Java / Generics / ClassCastException

Question

I have a LinkedList<T> which contains Object toArray() method:

public Object[] toArray()
{

    Object[] array = new Object[size];

    int c=0;
    for(Node<T> i = first;i != null;i=i.next)
    {
        array[c++] = i.data;
    }

    return array;
}

I would like to sort my LinkedList with a generic method: <T extends Comparable> void sort (List<T> list). To sort lists, i must represent them as array in this method:

T[] elements = (T[])list.toArray();`

However, i get ClassCastException at this line and i don't know why. Since the generic type of the method equivalent to the element's runtime type in the returned array, this cast is not a lie!

could you provide more code please? its hard to tell anything like that — Ariel Pinchover
– Ariel Pinchover, Commented Feb 6, 2016 at 13:56
List#toArray() returns Object[], not T[] (array keeps type of its elements, in this case it is Object). To sort a list, use Collections.sort(list), or provide T[] in toArray(T[] destination) — Alex Salauyou
– Alex Salauyou, Commented Feb 6, 2016 at 14:00
Why not just sort the list without changing it to an array using java.util.Collections: Collections.sort(list)? — whistling_marmot
– whistling_marmot, Commented Feb 6, 2016 at 14:37

Markus Wagner · Accepted Answer · 2016-02-06 14:30:13Z

2

toArray() returns Object[]. The type information is lost and you can't cast it back to T[]. If you want to keep the type information you can use the following. Then you give the method an predifined array which get filled. If you don't give anything - toArray will create a new Object[].

T[] elements = list.toArray(new T[list.size()]);

just filling an array (another writing style):

T[] elements = new T[list.size()];
list.toArray(elements);

or if you use Java 8:

T[] elements = list.stream().toArray(T[]::new);

edited Feb 6, 2016 at 14:30

answered Feb 6, 2016 at 14:11

Markus Wagner

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2 Comments

Beny Bosko Over a year ago

However, the complier says i cannot create generic array.

Markus Wagner Over a year ago

Of course, the T is only for demonstration. Outside, where you call the toArray method, you have to give a real type like String or Integer. Internally in the toArray method, the generics are used.

Stefan Dollase · Accepted Answer · 2016-02-06 16:00:49Z

2

The method LinkedList.toArray() creates a new array of the type Object[]. It does not create a new array of the type T[]. This is important, because even though the array only contains instances of T, you cannot assign the array to a variable of the type T[], because the array itself has the type Object[]. You can reproduce this error with the following code:

String[] array = (String[]) new Object[0];

If I understand you correctly, you want to convert the list into an array to be able to implement your own search function. Given a List<T>, that contains elements of the type T you want to convert this list into an array of the type T[]. However, you cannot simply call new T[list.size()], since Java looses the generic type information at compile time. To create the correctly typed array, you need to use the reflection method Array.newInstance().

Here is an example:

@SuppressWarnings("unchecked")
private <T extends Comparable<T>> void sort(List<T> list, Class<T> clazz) {
    T[] array = list.toArray((T[]) Array.newInstance(clazz, list.size()));
    // sort array and write result to the list
}

And here the usage:

    List<String> list = new LinkedList<String>();
    // populate the list
    sort(list, String.class);
    System.out.println(list); // -> the sorted list

edited Feb 6, 2016 at 16:00

answered Feb 6, 2016 at 14:12

Stefan Dollase

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2 Comments

Beny Bosko Over a year ago

Thanks for the extra work. I will do some further research in these stuff

Stefan Dollase Over a year ago

Always happy to help. The keyword is type erasure.

Burkhard · Accepted Answer · 2016-02-06 14:00:43Z

1

You should use T[] toArray(new T[list.size()]) instead. No need to cast.

answered Feb 6, 2016 at 14:00

Burkhard

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3 Comments

Alex Salauyou Over a year ago

@BenyBosko because T[] is "array of Ts", not "array of Objects". Type erasure isn't applied.

Beny Bosko Over a year ago

But i thought the underlying type is T and not Object in the returned array! Sorry, it is not clear.

Alex Salauyou Over a year ago

@BenyBosko this method appeared far before generics were introduced, when List was just List. Now it is impossible to change return type to T[] because of backward compatibility.

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