Question

This is a very straightforward task, but I feel I’m overlooking something. I have multiple objects that I’m trying to add to an ArrayList, and each of them has an identifying name in the form of a String. I need to be able to find (interact) with the objects in the ArrayList by calling the string name. So I tried this:

In my item class I have:
private String itemName;

public Item(String name)
{
    itemName = name;
}

So I can give it a name to be used by the user.

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Then in my class that interacts with the object, I create an ArrayList:

private ArrayList<Item> items = new ArrayList<Item>();

I add an object to the arrayList first by it’s actual object name, but I need to be able to interact with it using it’s String name, so I tried this:

public void removeItem(String itemName)
{
    for (int i = 0; i < items.size(); i++)
    {
        if (items.get(i).toString() == itemName)
        {
            items.remove(i);
        }
        break;
    }

}

But it’s not removing the item. If all of this is confusing, in essence I’m trying to create an OBJECT that I can give a STRING name (like I did with the item above), then have the ability to add the OBJECT to an ArrayList, and then finally be able to remove, or get, or do something with the OBJECTS in the ArrayList by calling the STRING name. I know I need to iterate through the ArrayList, but I can’t actually get the object.

Thanks for any help.

Answer 1

You are dong three mistakes here:

• You are using items.get(i).toString() which will not give you itemName for your Item. It will just give you a string representation of your Item class, returned by Object class’s toString method, if you don’t override one. However, this might work, if you have overriden a toString method, and returned the itemName from that. But, that I don’t see. And even if you have overriden that, I suggest you to have getter and setter for your itemName field, and use that to return the itemName.

• You are comparing strings using == operator, which will not give you correct result. You should always compare the string using equals method.

So, your if statement should look like:

 if (items.get(i).getName().equals(itemName))

• 3rd problem is, you are trying to modify the List that you are iterating upon. This will not work out, and may throw ConcurrentModificationException. You should use Iterator to remove elements from the List while iterating.

See for more details about those two problems, and how to solve them:

• How do I compare strings in Java?
• Iterating through a Collection, avoiding ConcurrentModificationException when removing in loop

Further, you can consider overriding equals method in your class, and then you can directly compare your instances using equals method.

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Now, having pointed out the some logical problems with your code, it’s time to point out some design problems.

Given your requirement, it seems like you need to use a HashMap, rather than a List of some custom type storing your attribute. You can create a map like this:

Map<String, Integer> map = new HashMap<String, Integer>();

which will contain the mapping of itemName to respective Item, and then getting the Item for a particular itemName is as simple as map.get(itemName).