ClassCastException when taking element from the ArrayList

I have an ArrayList and when I’m trying to access some objects I get ClassCastException: class java.lang.Double cannot be cast to class java.lang.String. But ArrayList only has Strings in it…

Here’s the fragment of the code.

    ArrayList<ArrayList<String>> listDefect = getListSortingDefectWithoutMTPGrouping(party1, party2);
 
    for (int i = 0; i < listDefect.size(); i++) {
            ArrayList<String> list = listDefect.get(i);
            <...>
            cell.setCellValue(list.get(2));

getClass() returns String for this element, and if I’m trying to print it with System.out.println I’m getting the same exception, but the element is still being printed.

I tried to play dirty and erased the type from the List.

 for (int i = 0; i < listDefect.size(); i++) {
        ArrayList list = listDefect.get(i);
        <...>
        cell.setCellValue(list.get(2));

It didn’t help.

The method I’m calling to collect data also returns only Strings and throws no exceptions.

private ArrayList<ArrayList<String>> getListSortingDefectWithoutMTPGrouping(String party1, String party2) 

I don’t know what can be the problem, can anyone help?

>Solution :

I think you should check how your ArrayLists inside listDefect are populated. It is definitely possible in Java to have a List<String> that contains Double values instead of Strings by doing a few casts. Check out this piece of code:

import java.util.List;

public class MyClass {
    public static void main(String args[]) {
        
        Double d = 10.0D;
        var l = List.of(d);
        List<?> l1 = (List<?>)l;
        List<String> l2 = (List<String>)l1;

        String foo = l2.get(0);
    }
}

It runs completely fine until you try to assign l2.get(0) to a String variable. If you try to do so, it will throw

Exception in thread "main" java.lang.ClassCastException: class java.lang.Double cannot be cast to class java.lang.String (java.lang.Double and java.lang.String are in module java.base of loader 'bootstrap')
    at MyClass.main(MyClass.java:11)