java - Create multiple lists dynamically - 


java version "1.7.0_45"

i trying create mutliple lists contain same messages sent jid.

for example offlinemessageheaderlist contain many messages different , same jid's. want group ones have same jids in there own lists.

messagelist1 -> messages john messagelist2 -> messages peter etc.

then @ end check ones empty , add them grouplist.

however, solution not declare first , have more or less messages same jid.

code snippet checking , 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 have message list jid */             /* messagelist1 have jid message */             if(messagelist1.get(0).getjid().compareto(messageheader.getjid()) == 0) {                 /* add same jid list */                 messagelist1.add(messageheader);             }             else {                 /* jid different need add new list */                 if(messagelist2.get(0).getjid().compareto(messageheader.getjid()) == 0) {                     messagelist2.add(messageheader);                 }             }     /* same others */         }          /* add non-empty lists 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 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 way can in more dynamic way?

if understood correctly, can use below approach minimize number of objects , proper use of data structure.

1) create map<string, list<messageheader>> store message each jid. in map key jid , value list of messages jid.

2) when message comes, check if key jid present:

if yes - fetch value map jid, return list<messageheader> object , add new message list.

if no - create new entry within map, jid key , new arraylist<messageheader>

later iterate through map eligible jid inserting group list


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