Check if Object exists in ArrayList in Java

Question

I have the following List of objects:

private List<Object> teamlist = new ArrayList<Object>();

And I'm adding objects to the list like so:

teamlist.add(new MCWarTeam(args[0], joinkey));

Now the objects in the list have no name, but can be referenced by using the list, right? Before I add a new element to the list, how can I check if an object with a certain attribute already exists? This is the constructor of the Objects:

public MCWarTeam(String teamname, String joinkey){
    this.teamname = teamname;
    this.joinkey = joinkey;
}

I want to check if there already is a team with the name teamname. Alternatively, is there a better way to store the Objects? Before, I just used a HashMap to add the teamname and joinkey and it worked just fine, but figured using Objects instead would be a better way to do it.

Here is the important code for the event handler:

        else if (cmd.getName().equalsIgnoreCase("createTeam")) {
        if (args.length > 0 && args.length < 3) {
            String joinkey = "";
            if (args.length > 1)
                joinkey = args[1];

            String teamname = args[0];

            MCWarTeam newTeam = new MCWarTeam(teamname, joinkey);
            if (!teamlist.containsKey(teamname)) {
                teamlist.put(teamname, newTeam);
                sender.sendMessage("Created new team \"" + teamname + "\" with join key \"" + joinkey + "\" successfully! Teams:");

                sender.sendMessage("All teams:");
                for (String key : teamlist.keySet()) {
                    sender.sendMessage(key);
                }

            } else
                sender.sendMessage("Team already exists!");
            return true;
        }
        return false;
    }

    else if (cmd.getName().equalsIgnoreCase("joinTeam")) {
        if (args.length > 0 && args.length < 3) {
            String joinkey = "";
            if (args.length > 1)
                joinkey = args[1];

            String teamname = args[0];

            if (teamlist.containsKey(teamname)) {
                String teamKey = teamlist.get(teamname).getJoinKey();
                if (joinkey == teamKey) {
                    teamlist.get(teamname).addPlayer(playername);
                    Bukkit.broadcastMessage("MCWar: " + playername + " joined Team \"" + teamname + "\" successfully!");
                } else
                    sender.sendMessage("Join key incorrect!");
            } else {
                sender.sendMessage("Team doesn't exist! Teams:");
                for (String key : teamlist.keySet()) {
                    sender.sendMessage(key);
                }

            }
            return true;
        }
        return false;
    }

Basically, if it returns false, the user will get a message explaining the correct usage of the command he entered.

Do you override equals in MCWarTeam? If so, just use List.contains. — Andy Turner
– Andy Turner, Commented Oct 25, 2015 at 11:46
You probably want to type the list to <MCWarTeam> instead of <Object> as well. Also I don't know how you figured that an ArrayList is somehow inherently better than a HashMap, but I hope you based that on something real and not just a gut feeling. — Kayaman
– Kayaman, Commented Oct 25, 2015 at 11:48

Community · Accepted Answer · 2018-07-24 15:04:25Z

8

Java's List<T> has a boolean contains(Object) method, which is handy for situations when you wish to avoid duplicates:

if (!teamlist.contains(newTeam)) {
    teamlist.add(newTeam);
}

MCWarTeam class must implement equals in order for this to work. When you override equals, you must also override hashCode.

@Override
public boolean equals(Object obj) {
    if (!(obj instanceof MCWarTeam))  {
        return false;
    }
    MCWarTeam other = (MCWarTeam)obj;
    return teamname.equals(other.teamname)
        && joinkey.equals(other.joinkey);
}
@Override
public int hashCode() {
    return 31*teamname.hashCode()+joinkey.hashCode();
}

I'm just looking to check if an Object with the same teamname already exists, but not care about the joinkey?

If joinkey is not part of your object's state that influences equality, it is usually not a good idea to keep it as part of the object as a field. For example, if joinkey is something transient which you use to "connect" teams to other things, making a HashMap<String,MCWarTeam>, using joinkey as the key to the map, and removing joinkey from MCWarTeam should be a good idea.

edited Jul 24, 2018 at 15:04

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answered Oct 25, 2015 at 11:48

Sergey Kalinichenko

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3 Comments

Martin Over a year ago

joinkey is something like a password the user will have to enter to become part of a team but I see how the name might be misleading. Can you explain to me how MCWarTeam other = (MCWarTeam)obj; return teamname.equals(other.teamname); works to see if there already is an object with the same name in the List?

