Generic abstract classes in Java

Question

I'm trying to make a simple little class where I can make something similar to a macro in C/C++. Basically I want to be able to call the following code when I need to perform a few actions though a list of objects, like so:

new IteratableList{
   void action() {
       // do something
   }
}.perform();

I haven't programmed in Java in quite a while, and I can't figure out what is wrong with my code when I try to add the abstract action method:

import java.util.List;
import java.util.Iterator;

public abstract class IteratableList<T> {
    public void perfrom(List<T> ls) {
        Iterator<T> l = ls.iterator();
        while (l.hasNext())
            action(l.next());
    }

    private abstract void action(<T> obj);
}

My code doesn't get any errors until I type that last method. I get errors telling me to remove abstract from the method header, and when I change that I get a list of other errors. I feel like I missed a fundamental thing here. Does any one see a glaring error?

Jeroen Vannevel · Accepted Answer · 2013-11-26 02:44:12Z

7

private abstract void action(<T> obj);

should be

protected abstract void action(T obj);

When you have a generic method you have to define it in the method signature as such:

protected abstract <T> void action(T obj);

unless (and this is the case here) the generic parameter is defined at class level.

You should also remove the private modifier and make it protected instead. abstract indicates the method has to be overridden, but private indicates subclasses can't access it. protected expands the accessibility of private to subclasses.

edited Nov 26, 2013 at 2:44

answered Nov 26, 2013 at 0:35

Jeroen Vannevel

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4 Comments

Jeroen Vannevel Over a year ago

Why was I downvoted? A comment to clarify would be a lot more helpful.

Elliott Frisch Over a year ago

Jeroen - because you can't have a private abstract method. Since (by definition) it would have to be protected or public or package for a subclass to override it. I objected to this line private abstract <T> void action(T obj);.

Jeroen Vannevel Over a year ago

@ElliottFrisch: that's right underneath it. I focused on the faulty generic assignment rather than the accessor at first. I'll edit it to prevent misunderstandings.

Elliott Frisch Over a year ago

Sorry. I must have missed it the first time.

Elliott Frisch · Accepted Answer · 2013-11-26 00:36:50Z

0

Instead of

private abstract void action(<T> obj);

I think you want

protected abstract void action(T obj);

The syntax for the generic was wrong, and you can't have a private abstract method.

answered Nov 26, 2013 at 0:36

Elliott Frisch

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