java - Why make a collection unmodifiable? -

i know how make collection unmodifiable in java dont understand need such method exist. can please explain example scenario want make collection unmodifiable?

thanks in advance

take @ scenario. there application creates 2 users, , wants notify them something. users name different peter should notification.

so have user.class:

public class user {     private string name;     private integer id;      public user(final integer id, final string name) {         this.id = id;         this.name = name;     }     public string getname() {         return name;     }      public integer getid() {         return id;     } } 

the users stored in special holder class (containing map):

public class usersholder {     private static map<integer, user> usersmap = new hashmap<integer, user>();      public static void adduser(final user user) {         usersmap.put(user.getid(), user);     }      public static map<integer, user> getusersmap() {         return usersmap;         //return collections.unmodifiablemap(usersmap);     } } 

then have userscreator creates users , stores them in map:

public class userscreator {     public static void createusers() {         usersholder.adduser(new user(1, "peter"));         system.out.println("created user " + usersholder.getusersmap().get(1).getname());         usersholder.adduser(new user(2, "paul"));         system.out.println("created user " + usersholder.getusersmap().get(2).getname());     }      public static void main(string[] args) {         userscreator.createusers();         system.out.println("number of users before notification: " + usersholder.getusersmap().size());         new usersnotificator().notifyallusersbutpeters(usersholder.getusersmap());         system.out.println("number of users after notification: " + usersholder.getusersmap().size());     } } 

and notificator notifies peters:

public class usersnotificator {     public void notifyallusersbutpeters(final map<integer, user> map) {         //we don't need peters, we'll remove them list;         iterator<entry<integer, user>> iterator = map.entryset().iterator();         while (iterator.hasnext()) {             if (iterator.next().getvalue().getname().equals("peter")) {                 iterator.remove();             }         }         //now can notify list;         notifyusers(usersholder.getusersmap());     }      private void notifyusers(map<integer, user> map) {         (final user user : map.values())         system.out.println("notifyingusers: " + user.getname());     } } 

now - notificator presented map , may modify it, does. doesn't know shouldn't modify it's global usersmap. in effect removes users name peter. it's own purposes, results visible every other class using usersholder.

the result follows:

created user peter created user paul number of users before notification: 2 notifyingusers: paul number of users after notification: 1 

when returning unmodifiablemap in usersholder removal not possible. way create new map users notify, our usersholder safe.

this example bit big, sorry that, failed think of/create somehting shorter.

unmodifiable map helps keep classes immutable safe(as presented in example) in multithreaded enviroment.