Java Cant add subclassed object to list

Question

Parent class:

public class Action
    {
        private String eventId;
        private List<ActionArgument> arguments;
        //action to perform when this action is done
        private List<? extends Action> onCompleteActions;

        public Action(){}

        public Action(String eventId, List<ActionArgument> arguments, List<? extends Action> onCompleteActions)
        {
            this.eventId = eventId;
            this.arguments = arguments;
            this.onCompleteActions = onCompleteActions;
        }

        public String getEventId()
        {
            return eventId;
        }
        public void setEventId(String eventId)
        {
            this.eventId = eventId;
        }
        public List<ActionArgument> getArguments()
        {
            return arguments;
        }
        public void setArguments(List<ActionArgument> arguments)
        {
            this.arguments = arguments;
        }
        public List<? extends Action> getOnCompleteActions()
        {
            return onCompleteActions;
        }
        public void setOnCompleteActions(List<? extends Action> onCompleteActions)
        {
            this.onCompleteActions = onCompleteActions;
        }
    }

extended class:

public class UserDefinedAction extends Action
{
    // body not important
}

Some service:

private boolean arrangeBefore(List<? extends Action> defaultActions, UserDefinedAction action)
    {
        String actionToFind = action.getDoBefore();
        boolean actionFound = false;
        for(int i = 0; i < defaultActions.size(); i++)
        {
            if(defaultActions.get(i).getEventId().toUpperCase().equals(actionToFind.toUpperCase()))
            {
                defaultActions.add(i, action);
                return true;
            }
...

So I have an error here: defaultActions.add(i, action); which says "

add
(int,
capture<? extends com.myPackage.Action>)
in List cannot be applied
to
(int,
com.myPackage.UserDefinedAction)

"

Can someone explain to me why this doesnt work?

If I do that I get this error:add (int, capture<? extends com.myPackage.Action>) in List cannot be applied to (int, com.myPackage.Action) — jensengar
– jensengar, Commented Aug 8, 2013 at 23:00
You can't add anything inside a list with upper bounds - List<? extends Action>. You can only add null, or the elements you previously fetched from it. — Rohit Jain
– Rohit Jain, Commented Aug 8, 2013 at 23:00
@RohitJain: "or the elements you previously fetched from it." How does the compiler keep track of that? — Thilo
– Thilo, Commented Aug 8, 2013 at 23:44

Andy Thomas · Accepted Answer · 2013-08-08 23:24:24Z

1

A simple possibility to consider is:

Make a defensive copy of the input list in the constructor.
Store it simply as a List<Action>.

In general, it's useful to remember the PECS acronym - Producer Extends, Consumer Super. If you want to add elements to a List with a bounded type variable or wildcard, you'd want to express it with super rather than extends. For example, List<? super A> means that the list member type is some superclass of an A.

edited Aug 8, 2013 at 23:24

answered Aug 8, 2013 at 23:12

Andy Thomas

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2 Comments

Thilo Over a year ago

+1. List<Action> should work well here. Note that a List<Action> can store subclasses of Action already, no need for a wildcard just for that.

Andy Thomas Over a year ago

Right. In this particular case, it doesn't seem called for. You could accept an arbitrary List<? super Action> in the constructor, or have a getter that accepts as an argument the List<? super Action> to be filled -- but I don't see how either would be particularly helpful.

rgettman · Accepted Answer · 2013-08-08 23:06:43Z

0

You cannot pass an argument of any type to a method with a wildcard in it. Here, you have a List<? extends Action>, and you're attempting to add a UserDefinedAction.

But Java doesn't know the exact generic type of your List; it assumes it could be anything, even something like List<SomeOtherAction>. It can't allow you to add a UserDefinedAction to your list for that reason. For this reason, Java disallows you to call such a method.

To get around this, you will need to define a generic type parameter on your Action class:

public class Action<A extends Action>

Then you can replace all <? extends Action> in that class with <E>. This way, an Action<Action> will be able to handle a List<Action>.

Next, define A in UserDefinedAction to be UserDefinedAction. This way, a UserDefinedAction can handle a List<UserDefinedAction>, which will allow you to add a UserDefinedAction.

public class UserDefinedAction extends Action<UserDefinedAction>

Alternatively, you could, for more flexibility, define UserDefinedAction this way:

public class UserDefinedAction<A extends UserDefinedAction> extends Action<A>

... and use a UserDefinedAction<UserDefinedAction> instance. This would allow further subclasses of UserDefinedAction to define the generic type parameter A the way they want.

edited Aug 8, 2013 at 23:06

answered Aug 8, 2013 at 23:00

rgettman

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2 Comments

jensengar Over a year ago

So what would you recommend for what im trying to do?

jensengar Over a year ago

Sorry, that wasnt there when I posted my previous comment.

Luiggi Mendoza · Accepted Answer · 2013-08-08 23:17:26Z

0

You can change the signature of the method and use generics in it:

public <A extends Action> void arrangeBefore(List<A> actionList, A action) {
    actionList.add(action);
}

Note that this change assumes that the methods used from A action in arrangeBefore belongs to Action class only.

edited Aug 8, 2013 at 23:17

answered Aug 8, 2013 at 23:12

Luiggi Mendoza

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