Why won't Java call the (List<Object>) method, if I have an (Object...) one?

Question

I have the following class which stores a list of object arrays.

public class Test {
    private List<Object[]> list = new ArrayList<Object[]>();

    public void addList(Object... obj) {
        list.add(obj);
    }

    public void addList(List<Object> lst) {
        list.add(lst.toArray());
    }
}

When I call the following, the overloaded method addList(Object... obj) is called but I want the addList(List<Object> lst) to be called. How can I do this?

public class Main {
    public static void main(String[] args) {
        Test testObj = new Test();
        List<String> myStrings = new ArrayList<String>();
        myStrings.add("string 1");
        myStrings.add("string 2");
        myStrings.add("string 3");

        // The variable argument method is called but this is a list!
        testObj.addList(myStrings);

    }    
}

Answer 1

Change List<Object> to List<?> to capture lists of any type of object. I tried this and it printed "in List":

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

public class Test {
    private List<Object[]> list = new ArrayList<Object[]>();

    public void addList(Object... obj) {
        System.out.println("in object");
        list.add(obj);
    }

    public void addList(List<?> lst) {
        System.out.println("in List<?>");
        list.add(lst.toArray());
    }

    public static void main(String[] args) {
        Test testObj = new Test();
        List<String> myStrings = new ArrayList<String>();
        myStrings.add("string 1");
        myStrings.add("string 2");
        myStrings.add("string 3");

        // The variable argument method is called but this is a list!
        testObj.addList(myStrings);

    } 
}

Answer 2

It's problem of Java Generic. You cannot assign List<String> to List<Object>.

See also: Java Reference assignment with generic lists

Answer 3

Rewrite the type of you non-variadic method to use a wildcard:

public void addList(List<?> lst) {
    list.add(lst.toArray());
}

Then List<String> will be a subtype of the parameter type.

Answer 4

List<String> is not a subclass of List<Object>. So that overload will never be called, even if you remove the ... variant.

Answer 5

Change your method to

public void addList(List<?> lst) {
    list.add(lst.toArray());
}