Java - How to check if value of an array contains specified value as attribute

Question

I have an array of Player objects. The players have names and when i add a player, i want to check if the playername already exists. Following code never throws the exception, it just adds duplicate players.

public void addPlayer(String name, boolean gender, int index)  throws RuntimeException {
    List<String> names = new ArrayList<>();

    if (names.contains(name))
        throw new DuplicatePlayerException();
    else {
        players[index] = new Player(name, gender);
        names.add(name);
    }
}

issue in this line List<String> names = new ArrayList<>(); every time you create new ArrayList. — Akash Shah
– Akash Shah, Commented Mar 5, 2019 at 9:41
Because every time you create new List<String> names which is empty. It should be a class-level-variable — Michał Ziober
– Michał Ziober, Commented Mar 5, 2019 at 9:41
Using a boolean for gender is somehow from the last century... names cannot contain the given name in the method because it will always be empty before your if statement that checks if the name is contained. Make it a class attribute... — deHaar
– deHaar, Commented Mar 5, 2019 at 9:41

Stultuske · Accepted Answer · 2019-03-05 09:42:17Z

public void addPlayer(String name, boolean gender, int index)  throws RuntimeException {
    List<String> names = new ArrayList<>(); // you create a new instance of the list each time you call it, so it'll always be empty

    if (names.contains(name)) // your empty list does not contain anything
        throw new DuplicatePlayerException();
    else {
        players[index] = new Player(name, gender);
        names.add(name);
    }
}

You'll need to change your method, to work with an instance level list:

private List<String> names = new ArrayList<>();
public void addPlayer(String name, boolean gender, int index)  throws RuntimeException {
    if (names.contains(name))
        throw new DuplicatePlayerException();
    else {
        players[index] = new Player(name, gender);
        names.add(name);
    }
}

so the contents of names won't be erased each time you call your method.

Ruslan · Accepted Answer · 2019-03-05 09:55:56Z

1

You could consider Set<Player> instead of array. Set by definition can not contain the duplicates. Assuming Player has implemented equals/hashcode your code might look like:

Set<Player> players = new HashSet<>();

public void addPlayer(Player player)  throws RuntimeException {
    if (!players.add(player)) {
        throw new DuplicatePlayerException();
    }
}

Set::add returns true if the set did not already contain the element

edited Mar 5, 2019 at 9:55

answered Mar 5, 2019 at 9:47

Ruslan

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3 Comments

Stultuske Over a year ago

even though this is indeed a good remark, if he had used a Set instead of a List, he still would have had the same problem

Ruslan Over a year ago

@Stultuske I suggest to use Set instead of array. There is no List in my answer

Stultuske Over a year ago

I was referring to the code of the OP, where he used a list. In your answer, you fixed his error, yet you don't speak of that, you only explain the benefit of using a Set (after fixing his error)

Aris Mist · Accepted Answer · 2019-03-05 09:53:45Z

0

String Names = "John";
    if (Names.toLowerCase().contains("john")){
        System.out.println("yes");
    }

You can also use .toLowerCase().contains() to include case sensitive inputs.

answered Mar 5, 2019 at 9:53

Aris Mist

894 bronze badges

1 Comment

Stultuske Over a year ago

This would make a good comment, but it is not related to the actual problem the OP encountered.

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