Create multiple lists dynamically

Question

java version "1.7.0_45"

I am trying to create mutliple lists that will contain the same messages sent from the JID.

For example offlineMessageHeaderList will contain many messages from different and the same JID's. I want to group all the ones that have the same JIDs in to there own lists.

messageList1 -> all messages from John
messageList2 -> all messages from Peter
etc.

Then at the end check which ones are empty and add them to the groupList.

However, my solution is not good as I declare everything first and I could have more or less messages with the same JID.

code snippet for checking and adding

    List<List<MessageHeader>> groupList = new ArrayList<>();
    List<MessageHeader> messageList1 = new ArrayList<>();
    List<MessageHeader> messageList2 = new ArrayList<>();
    List<MessageHeader> messageList3 = new ArrayList<>();
    List<MessageHeader> messageList4 = new ArrayList<>();

    if(offlineMessageHeaderList.size() > 0) {
        for(OfflineMessageHeader header : offlineMessageHeaderList) {
            log.log(Level.INFO, "JID: " + header.getJid());
            log.log(Level.INFO, "Stamp: " + header.getStamp());
            log.log(Level.INFO, "User: " + header.getUser());

            MessageHeader messageHeader = new MessageHeader(header.getJid(), header.getStamp(), header.getUser());

            /* Check do we have message list already for this JID */
            /* Does this messageList1 already have this JID message */
            if(messageList1.get(0).getJid().compareTo(messageHeader.getJid()) == 0) {
                /* then we add all the same Jid to this list */
                messageList1.add(messageHeader);
            }
            else {
                /* The JID was different so need to add it to new list */
                if(messageList2.get(0).getJid().compareTo(messageHeader.getJid()) == 0) {
                    messageList2.add(messageHeader);
                }
            }
    /* Do the same for the others */
        }

        /* Add all the non-empty lists to the groupList */
        if(!messageList1.isEmpty()) {
            groupList.add(messageList1);
        }

        if(!messageList2.isEmpty()) {
            groupList.add(messageList2);
        }

        if(!messageList3.isEmpty()) {
            groupList.add(messageList3);
        }

        if(!messageList4.isEmpty()) {
            groupList.add(messageList4);
        }

    }

class of the MessageHeader
    public class MessageHeader {
        private String mJid;
        private String mStamp;
        private String mUser;

        public MessageHeader(String jid, String stamp, String user) {
            mJid = jid;
            mStamp = stamp;
            mUser = user;
        }

        public String getJid() {
            return mJid;
        }

        public String getStamp() {
            return mStamp;
        }

        public String getUser() {
            return mUser;
        }
    }

Is there any way I can do this in a more dynamic way?

Gyanendra Dwivedi · Accepted Answer · 2015-09-30 07:24:02Z

If I understood correctly, you can use the below approach to minimize number of objects and proper use of data structure.

1) Create a Map<String, List<MessageHeader>> to store all the message for each JID. In the Map the key is JID and value is the list of the messages for the JID.

2) When a message comes, check if a key with JID is present:

if yes - fetch the value from the Map for the JID, which would return a List<MessageHeader> object and add your new message into the list.

if No - Create a new entry within Map, with JID as key and a new ArrayList<MessageHeader>

Later you could iterate through the map to get eligible JID for inserting into group list

Ankush soni · Accepted Answer · 2015-09-30 06:42:06Z

Don't create an object like that, create a reference and when you think you have data to add in the list than create an Object i.e.

List<MessageHeader> messageList1;// Creating the reference or pass null to it
List<MessageHeader> messageList2;;// Creating the reference or pass null to it

and when your check it true create an object i.e.

if(messageList1.get(0).getJid().compareTo(messageHeader.getJid()) == 0) {
                /* then we add all the same Jid to this list */
                messageList1 = new ArrayList<>(); // Create an Object
                messageList1.add(messageHeader);
            }
            else {
                /* The JID was different so need to add it to new list */
                if(messageList2.get(0).getJid().compareTo(messageHeader.getJid()) == 0) {
                    messageList2 = new ArrayList<>(); // Create an Object
                    messageList2.add(messageHeader);
                }
            }

In that way you are avoiding unnecessary Object creation, ArrayList Object.

Note: In your code you are running a FOR loop so make sure you create an ArrayList Object 1 single time otherwise it will create a new Object always, recommend to enhance your if Condition.

Thanks for the answer. But not quite what I was looking for. As I don't know how many type messageList I will need. It all depends on the JIDs in the offlineMessageHeaderList. I am looking to group all the JIDs with the same name into there separate lists. However, I could have up to 10 different ones. So I could have up to 10 lists. I hope I am more clear. I am working on another solution now to sort the offlineMessageHeaderList first to see how many different JIDs there are first. Thanks.

Collectives™ on Stack Overflow

Create multiple lists dynamically

2 Answers 2

Comments

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Related