How to create a custom list of objects in Java?

Question

Arraylists use the following syntax: ArrayList<Object> = new ArrayList<Object>; I need to add a constructor to my GameList class that allows you to specify what type of list to create. I don't understand how to make my class capable of being defined like this:

GameList<objectType> = new GameList();

All of my objects in the game will descend from the gameobject class.

public class GameObject
{
    String name;
    public GameObject
    {
        name = "Stat";
    }
    public String getName()
    {
        return name;
    }
    public void setName(String newName)
    {
        name = newName;
    }
}

public class GameList
{
    GameObject[] theList;
    public GameList(int size)
    {
        theList = new GameObject[size];
    }
    public GameObject parseList(String objectName)
    {
        for(int i = 0; i < theList.length; i++)
            if(theList[i].getName() == objectName)
                return theList[i];
        return null;
    }
}

Do you really need to create your own custom list? It looks more like you'd want a HashMap, if you're going to fetch objects by their name. — Kayaman
– Kayaman, Commented Jul 23, 2013 at 13:28
I'm sure your correct but I'm not familiar with a hashMap I just figured I would make my own class for the matter. Can you tell me more about them? — CodeManiak
– CodeManiak, Commented Jul 23, 2013 at 13:31
Maps allow you to associate keys to values. Since you're using objectName as a key, you don't want a List type data structure. You can simply do map.put("someName", myObject) and map.get("someName"); Implementing the basic data structures yourself is good for practice, but they have been implemented already in the language. — Kayaman
– Kayaman, Commented Jul 23, 2013 at 13:34
Awesome! Thanks for that information! Being a programmer is about the "Right kind of lazy", and I definitely need more experience using the data structures that already exist. — CodeManiak
– CodeManiak, Commented Jul 23, 2013 at 13:37
How does this syntax look to you? Map myMap = new Map(GameObject myObject, String objectName); — CodeManiak
– CodeManiak, Commented Jul 23, 2013 at 13:43

Kayaman · Accepted Answer · 2013-07-23 13:30:41Z

5

What you're looking for is Generics. The syntax would be

public class GameList<T> {
    T[] theList;
    ...

answered Jul 23, 2013 at 13:30

Kayaman

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4 Comments

CodeManiak Over a year ago

Interesting. I have more experience with C++ so I learned this as "templating". Now I know how to template in java also! Thanks!

Andy Thomas Over a year ago

How will the class create a new T[], similar to the OP's constructor GameList(int size)?

Kayaman Over a year ago

One way would be to not type it as a T, but instead as an Object[] (this is how ArrayList does it). Another way is to use a Class object passed in to create the array via reflection (this is more cumbersome).

Andy Thomas Over a year ago

Agreed, it would be simplest and easiest to understand to replace the T[] field declaration in the answer with Object[], and to create an Object[] in the constructor.

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