Getting latest/newest HashMap entries

Question

I have a HashMap<String, String>, and I'm inputting an entry into the HashMap everytime a player speaks for example. Bob492 says 'Hi'. and I will be inputting 'Bob492', and 'Hi' as its value.

Now I need a way to capture the last 5 message into a their own string, one string for each.

How would I do this?

EDIT: Thanks to a reply, it seems HashMap is not the best way to do this but a queue might be better. How would I achieve this, for example.

This is a dialog. Bob: Hey Bobby: Oh there you are! Bobby: I was looking for you Joseph: Yeah, I know I know. James: I wasn't talking to you! Joseph: Who cares?!?

And that will all be put in a queue, and I can easily fetch their username and message somehow (I don't know). Only can see 5 from that 6 messages, so Bob's message would be gone.

EDIT2: I want it similar to this: A minecraft server

Did you consider having several data structures (e.g. an hash map and a fifo queue)? — Basile Starynkevitch, Aug 01 '13 at 11:47
I am confused, what do you mean as I have never heard of a fifo queue. — Lewes, Aug 01 '13 at 11:48
the key can always hold one value, the one that is updated latest. or are you trying to achieve queue functionality here? — Anish Mohile, Aug 01 '13 at 11:49

Narendra Pathai · Answer 1 · 2013-08-01T13:20:01.200

2

You can have a Map of CircularFifoBuffer of Apache Collections.

//maps username to last 5 messages
Map<String,CircularFifoBuffer> map = new HashMap<String,CircularFifoBuffer>();

//putting the values

if(map.get(username) == null){
    CircularFifoBuffer last5messages = new CircularFifoBuffer(5);
    last5messages.add(message);
    map.put(last5messages);
}else{
    //similar code
}

What is CircularFifoBuffer?

CircularFifoBuffer is a first in first out buffer with a fixed size that replaces its oldest element if full.

So that automatically stores the last 5 messages, removing the oldest message from the buffer.

EDIT:

As per my understanding from the comment:

Let me show you what I mean Bob: Hi Bob: I'm bored :l Joseph: Oh heyyy Josephary: Hey Joseph and bob Joseph: Oh how are you two?

class MessageEntry{
    String userName;
    String message;

    public MessageEntry(String userName, String message){
       //store
    }

    public String getUsername(){
        return userName;
    }

    public String getMessage(){
        return message;
    }

}

//just a buffer to store last 5 messages received
CircularFifoBuffer last5messages = new CircularFifoBuffer(5);


//for storing last 5 messages
last5messages.add(new MessageEntry(userName,message));


//for retrieving elements:
Object[] messages = last5Messages.toArray();
//gives you messages that you can use any way you want


for(Object message : messages){
   MessageEntry entry = (MessageEntry)message;

   String userName = entry.getUsername();
   String message = entry.getMessage();

}

edited Aug 01 '13 at 13:20

answered Aug 01 '13 at 11:56

Narendra Pathai

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Then how would I retrieve a message, such as the 2nd message? – Lewes Aug 01 '13 at 12:03
2nd message of which user? – Narendra Pathai Aug 01 '13 at 12:09
Let me show you what I mean Bob: Hi Bob: I'm bored :l Joseph: Oh heyyy Josephary: Hey Joseph and bob Joseph: Oh how are you two? – Lewes Aug 01 '13 at 12:11
Check the updated answer. I guess you just need to store last 5 messages that were received and not last 5 messages per user. So i have edited to add that code too. – Narendra Pathai Aug 01 '13 at 12:18
Is there anyway of storing the players name and their messages, but not combined, since I need to customize the messages afterwards. – Lewes Aug 01 '13 at 12:25
I have edited code to add `MessageEntry` class. Check it out. – Narendra Pathai Aug 01 '13 at 12:32
How would I retrieve the username and the message aswell? – Lewes Aug 01 '13 at 12:37
Have edited to add that part too! But really you should try something by yourself. You just had to loop the messages and cast to get username and password. Nyways ther you go. – Narendra Pathai Aug 01 '13 at 13:20

score 0 · Answer 2 · answered Aug 01 '13 at 11:54

Unfortunately HashMap does NOT retain the order in witch the data was added.For you specific case you should use a LinkedHashMap because this kind of map is exactly like HashMap but keeps the order and you can iterate over it to get the last entries. But keep in mind that you cannot have multiple values for a single key so i dont think that you are looking for a map but for a Queue

score 0 · Answer 3 · answered Aug 01 '13 at 11:56

0

Is there any reason you are using a HashMap for this? As the HashMap will remove the last string the played says when a player speaks again. It sounds to me like you need a different datastructure to store your data, such as a fifo queue.

answered Aug 01 '13 at 11:56

Filip

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score 0 · Answer 4 · answered Aug 01 '13 at 12:03

HashMap do not support such behavior. What you need to do is to create or find a class that will do the logic part of your code. The logic says that you need store in some place five messages. As simple MessageBuffer, should do the job for you.

class MessagesBuffer {

 private String[] recent;

 public MessagesBuffer(int size) { 
   this.recent = new String[size];
 }

 public void addMessage(String messsage) {

   for(int i = 1; i < recent.lenght; i++) {
     recent[i] = recent[i-1];
   }

   this.recent[0] = message;

 }

/*Other implementation details*/

}

When you have a structure that do the logic, you can use Map to bind it with a person.

Map<User,MessageBuffer> recent = new HashMap<>();

But then I would require the playersname to retrieve the data, which I wouldn't know. I just need a way to store playersname and their message. And I send the player back a maximum of 5 newest messages. — Lewes, Aug 01 '13 at 12:06
Then do not use the Map, And compose the message to form `User + ":" message` and put it to buffer. — Damian Leszczyński - Vash, Aug 01 '13 at 12:31

score 0 · Answer 5 · answered Aug 01 '13 at 12:13

You can use LinkedHashMap, this

    LinkedHashMap m = new LinkedHashMap(16, 0.75f, true) {
        protected boolean removeEldestEntry(java.util.Map.Entry eldest) {
            return size() > 5;
        };
    };

makes sure that the map contains 5 most recent entries

    m.put("1", "1");
    m.put("2", "2");
    m.put("3", "3");
    m.put("4", "4");
    m.put("5", "5");
    m.put("6", "6");
    System.out.println(m);

output

{2=2, 3=3, 4=4, 5=5, 6=6}

Getting latest/newest HashMap entries

5 Answers5