How to cast generic List types in java?

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).

---

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.

Answer 2

I wonder what you might need that for because LinkedList extends List so there’s no need to cast.

Answer 3

LinkedList implements List, so you can implement

List< String > list1 = new LinkedList< String >();

Or do you want to cast from LinkedList< String > to List< int >? in this case you have to pick every single element and convert it to an integer.

Answer 4

List is an interface, LinkedList is a concrete implementation of that interface. Much of the time an implicit cast will work, assign a LinkedList to a List, or pass it to a function expecting a List and it should just `work'.

An explicit cast can also be done if necessary.

//This is valid
List<Customer> myList = new LinkedList<Customer>();

//Also Valid
List<Customer> myList = (List<Customer>) new LinkedList<Customer>();

Answer 5

>    public List<Object> getList()

Why are you returning List<Object>? You might as well return List (without generics) since that is equivalent but would make the following code work:

LinkedList<Customer> theList = new LinkedList<Customer>();

public List getList() {
    return theList;
}

Casting between Lists with different generic types is tricky and seems unnecessary here.

Of course you should be returning type List<Customer> ...

Answer 6

You should return a List<?> from your method. Intuitively, getList() returns a list so that the caller can retrieve the items inside. List<?> (which is equivalent to List<? extends Object>) allows that functionality. However, you won't be able to put anything into it via the returned list, because that would not be type safe; but I don't think that is what you need anyway.

public List<?> getList()
{
    return theList;
}