can you cast arrays? (c++)

nadroj · 10-10-2005, 09:49 PM

EDIT: SORRY, THIS IS JAVA NOT C++! (lol, i got my work confused)
ill try and be brief.. i have a class named Products, which stores a list of products into a HashMap.

I have the following function in it:

Code:

public static Product[] getProducts() {
	return (Product[]) products.values().toArray();
}

it returns (or is supposed to

) an array of Products, which are stored in the HashMap.

- 'products' is the HashMap of 'Product' objects. the .values returns a collection of the elements. the .toArray() is called on the collection object which was returned by .values(), and it returns (obviously) an array.. of _objects_.

In an executable class, i test it with the following (eventually):

Code:

Product[] products = (Product[]) Product.getProducts();
	for (int i = 0; i < products.length; i++)
		System.out.println(products[i]);
	System.out.println();

but i get a classCastException.

However, when i change the code from my Products class to return an array of objects (without the casting obviously)

Code:

public static Object[] getProducts() {
	return products.values().toArray();

and change the tester code to:

Code:

Object[] products = Product.getProducts();
	for (int i = 0; i < products.length; i++)
		System.out.println(products[i]);
	System.out.println();

it does work.
any clues?

vladmihaisima · 10-11-2005, 01:36 AM

It is always helpfull to include all the source and error message (maybe you missed something). Looking over your description everything looks ok (and I saw that on many occasions when people described bugs they desribed perfectly correct things, omiting what was important).

Also you could have changed the title of the thread.

spooon · 10-11-2005, 02:19 AM

Try "return (Product[])products.values().toArray(new Product[0]);" because of weird ways in the way that the toArray() methods work.

However, I think that a better way is to just return the Collection "products.values()". Then you can iterate through it like this (Java 1.5):

Code:

public static Collection<Product> getProducts() {
	return products.values();
}

Code:

Collection<Product> products = Product.getProducts();
for (Product prod : products)
	System.out.println(prod);

or this (Java 1.4 and below):

Code:

public static Collection getProducts() {
	return products.values();
}

Code:

Collection products = Product.getProducts();
for (Iterator i = products.iterator(); i.hasNext(); ) {
	Product prod = (Product)i.next();
	System.out.println(prod);
}

nadroj · 10-11-2005, 06:16 AM

alright, thanks guys.

vlad- i did try editing post and changing title, but it only changed it at the top of my thread, not in the forums page where people will see it.

spooon-
that would be odd.. ill try the 'new Product[0]' method first later today after class. also, i thought about just returnint a collection of some sort, Vector for ex., but the method requuires that an 'array..' is returned.

nadroj · 10-12-2005, 03:56 PM

alright.. i guess the method was the first one you suggested, with the '... new Product[0]...'
i asked my teacher about this and thats what he suggested too.
can you not cast arrays, though? or how does this actually solve the problem? (the new Product[0]...)
Thanks though.

vladmihaisima · 10-12-2005, 04:39 PM

(back with a more documented answer)

When you cast between two objects like obj1 = (SomeClass) obj2, obj2 must be of type SomeClass or be a type that inherits SomeClass (otherwise the case would not make any sense).

This is also true for arrays. In the following example the line " v = (String[]) x;" will give an error, because x is Object[] and Object[] does not inherit String[]. On the other hand y contains an object which type is String[] - so no problem at the conversion.

Bottom line is that this behavior is consistent with the strong typing of the language. If you would be allowed to do that cast the runtime could not gurantee that v[0] will not return a bogus result (because you could have added anything in x not just strings)

Code:

class a {
    public static void main(String a[]) {
        Object x[] = new Object[10];
        Object y[] = new String[10];
        String v[];

        v = (String[]) y;
        v = (String[]) x;
    }
}

Now a question. Do you think you would be allowed to do something like "y[1] = new Integer(5);" (for the above example)

Hope this helps

nadroj · 10-12-2005, 04:52 PM

id say no. because you made (demoted?) y to reference an array of Strings, rather than objects. you could, however, store the '..new Integer(x)...' into x, correct?

just before you posted this, coincidently, i was on the java site (http://java.sun.com/j2se/1.4.2/docs/....Object[])) reading about the toArray method in the collection interface, and it kinda made sense of what was said earlier.
Thanks again!

Code:

toArray

public Object[] toArray(Object[] a)

...

    Parameters:
        a - the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose

...

vladmihaisima · 10-13-2005, 01:34 AM

Yap, that was my point

.