Java superclass variable and subclass copy constructor issue

Question

I know that in Java due to inheritance, a superclass type variable can hold a reference to a subclass object. Example, say ClassA is the superclass to ClassB. You could set an element of a ClassA[] to reference a ClassB object. I'm running into a problem doing this while using a copy constructor of the subclass.

public Items copyFromMasterInventory(String str){
        Items itemToReturn = null;
        int length = masterInventory.length;
        int iterator = 0;
        while (iterator < length){
            String name = masterInventory[iterator].getName();
            if (name.equals(str)){
                if (name.getClass().equals(Collectible.class)){
                    itemToReturn = new Collectible(masterInventory[iterator]); 
                 }
                if (name.getClass().equals(Props.class)){
                    itemToReturn = new Props(masterInventory[iterator]);
                }
            }
        }
        return itemToReturn;
    }

In this case, there are three class. Items, Props, and Collectible. Props and Collectible are subclasses of Items. masterInventory is an array of Items storing both Collectible and Props objects. The purpose of this method is to create and return a copy of an item in the masterInventory array. To avoid .clone(), I've created copy constructors for Collectible and Props. But the way I have it now shows an incompatible types error, that Items cannot be converted to Collectible or Props, even though the object stored in that element is a Collectible or Props object. If been reading and searching for hours but can't seem to come up with a solution. Does anyone know a way around this issue? All help is appreciated.

You can do a deep copy if you want avoid clone(), here is an Article about it : javatechniques.com/blog/faster-deep-copies-of-java-objects — hoomb
– hoomb, Commented Nov 20, 2015 at 11:52
I admit, with this error popping up I had forgotten to add the line to increment the iterator. The problem crops up in the lines itemToReturn = new Collectible(masterInventory[iterator]); and itemToReturn = new Props(masterInventory[iterator]); — shadowfox
– shadowfox, Commented Nov 20, 2015 at 12:14

rsp · Accepted Answer · 2015-11-20 11:54:20Z

1

You could add an abstract method Item getCopy() to the Item class, implement it in both Props and Collectible, and call it in de while loop:

itemToReturn = masterInventory[iterator].getCopy();

the benefit here is that you do not need the condition on class.

answered Nov 20, 2015 at 11:54

rsp

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1 Comment

shadowfox Over a year ago

Thanks! Although Eran's answer did work beautifully, I'm going with this method as there are still a couple of Items subclasses I plan to add and using an abstract method implementation for each and cutting out the class condition makes the code more readable

Eran · Accepted Answer · 2015-11-20 11:53:17Z

1

The solution is simple - casting :

if (masterInventory[iterator] instanceof Collectible) {
    itemToReturn = new Collectible((Collectible) masterInventory[iterator]); 
}
if (masterInventory[iterator] instanceof Props) {
    itemToReturn = new Props((Props) masterInventory[iterator]); 
}

answered Nov 20, 2015 at 11:53

Eran

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