CODEWITHSUNDEEP
javaarraylist
TheBook
TheBook

Reputation: 1708

Type mismatch when creating List of objects

I am trying to part the mainListarraylist to 3 sublist Arraylist and then add it as sublist in other arraylist mainSublist but I am getting this error: How can I fix it?

I appreciate any help.

1)-Type mismatch: cannot convert from List to ArrayList

2) -The method add(RootCreator) in the type ArrayList is not applicable for the arguments (ArrayList)

Code:

    ArrayList<RootCreator> mainList = new ArrayList<RootCreator>();
    for (String key : names) {
        RootCreator rootcreat = join_line(path, key);
        mainList.add(rootcreat);
    }
    ArrayList<RootCreator> mainSublist = new ArrayList<RootCreator>();

    for(int i= 0 ; i < mainList.size(); i++){
        int index = i*3;    
        //the error 1 is here       
        ArrayList<RootCreator> sublist = mainList.subList(0, index);
         //error 2 is here
        mainSublist.add(sublist);
    }

Upvotes: 1

Views: 951

Answers (3)

Kylar
Kylar

Reputation: 9334

OK, let's look at the two errors here:

//the error 1 is here       
ArrayList<RootCreator> sublist = mainList.subList(0, index);

This is telling you that you're trying to assign something of the type List to a variable that's declared as an ArrayList. While an ArrayList is a subtype of List, it doesn't work both ways - you can assign an ArrayList to a List, but not vice versa. So let's change your declaration:

//the error 1 was here, but fixed.        
List<RootCreator> sublist = mainList.subList(0, index);

The second one is slightly different:

//error 2 is here
mainSublist.add(sublist);

Here, you've got a list defined to hold items of the type RootCreator - but you're not adding RootCreator items, you're adding a list of them. So if you want to have the list hold other lists, you need to specify that when you create it:

ArrayList<List<RootCreator>> mainSublist = new ArrayList<List<RootCreator>>();

But, if you're trying to just keep a single list and want to add everything from the sublists, then instead, change your code to:

//error 2 was here, but Fixed
mainSublist.addAll(sublist);

Upvotes: 0

Ezequiel
Ezequiel

Reputation: 3592

All List are not ArrayList, so when you use sublist you get a generic List. On the other hand, the method to add a collection to another collection is addAll instead add.

List<RootCreator> mainList = new ArrayList<RootCreator>();
for (String key : names) {
    RootCreator rootcreat = join_line(path, key);
    mainList.add(rootcreat);
}
List<RootCreator> mainSublist = new ArrayList<RootCreator>();

for(int i= 0 ; i < mainList.size(); i++){
    int index = i*3;    
    //the error 1 is here       
    List<RootCreator> sublist = mainList.subList(0, index);
     //error 2 is here
    mainSublist.addAll(sublist);
}

Upvotes: 1

rgettman
rgettman

Reputation: 178253

The subList method returns a List, not an ArrayList, and you cannot assign a List to an ArrayList. In fact, the List returned by the subList method is not an ArrayList.

If it needs to be an ArrayList, then create an ArrayList out of the sub list.

ArrayList<RootCreator> sublist = new ArrayList<>(mainList.subList(0, index));

If you just need a List, then make sublist a List instead.

List<RootCreator> sublist = mainList.subList(0, index);

You can't add an ArrayList<RootCreator> to an ArrayList<RootCreator>; you must add a RootCreator. If you want to add the elements of sublist to another list, then use the addAll method.

mainSublist.addAll(sublist);

Upvotes: 0

Related Questions