CODEWITHSUNDEEP
javagenericsclasscastexception
LunaVulpo
LunaVulpo

Reputation: 3221

ClassCastException Object cannot be cast to

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);        
    }
}

Upvotes: 2

Views: 5582

Answers (3)

shmosel
shmosel

Reputation: 50776

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.

Upvotes: 4

Rod_Algonquin
Rod_Algonquin

Reputation: 26198

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]);

    }
}

Upvotes: 0

Aleksander Lech
Aleksander Lech

Reputation: 1344

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];

Upvotes: 1

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