Question

Well, I have a class Customer (no base class).

I need to cast from LinkedList to List. Is there any clean way to do this?

Just so you know, I need to cast it to List. No other type will do. (I’m developing a test fixture using Slim and FitNesse).

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EDIT: Okay, I think I need to give code examples here.

import java.util.*;
public class CustomerCollection
{
    protected LinkedList<Customer> theList;

    public CustomerCollection()
    {
        theList = new LinkedList<Customer>();
    }

    public void addCustomer(Customer c){ theList.add(c); }
    public List<Object> getList()
    {
        return (List<? extends Object>) theList;
    }
}

So in accordance with Yuval A’s remarks, I’ve finally written the code this way. But I get this error:

CustomerCollection.java:31: incompatible types
found   : java.util.List<capture#824 of ? extends java.lang.Object>
required: java.util.List<java.lang.Object>
        return (List<? extends Object>)theList;
               ^
1 error

So, what’s the correct way to do this cast?

Answer 1

You do not need to cast. LinkedList implements List so you have no casting to do here.

Even when you want to down-cast to a List of Objects you can do it with generics like in the following code:

LinkedList<E> ll = someList;
List<? extends Object> l = ll; // perfectly fine, no casting needed

Now, after your edit I understand what you are trying to do, and it is something that is not possible, without creating a new List like so:

LinkedList<E> ll = someList;
List<Object> l = new LinkedList<Object>();
for (E e : ll) {
    l.add((Object) e); // need to cast each object specifically
}

and I’ll explain why this is not possible otherwise. Consider this:

LinkedList<String> ll = new LinkedList<String>();
List<Object> l = ll; // ERROR, but suppose this was possible
l.add((Object) new Integer(5)); // now what? How is an int a String???

For more info, see the Sun Java generics tutorial. Hope this clarifies.