Compare attributes of objects in a list

Question

I have a problem with my minecraft plugin. I want to add objects of player to a list. Before that, I want to check if the player already exist in my list.

Here an Example:

public class Player{

    public String playerName;
    public int Count = 0;

    public Player(String name){

         playerName = name;
    }
}

Here the main class:

public class mainClass{ 
   List<Player> = PlayerList;
   [...]
   if(!(*An object with attribute examplename in List exist*)){
        Player p = new Player("eamplename") 
        PlayerList.Add(p);
   }
}

Answer 1

I would suggest two ways to solve your problem. The first one is to stick with your approach of using List.

    List<Foo> list = new ArrayList<Foo>();

    boolean hasMyItem = false;
    String newName = "hello world";

    for (Foo foo : list)        // iterate over each item in the list
    {
        if (foo.name.equals(newName))
        {
            hasMyItem = true;
            break;            // get out of for loop
        }
    }

    if (!hasMyItem)
    {
        list.add(new Foo(newName));
    }
    else
    {
        // the player is already in the list ...
    }

In this code snippet, we are iterating over all of the items in the list until we find that the player already exists. If no such player exists in your list, it will exit with the value of hasMyItem being false, in which case you will add the new player to the list.

Iterating over all items in a list is a commonly used method and it's definitely good to know. However, you may consider using another data structure called Map<Key, Value>. Map associates a Key with a Value and store them together in the Map as like a list.

You could think Map<Key, Value> as labeling items. Key is a label for each item. Suppose you have a bunch of notebooks on your desk and you want to find a Math note. You can find it without much difficulty if you know the unique label of your math note, such as some texts or images on the cover page. In your example, the Key will be the username, and Value will be the player.

What's good with Map? It provides a simple way to look for Key values you have. The above codes can be more simplified if you use Map.

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

    Foo f1 = new Foo("name1");
    Foo f2 = new Foo("name2");
    Foo f3 = new Foo("name3");

    map.put("name1", f1);
    map.put("name2", f2);
    map.put("name3", f3);

    // will return true and this if statement will be executed
    if (map.containsKey("name1"))
    {
        // will be executed
    }

    // will return false because you don't have and this if statement will not be executed
    if (map.containsKey("some new name"))
    {
        // will not be executed
    }

There are other useful methods provided by Map<K,V>, which can be found here.

As a side note, declare every class member as private whenever you can, rather than default (which is when you don't specify anything) or public. There are many discussions of why you should do that, but basically it's to make protect your own codes safe from other people. You can search on this very easily but here are some links to it. Link1 Link2

I hope this can give you some good starting points.

Answer 2

Since it was pointed out that my use of Set was incorrect, I've decided on another approach (still using the overloaded equals method)

public class Player{
    public String playerName;
    public int Count = 0;
    public Player(String name){
        playerName = name;
    }
    public boolean equals(Player p){
        if(this==p) return true;
        if(this.playerName.equals(p.playerName) return true;
        return false;
}

And

public class mainClass{
    ArrayList playerList;
    public static void main(String[] args){
        playerList = new ArrayList<Player>();
        Player p = new Player("eamplename");
        checkList(p);
    }
    //check Player object against list and add if not exist
    //if desired, the returned boolean can confirm whether or not
    //player was successfully added
    public static boolean checkList(Player player){
        for(Player p : playerList){
            if(p.equals(player)) return false;
        }
        playerList.add(player);
        return true;
}