5

I'm having a bit of a problem grasping something - I might be going about this completely wrong.

I am trying to create a class which extends ArrayList but has several methods which increase the functionality (at least for the program I am developing.)

One of the methods is a findById(int id), which searches each ArrayList object for a particular id match. So far it's working, but it won't let me do for (Item i : this) { i.getId(); }

I don't understand why?

Full code:

public class CustomArrayList<Item> extends ArrayList<Item> {

    // declare singleton instance
    protected static CustomArrayList instance;

    // private constructor
    private CustomArrayList(){
        // do nothing
    }

    // get instance of class - singleton
    public static CustomArrayList getInstance(){
        if (instance == null){
            instance = new CustomArrayList();
        }
        return instance;
    }

    public Item findById(int id){
        Item item = null;
        for (Item i : this) {
            if (i.getId() == id) {
                      // something
         }
        }
        return item;
    }
    public void printList(){
        String print = "";
        for (Item i : this) {
            print += i.toString() + "\n";
        }
        System.out.println(print);
    }
}
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2 Answers 2

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7

Change

public class CustomArrayList<Item> extends ArrayList<Item> {

to

public class CustomArrayList extends ArrayList<Item> {

I suspect Item is the name of the class that you want to store in the list. By adding <Item> after CustomArrayList you're introducing a type parameter which shadows this class.

With the <Item> parameter, your code is equal to

public class CustomArrayList<T> extends ArrayList<T> {
    // ...
        for (T i : this) { i.getId(); }
    // ...
}

which obviously won't always work, as T may refer to any type.

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4 Comments

Awesome. Is that just the proper syntax, or was I trying to do something else? Thank you so much! Quick reply.
That is probably the proper syntax for your scenario. Updated the answer to explain why.
Thank you! That explains it very well.
To add to this, if you want to support various element types with common super type Item, use public class ItemArrayList<T extends Item> extends ArrayList<T> { ... }.
2

What is getId()? Presumably it's a method in some class, but we don't know which class.

If you've actually got a class called Item with a getId() method, which this is meant to be a list of, you simply need to stop your class from being generic. So instead of this:

public class CustomArrayList<Item> extends ArrayList<Item> {

you want:

public class CustomArrayList extends ArrayList<Item> {

Currently within your class, Item doesn't refer to a class called Item, it refers to a type parameter called Item.

Now personally:

  • I wouldn't avoid creating singletons unless you really have to
  • If you have to, I'd avoid creating them in the way you have (which isn't thread-safe)
  • I wouldn't extend ArrayList<> unless I really had to, preferring composition over inheritance
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2 Comments

I'm creating a singleton for the reason that I can only have one. I know that the way I'm creating it isn't thread safe - I would need to have a "lock method", correct? What aioobe posted solved my problem, but I'm still interested in creating a thread-safe singleton.
The preferred way to create a singleton in Java 1.5+ is to use an Enum. See for instance this question: stackoverflow.com/questions/70689/…

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