Sergey Kalinichenko Over a year ago

@MartinHoffmann teamlist.contains(newTeam) method calls equals(Object) method under the hood, comparing newTeam to what's already on the list.

Martin Over a year ago

how can I get the String joinkey from a teamname? I tried the following: String teamKey = teamlist.get(teamlist.indexOf(teamname)).getJoinKey(); and tried using a test MCWar-object with the same name in place of teamname in indexOf(), but that didn't work either, do I have to @Overwrite indexOf() as well?

Mick Mnemonic · Accepted Answer · 2015-10-25 16:10:27Z

4

Based on the description and your comments to other answers, it seems like a good idea to not use a List, but instead store your data in a Map<String, MCWarTeam>, which maps team names into MCWarTeam objects:

private Map<String, MCWarTeam> teams = new HashMap<>();

You can add a team, checking whether a team with the same name already exists, like this:

String teamName = args[0];

if (!teams.containsKey(teamName)) {
    teams.put(teamName, new MCWarTeam(teamName, joinKey));
} else {
    // do what you want when the team name was already in the map
}

Retrieving an MCWarTeam object based on team name, e.g. for accessing the joinKey attribute, is easy:

String joinKey = teams.get(teamName).getJoinKey();

Note that using this approach, you shouldn't implement equals or hashCode in MCWarTeam, because you aren't gonna need it; as your map keys are team names, containsKey operates on String objects which already have well-defined equals and hashCode semantics.

edited Oct 25, 2015 at 16:10

answered Oct 25, 2015 at 15:59

Mick Mnemonic

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6 Comments

Martin Over a year ago

This really seems like the best way I've seen to do this. I'll try it when I'm at home, thanks

Martin Over a year ago

I'm getting a weird error here. In a function, I add teams with teamlist.put(teamname, newTeam);. Now every time I check if an entry exists, via teamlist.containsKey(teamname), it appears empty. No results show up when I let it run through a loop, but when I run the exact same loop after adding an element, all the added elements show up. EDIT: this is when I call /createteam and /jointeam after each other prntscr.com/8vpdnk

Mick Mnemonic Over a year ago

It's hard to say what might be happening there without seeing the code. Could you edit your question and post the code where you add teams / check for entries?

Martin Over a year ago

I did, thanks. The HashMap teamlist is defined above.

Mick Mnemonic Over a year ago

One thing I noticed is that you're comparing the join key Strings with ==; you should use equals instead. You could try changing the comparison to joinkey.equals(teamKey) and see if it has any effect. Other than that, it's still hard to say what your actual problem is. I'm not familiar with Bukkit or its event handling framework, but there might also be some timing issues playing a part here if the events are handled asyncronously. Posting more code will help tracking the problem down. Also, please use braces ({ }) in all ifs and elses (even ones containing a single statement).

|

Eran · Accepted Answer · 2015-10-25 11:49:08Z

2

In order to search for an MCWarTeam instance in the ArrayList, you'll first have to override equals in order to define what it means for two MCWarTeam instances to be equal to each other. Then you can use indexOf(team) or contains to determine whether a instance is in the List.

However, such a search would take linear time, so a HashSet may be better for your needs (for that purpose you'll need to override both equals and hashCode, and you'll be able to find if an object is in the Set in constant time).

edited Oct 25, 2015 at 11:49

answered Oct 25, 2015 at 11:47

Eran

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Comments

Sleiman Jneidi · Accepted Answer · 2015-10-25 11:52:31Z

2

If you implement MCWarTeam equals method properly, then contains should tell you if the object exists.

 boolean exists = teamlist.contains(member);

And as @Eran mentioned a HashSet would give you O(1) lookup where list contains is O(n), the only thing is that HashSet doesn't allow duplicates. And Yes, use the actual type rather than Object

List<MCWarTeam> teamlist = new ArrayList<>();

edited Oct 25, 2015 at 11:52

answered Oct 25, 2015 at 11:47

Sleiman Jneidi

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1 Comment

Mick Mnemonic Over a year ago

A Set may be impractical for the OP because even though contains performs well, the only way to retrieve elements is through iteration; there is no get method. A Map<String,MCWarTeam> might be the best alternative for this reason.

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