Use list Addall add list to another

In this code, the last line (LIST1. Addall (List2);) Build compile time error

I do realize that if this is allowed, you can add to the same list instances of different classes But I don't know what is the right way

My GetList () method needs to return a subclass of baseClass. I can't change that requirement

I have two lists that are declared in the same way and contain objects of the same type. I need to add one to the other list Any ideas in this regard will be appreciated

BaseClass. java：

package main;

public class BaseClass {}

Main. java：

package main;

import java.util.ArrayList;
import java.util.List;

public class Main {

    private static List<? extends BaseClass> getList() {

        return new ArrayList<>();
    }

    public static void main(String[] args) {

        List<? extends BaseClass> list1 = getList();

        List<? extends BaseClass> list2 = getList();

        list1.addAll(list2); // Compile-time error: "The method addAll(Collection<? extends capture#3-of ? extends BaseClass>) in the type List<capture#3-of ? extends BaseClass> is not applicable for the arguments (List<capture#4-of ? extends BaseClass>)".
    }

}

Solution

You can create a new list < baseClass > and add two lists:

List<BaseClass> merged = new ArrayList<>();
merged.addAll(list1);
merged.addAll(lise2);

But it might be a better idea to have GetList return list < baseClass >