ClassCastException Object cannot be cast to

Question

I don't understand why I get ClassCastException on below code in line:

for(int i = 0; i < k.t.length; i++)

So problem is that in method addElement I make replacing of array elements by objects with type T. In my opinion in array should be objects with type T. And compiler doesn't protest for that. But in run-time JVM cannot cast despite in array is really objects with type T (in case below String), why JVM cannot use polymorphism?

But when I change the T[] t; to Object[] t;

and remove cast in constructor it run correctly without any errors, why?

public class MyCollection<T> {

    T[] t;

    MyCollection( int size){
        t = (T[]) new Object[size];
    }

    boolean addElement(T e, int i){        
        if(i < t.length){
            t[i] = e;
            return true;
        }
        return false;            
    }

    public static void main(String[] ss){

        MyCollection<String> k = new MyCollection<String>(3);

        k.addElement("a",0);
        k.addElement("b",1);
        k.addElement("c",2);

        for(int i = 0; i < k.t.length; i++)
            System.out.println(k.t[i]);     

        //for(String s : (String[])k.t)
        //    System.out.println(s);        
    }
}

http://stackoverflow.com/questions/529085/how-to-generic-array-creation — merlin2011, Jun 08 '14 at 08:21
@algui91 The problem isn't on System.out.println(k.t[0]); but on for(int i = 0; i < k.t.length; i++) — LunaVulpo, Jun 08 '14 at 08:44

shmosel · Accepted Answer · 2014-06-08T19:56:04.480

The problem is that you're casting Object[] to T[], and then you're exposing the underlying array. The only reason this works altogether is because the type erasure of T is Object. But since in our case T is being interpreted as String, when we access the array externally, we're trying to cast it to String[], which is incorrect. In order to avoid this issue, you should make the array private and provide accessor methods to retrieve elements. By doing that, you only cast individual elements to their correct type without making assumptions about the underlying array.

public class MyCollection<T> {

    private T[] t;

    MyCollection( int size){
        t = (T[]) new Object[size];
    }

    boolean addElement(T e, int i){        
        if(i < t.length){
            t[i] = e;
            return true;
        }
        return false;

    }

    T getElement(int index) {
        return t[index];
    }

    int getLength() {
        return t.length;
    }

    public static void main(String[] ss){

        MyCollection<String> k = new MyCollection<String>(3);

        k.addElement("a",0);
        k.addElement("b",1);
        k.addElement("c",2);

        for(int i = 0; i < k.getLength(); i++)
            System.out.println(k.getElement(i));     

        //for(String s : (String[])k.t)
        //    System.out.println(s);        
     }
}

Note that Java's Collection interface demonstrates the same behavior. Collection.toArray() returns Object[] regardless of the type of E. The only available workaround is Collection.toArray(T[]), where you're forced to pass an array with a fixed type, which can then be either be populated or copied.

It would also break type-safety to allow such a cast (Object[] to T[]), as there is no way to statically determine whether your Object-array contains only T's. — Seb, Jun 08 '14 at 20:00
@SebastianOkon As long as the array is internally managed, you can be certain it will always contain `T`s. In that case, using an array of `T` lets you skip casting values to `T` upon retrieval. — shmosel, Jun 08 '14 at 20:07
I did not meant to disagree to you. If it is implemented like this it is fine. Nevertheless this is potentially dangerous as you do not get any warning in case you make some changes/extensions later. — Seb, Jun 08 '14 at 20:27

score 1 · Answer 2 · answered Jun 08 '14 at 08:17

1

Check again the line of the problem. In my opinion the exception will be thrown because of:

    for(String s : (String[])k.t)

You`re trying to cast to String[] here, while the array is defined as Object[]:

t = (T[]) new Object[size];

answered Jun 08 '14 at 08:17

Aleksander Lech

1,105
12
16

But he says the error is in the first loop `for(int i = 0; i < k.t.length; i++)` – Alejandro Alcalde Jun 08 '14 at 08:18
1

The sample problem occurs in the first loop. It's really equivalent to `for (int i = 0; i < ((String[])k.t).length); i++)`. – Oliver Charlesworth Jun 08 '14 at 08:19
You can look how own collection with array can be made in http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/ArrayList.java – hexin Jun 08 '14 at 08:26

score 0 · Answer 3 · answered Jun 08 '14 at 08:34

0

you can Create a new instance of array using the reflection to avoid ClassCastException

example:

    import java.lang.reflect.Array;

public class MyCollection<T> {

    T[] t;

    MyCollection(Class<T> clazz, int size) {
        t = (T[]) Array.newInstance(clazz, size);

    }

    boolean addElement(T e, int i) {
        if (i < t.length - 1) {
            t[i] = e;
            return true;
        }
        return false;

    }

    public static void main(String[] ss) {

        MyCollection<String> k = new MyCollection<String>(String.class, 3);

        k.addElement("a", 0);
        k.addElement("b", 1);
        k.addElement("c", 2);

        for (int i = 0; i < k.t.length; i++)
            System.out.println(k.t[0]);

    }
}

answered Jun 08 '14 at 08:34

Rod_Algonquin

26,074
6
52
63

I know that I can do that. But I'm instresting why I cannot do as I write. – LunaVulpo Jun 08 '14 at 08:46
So problem is that in method addElement I make replacing of array elements by objects with type T. In my opinion in array should be objects with type T. And compiler doesn't protest for that. But in run-time JVM cannot cast despite in array is really objects with type T (in case below String), why JVM cannot use polymorphism? – LunaVulpo Jun 08 '14 at 08:54
@LunaVulpo that is beause java doesnt know what T is and therefore cant use polymorphism due to that. – Rod_Algonquin Jun 08 '14 at 09:05

ClassCastException Object cannot be cast to

3 Answers3