered Mar 25, 2015 at 9:33

Florian Schaetz

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Viswanath Donthi · Accepted Answer · 2015-03-25 09:31:41Z

0

You can use addAll() method as:

combinedList.addAll(newsList);
combinedList.addAll(eventList);

answered Mar 25, 2015 at 9:31

Viswanath Donthi

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marcS · Accepted Answer · 2015-03-25 09:34:08Z

0

If you want a new list :

 ArrayList<NewsItem> combinedList = new ArrayList();
 combinedList.addAll(newsList);
 combinedList.addAll(eventList);

too late ..

answered Mar 25, 2015 at 9:34

marcS

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