Problem to store information in a hashMap<String, ArrayList<String>>

Question

I have a problem to store data in this hashmap, I'm programming in Java.

My system consists of some chats, in the hash map I have to insert chats as indexes and the list of users who are connected to a specific chat, my problem is the initialization of the hashmap, as I only have to enter the chats but the arraylists are empty because there is no user connected, only I cannot understand how to do this correctly.

This is a little sample of my code:

public class Master {
   private HashMap<String, ArrayList<String>> chatBox;

   public Master() {
      chatBox = new HashMap<String, ArrayList<String>>();
   }

   public insert() {
      FileReader fr;
      BufferedReader br;
      try {
        fr = new FileReader("listChat.txt");
        br = new BufferedReader(fr);
        while(true) {
            String topic = br.readLine();
            if(topic == null)
                break;
            chatBox.put(topic, null);
        }

    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } 
  }

So, what's the concrete problem? null is not an empty ArrayList. It's no ArrayList at all. An empty ArrayList is created using new ArrayList<>(). — JB Nizet
– JB Nizet, Commented Mar 18, 2019 at 9:57

Hichem BOUSSETTA · Accepted Answer · 2019-03-18 09:59:06Z

2

I will suggest you to change the code that way by creating an empty ArrayList when you add a new element in the hashmap:

while(true) {
        String topic = br.readLine();
        if(topic == null)
            break;
        chatBox.put(topic, new ArrayList<String>());
    }

When you will have to update this topic with messages, you get the value for the key "topic" and add new elements in the ArrayList

answered Mar 18, 2019 at 9:59

Hichem BOUSSETTA

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1 Comment

rusher83 Over a year ago

Thanks was just that mistake, I was getting lost in a glass of water.

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