ArrayList<A> conversion to ArrayList<B> 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<Listable> 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<Rule>();

And Rule implements Listable.

When I try to compile I get:

required: ListableList<Listable>,String
found: ListableList<Rule>,String
reason: actual argument ListableList<Rule> cannot be converted to ListableList<Listable> by method invocation conversion

Why is this, from what I've learned, this should work?? Thanks in advance:)

Answer 1

try to change your method signature of printList to:

public static void printList(ListableList<? extends Listable> list, String seperator){

Generic Types are not polymorphic, I.e., Listable<Listable> is not a super type of List<Rule> 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<? extends Listable> list, String seperator){
      list.add(whatever); // is not allowed

Useful Links

• An Awesome Tutorial for generics in java

Answer 2

Please use:

public static void printList(ListableList<? extends Listable> list, String seperator){

This means that list is a readonly collections, which contains subclasses of Listable.

ListableList<Rule> is not assign-compatible to ListableList<Listable> because you can add non Rule objects to a ListableList<Listable>.

Answer 3

Method declaration should be like printList(ListableList<? extends Listable> list, String seperator). Make it generic.