collections - java downcast results in method not applicable error -

i trying downcast function database objects. get:

the method getasstringarray(hashmap<long,dbobject>) in type dbobject not applicable arguments (hashmap<long,dbemployee>)

when try call getasstringarray(). here test code: dbobject base class:

public class dbobject  implements comparable<dbobject> {     protected long id;      public long getid() {         return id;     }      public void setid(long id) {         this.id = id;     }      public static dbobject[] getasarray(hashmap<long,dbobject> map) {         collection<dbobject> list = map.values();         dbobject[] ar = list.toarray(new dbobject[] {});         arrays.sort(ar);         return ar;     }      public static string[] getasstringarray(hashmap<long,dbobject> map) {         vector<string>vstrings = new vector<string>();         collection<dbobject> list = map.values();         dbobject[] ar = list.toarray(new dbobject[] {});         arrays.sort(ar);         (dbobject o : ar)             vstrings.add(o.tostring());         return (string[]) vstrings.toarray(new string[] {});     }      @override     public int compareto(dbobject another) {         return (int)(this.getid() - another.getid());     }  } 

child class dbemployee:

public class dbemployee extends dbobject {     private string first;     private string last;      public dbemployee(){}      public string tostring() {         return last + ", " + first;     } } 

and error:

public static void main(string[] args) {     hashmap<long,dbemployee>mapemployees = new hashmap<long,dbemployee>();     dbemployee.getasstringarray(mapemployees); } 

note: of dbobjects have own compareto() function (a few of objects have field in db override default sort order), of items sort id.

you need redefine method thusly:

public static dbobject[] getasarray(hashmap<long, ? extends dbobject> map) {     collection<? extends dbobject> list = map.values();     dbobject[] ar = list.toarray(new dbobject[] {});     arrays.sort(ar);     return ar; } 

a map<long, dbemployee> cannot used in place of map<long, dbobject>, because although has of same behaviour, of behaviour different. in particular, can't add object of type dbobject map<long, dbemployee>.

if method getasarray(hashmap<long, dbobject> map) allowed pass map<long, dbemployee>, able add dbobjects map inside method. method has no way know map not map of dbobjects. then, if held on reference map outside of method, have big problems. map no longer type-safe. map<long, dbemployee>, contain dbobjects not dbemployees.

the ? extends dbobject syntax allows specify can out of map without detailing can go in it. you'll notice if try add map inside getasarray(hashmap<long, extends dbobject> map) method, you'll compile error.

(there equivalent syntax, ? super dbemployee, let put things in map without knowing come out).