Exception thrown when using generic arrays

Question

I'm having an ArrayStoreException which I don't understand in following scenario:

file List.java:

import java.lang.reflect.Array;
class List<K> {
    K[] _list;
    K _dummy;
    int _size, _index;

    public List(int size) {
        _size = size;
        _index = 0;
        Class<?> cls = getClass();

        // as following is not allowed
        // _list = new K[size]; -->  cannot create a generic array of K
        // I'm doing the following instead
        _list = (K[])Array.newInstance(cls,size);
    }

    public void add(K obj) {
        _list[_index++] = obj;  // HERE'S THE EXCEPTION
                        //      java.lang.ArrayStoreException ??

        // IF I ASSIGN _dummy INSTEAD
        _list[_index++] = _dummy;   // NO ERROR
    }
} // class List

file mainLists.java:

public class mainLists {    
    public static void main(String[] args) {        
        List<String> list = new List<String>(5);
        list.add("test");
    }
}

What the docs say about ArrayStoreException: "Thrown to indicate that an attempt has been made to store the wrong type of object into an array of objects"

but I'm passing a String type "test" to the method add() no?

what is the problem here?

thx

CHris

JB Nizet · Accepted Answer · 2014-02-21 13:22:46Z

4

Your array is an array of List, and not an array of K, since you instantiate it with

Array.newInstance(cls,size);

where cls is initialized with

Class<?> cls = getClass();

which returns the current object class, i.e. the class of this.

You can simply use an Object[].

answered Feb 21, 2014 at 13:22

JB Nizet

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Pshemo · Accepted Answer · 2014-02-21 13:29:58Z

2

Class<?> cls = getClass(); will use current instance (this) to get Class object so later you are creating array of Lists which will not accept Strings.

Maybe instead of guessing what type of array to create let user pass Class in List constructor which will be same as specified K type.

public List(int size, Class<K> clazz)

edited Feb 21, 2014 at 13:29

answered Feb 21, 2014 at 13:22

Pshemo

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