How to get size of an arrayList which is within the hashmap

Question

I have one hashmap that contains Player and an ArrayList of Locations. How do I get the size of initialized ArrayList variables?

For an example: I have an if state that requires next condition: if(X==2) X is supposed to be the size of initialized variables within the ArrayList

How can I access the size value of an arraylist that belongs to the hashmap?

HashMap<Player, ArrayList<Location>> who = new HashMap<Player, ArrayList<Location>>();

Iterate through the keys of the hashmap via https://stackoverflow.com/questions/46898/how-do-i-efficiently-iterate-over-each-entry-in-a-java-map, and then for each value, you can get the length of arraylist — Devesh Kumar Singh, Apr 03 '19 at 19:15

jonathan Heindl · Answer 1 · 2019-04-04T08:03:12.343

0

size : fist we get the list for the player which we get by calling get with the playername then (checking null just cause it might not have been initialized ) and the actual .size() function to get the amount

List<Location> locationList=who.get("<playerName>");
if(locationList!=null&&locationList.size()==2){
    ...
}

edited Apr 04 '19 at 08:03

answered Apr 03 '19 at 19:04

jonathan Heindl

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Even though this may answer the question, there is no explanation of your code. Please update your answer to provide an explanation of what you are doing. Thanks! – Miroslav Glamuzina Apr 04 '19 at 02:37

score 0 · Accepted Answer · edited Apr 03 '19 at 19:16

0

This is how your code should look like

if(who.get(player).size() == 2) {
    who.get(player).get(0);
    who.get(player).get(1);
}

To keep a null check on this, you can do something like this:

ArrayList<Location> locations = who.get(player);

if(locations != null && locations.size() == 2) {
    locations.get(0);
    locations.get(1);
}

I hope this is what you're looking for.

edited Apr 03 '19 at 19:16

CryptoFool

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answered Apr 03 '19 at 19:04

Safeer Ansari

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Possible `NullPointerException`. – Abra Apr 03 '19 at 19:07

CryptoFool · Answer 3 · 2019-04-03T19:25:34.253

Here's how you could check your whole data structure for each Location list having two entries:

public class Test {
    public static void main(String... args) {

        // Representation of your data, which would obviously come fromo elsewhere.
        HashMap<Player, ArrayList<Location>> who = new HashMap<Player, ArrayList<Location>>();

        for (Player p : who.keySet()) {
            ArrayList<Location> locations = who.get(p);
            if (locations.size() == 2) {
                // Do something
            }
            else {
                // Do something else
            }
        }
    }
}

score 0 · Answer 4 · answered Apr 03 '19 at 19:40

There are multiple options to achieve this. It matters in which case you want to check that. E.g. if you only want to check if all players have two locations you can use Java Stream for that:

boolean allTwoLocations = who.values().stream().allMatch(locations -> locations.size() == 2);

If you want to check only a single player it matters if you already have the reference to that player or only an id or username of it. If you have an object of the player you want to check its as simple as this: who.get(player). Remember to check if the player exists in the map to prevent a NullPointerException. You then can access the locations of that player:

if (who.containsKey(player) && who.get(player).size() ==2) {
    // do whatever you want
}

If you only have a property of that player (id, username, etc.) I also would recommend to use a Stream for that:

boolean userExistsAndTwoLocations = who.entrySet().stream()
        .filter(p -> p.getKey().getId() == id).findFirst()
        .map(Map.Entry::getValue)
        .filter(locations -> locations.size() == 2)
        .isPresent();

How to get size of an arrayList which is within the hashmap

4 Answers4