error: incompatible types: List cannot be converted to ArrayList

Question

I have found a few questions similar to the problem I am facing, but I couldn't find solution. Example: Incompatible types List of List and ArrayList of ArrayList, Not able to understand how to define a List of List in java

The program should return list of lists. So, I declared a list of lists and then trying to add arraylists to it.

allsubsets = new ArrayList<List<Integer>>();

But, when I am trying to access each arraylist item from the list of lists as below, I get the error: incompatible types: List<Integer> cannot be converted to ArrayList<Integer>

for(ArrayList<Integer> subset:allsubsets)

When I try to convert the line to for(List<Integer> subset:allsubsets), it throws error that add, addAll don't exist for List type, which makes sense. Please help me understand how to access elements of list of lists in this case.

public List<List<Integer>> subsets(int[] nums) {
    List<Integer> arrayList = new ArrayList<Integer>();
    for(int i:nums) {
        arrayList.add(i);
    }
    return subsets(arrayList,nums.length);
}

public List<List<Integer>> subsets(List<Integer> arrayList, int index) {
    List<List<Integer>> allsubsets;
    if(index == -1) {
         allsubsets = new ArrayList<List<Integer>>();
         allsubsets.add(new ArrayList<Integer>());
    }
    else {
        allsubsets = subsets(arrayList, index-1);
        int item = arrayList.get(index);
        List<List<Integer>> moresubsets =  new ArrayList<List<Integer>>();
        for(ArrayList<Integer> subset:allsubsets) { 
        //The line above throws error as I created list of lists

            List<Integer> newsubset = new ArrayList<Integer>(); //create new subset
            newsubset.addAll(subset); // add all old items
            newsubset.add(item); // add new item
            moresubsets.add(newsubset); //add subset to moresubsets
        }
        allsubsets.add(moresubsets); // add to actual one
    }
  return allsubsets;
}

Note: If I change the return type to arraylist of arraylists, it works for me. But, I want to make it work for the list of lists

I am confused, `addAll` is being invoked on `List newsubset` — Scary Wombat, May 18 '18 at 02:12
What's the issue? "addAll don't exist for List type" is a [lie](https://docs.oracle.com/javase/8/docs/api/java/util/List.html#addAll-java.util.Collection-). — shmosel, May 18 '18 at 02:21
@ScaryWombat newsubset is initiated as arraylist. So addAll and add can be used. Please correct me if I am wrong. — zealouscoder, May 18 '18 at 02:22
They should work for any list type. Please provide a [mcve] demonstrating the issue. — shmosel, May 18 '18 at 02:22

Mạnh Quyết Nguyễn · Accepted Answer · 2018-05-18T02:28:17.810

4

Correct way to iterate your list of list should be:

    for(List<Integer> subset:allsubsets) {

instead of:

    for(ArrayList<Integer> subset:allsubsets) {

List<List<Integer>> allsubsets is declared as List of List, but the implementation is unknown.

Only you know the type of nested List is ArrayList, so either change foreach to use List<Integer> or manually cast your List<Integer> to ArrayList<> (this is not preferred)

One more thing:

    allsubsets.add(moresubsets); // add to actual one

This try to add a List of List (List<List<Integer>>) as element which should be List<Integer> hence compile error.

Change that statement to:

  allsubsets.addAll(moresubsets);

edited May 18 '18 at 02:28

answered May 18 '18 at 02:12

Mạnh Quyết Nguyễn

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But, when I change to List, I get error that list cannot have add and addAll functions. And I need list of arraylists and not list of lists. – zealouscoder May 18 '18 at 02:20
Did you use `for(List subset:allsubsets) {` as I suggested? It should work – Mạnh Quyết Nguyễn May 18 '18 at 02:21
Yes, I did as I mentioned in the question itself, and I get error: no suitable method found for add(List>) – zealouscoder May 18 '18 at 02:24
This resolved my issue. I missed the point that I am adding list of lists. Thanks – zealouscoder May 18 '18 at 02:27

StaticBeagle · Answer 2 · 2018-05-18T02:38:14.370

Let's try expanding that enhanced for loop into more basic components:

for(ArrayList<Integer> subset:allsubsets) { 
    //The line above throws error as I created list of lists
}

// this is roughly equivalent to 
Iterator<List<Integer>> it = allsubsets.iterator();
while(it.hasNext()) {
    ArrayList<Integer> subset = it.next(); // Error
    // Since the iterator was defined as an iterator to a List<List<Integer>>,
    // it.next() will return the next element in allsubsets
    // which happens to be an List<Integers>.
    // You can't assign a reference of a parent type to a child. However 
    // the opposite is perfectly fine, assigning a reference of a child type
    // to a parent.

    // If we change subset to be a List<Integer> i.e.
    // for(List<Integer> subset : allsubsets)
    // then we are assigning a reference of a List<Integer> to a List<Integer> 
    // so no problem.
}

Thank you for explaining in detail. – zealouscoder May 18 '18 at 02:36 — zealouscoder, May 18 '18 at 02:36
@ZealousCoder glad I could help. – StaticBeagle May 18 '18 at 02:38 — StaticBeagle, May 18 '18 at 02:38

score 0 · Answer 3 · answered May 18 '18 at 02:33

I prefer to share with you the code I did for managing the same type of Object List you are trying to handle. Hope this helps.

    public static void main(String[] args) {

    List<List<Integer>> allsubsets = setSubsets();

    List<List<Integer>> allsubsets2 = new ArrayList<>();

    allsubsets2.addAll(allsubsets);
    int i= 0;
    for (List<Integer> test : allsubsets2) {
        System.out.println(i + " Lista");
        for (Integer integer : test) {
            System.out.println(integer);
        }
        i++;
    } 
}
public static List<List<Integer>> setSubsets() {
    List<List<Integer>> allsubsets = new ArrayList<List<Integer>>();
    List<Integer> listInteger1 = new ArrayList<>();
    List<Integer> listInteger2 = new ArrayList<>();
    for (int i = 0; i < 100; i++) {
        listInteger1.add(i);
    }
    for (int i = 1010; i < 1110; i++) {
        listInteger2.add(i);
    }
    allsubsets.add(listInteger1);
    allsubsets.add(listInteger2);
    return allsubsets;
}

error: incompatible types: List cannot be converted to ArrayList

3 Answers3