Cannot convert ArrayList<ArrayList<Integer>> to ArrayList<ArrayList<Object>>

Question

I have a function foo which accept a parameter ArrayList<ArrayList<Object>> . When I tried to call the function by passing a variable with type of ArrayList<ArrayList<Integer>> , the compiler shows error message that says :

incompatible types: 
java.util.ArrayList<java.util.ArrayList<java.lang.Integer>> 
cannot be converted into  
java.util.ArrayList<java.util.ArrayList<java.lang.Object>>

What should I change/do in order to make the function accept parameter(s) with 2D ArrayList with any type? Thanks in advance.

Sample code

public static ArrayList<ArrayList<Object>> foo (ArrayList<ArrayList<Object>> parameter)
{
    //do something
}

Calling the function

ArrayList<ArrayList<Integer>> parameter;
//do something with the parameter
ArrayList<ArrayList<Integer>> product = foo(parameter);//red line under parameter indicate it has error

Answer 1

Make it generic:

public static <T> ArrayList<ArrayList<T>> foo (ArrayList<ArrayList<T>> parameter) {
  //do something

  // you probably want to create a new 2D ArrayList somewhere around here
  ArrayList<ArrayList<T>> ret = new ArrayList<>();

  //do more somethings
}

Answer 2

There are 2 solutions:

1.Use generic (and btw should use List instead of ArrayList)

public static <T> List<List<T>> foo(List<List<T>> parameter) {
    //do something
}

public void test() {
    List<List<Integer>> parameter;
    //do something with the parameter
    List<List<Integer>> product = foo(parameter);
}

2.Use nested wildcards:

public static List<? extends List<? extends Object>> foo(List<? extends List<? extends Object>> parameter) {
    //do something
}

public void test() {
    List<List<Integer>> parameter = new ArrayList<>();
    //do something with the parameter
    foo(parameter);
}