ArrayList conversion to ArrayList when A implements B Java

Question

I'm quite new to Java, and I have a question about method invocation conversion. I've made a new class that extends ArrayList called ListableList - but that does not seem to be the problem. The problem is I have a method that looks like this ``` public static void printList(ListableList list, String seperator){ for(Listable item : list){ System.out.println(seperator); System.out.println(item.toList()); } System.out.println(seperator); } ``` I call this method like this: ``` Output.printList(rules, ".."); ``` Where rules is initialized as ``` rules = new ListableList(); ``` And Rule implements Listable. When I try to compile I get: ``` required: ListableList,String found: ListableList,String reason: actual argument ListableList cannot be converted to ListableList by method invocation conversion ``` Why is this, from what I've learned, this should work?? Thanks in advance:)

Accepted Answer

try to change your method signature of printList to: ``` public static void printList(ListableList list, String seperator){ ``` Generic Types are not polymorphic, I.e., `Listable` is not a super type of `List` even though Rule is a `sub-type` of Listable. You will have to use generics with upper bounded wildcards in your method signature to inform that your List can accept anything which is a sub-type of Listable. Note that you cannot add anything into your list if you use `Generics with upperbounded wildcards`. I.e, ``` public static void printList(ListableList list, String seperator){ list.add(whatever); // is not allowed ``` Useful Links - An Awesome Tutorial for generics in java

ArrayList<A> conversion to ArrayList<B> when A implements B Java

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