CODEWITHSUNDEEP
javaarraysgenerics
Chris
Chris

Reputation: 117

Exception thrown when using generic arrays

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

Upvotes: 3

Views: 80

Answers (2)

Pshemo
Pshemo

Reputation: 124275

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)

Upvotes: 2

JB Nizet
JB Nizet

Reputation: 692181

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[].

Upvotes: 4

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