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

Answer 1

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;
}

Answer 2

>    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 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

just put

public static List<Object> getList() {
    List l = test;
    return l;
}

Answer 5

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 6

If your list is of a generic type for eg

ArrayList<String> strList = new ArrayList<String>(); strList.add("String1"); Object o = strList;

then Following method should work

public <T> List<T> getListObjectInBodyOfType(Class<T> classz, Object body) {
    if(body instanceof List<?>){
        List<?> tempList = (List<?>)body;
        if(tempList.size() > 0 && tempList.get(0).getClass().equals(classz)){
            return (List<T>) body;
        }
    }
    return new ArrayList<T>();
}

How to use it?

List<String> strList1 = getListObjectInBodyOfType(String.class, o);

as I mentioned before it works if the Object contains generic list, this won't work if you pass a non-generic list with mixed type of elements