Java Custom Generic List: add() method error

Question

I have a task in which I have to implement an add() method for a custom generic list. In my code i have the following structure:

public abstract class MyGenericListAbstract<T> {
    protected transient T head;
    protected transient T tail;
    protected transient int size;

    ...
}

public final class MyEmptyList<T> extends MyGenericListAbstract {

private T[] list;

... 
public final void add(T e)
{
    this.getList()[this.size()] = (T) e;
}

...

private T[] getList()
{
    return this.list;
}

}

The problem I have for now is that when i try to do this:

public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        MyGenericListAbstract <Integer> list0   = new MyEmptyList();
        list0.add(new Integer(3));
    }

}

I get the following error:

enter image description here

and I cannot figure out why ...

Can anyone please help me?

The entire code is available here, also some documentation about the task here. If you spot some flaws, I would be happy to hear them!

Thanks!

This is unfortunate to start with: public final class MyEmptyList<T> extends MyGenericListAbstract. That's using the raw type MyGenericListAbstract. Try public final class MyEmptyList<T> extends MyGenericListAbstract<T>. Likewise you should probably use MyGenericListAbstract <Integer> list0 = new MyEmptyList<>();. Avoid raw types as far as possible. — Jon Skeet
– Jon Skeet, Commented Jul 5, 2014 at 20:51
Why are you doing (T) e; whereas e is already a type of T? — Braj
– Braj, Commented Jul 5, 2014 at 20:54

joval · Accepted Answer · 2014-07-05 21:06:16Z

3

You hold a reference to MyGenericListAbstract <Integer> in list0 whereas it doesn't have an add method, as you previously said in a comment. The compiler doesn't see that method based on the list reference.

You should either change the reference type to MyEmptyList <Integer> or move the add(T e) method to MyGenericListAbstract class.

PS. The problem is not related with generics, it's just the basic Java inheritance principle.

edited Jul 5, 2014 at 21:06

answered Jul 5, 2014 at 20:59

joval

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