android studio add markers to google maps from sqlite database -

in android app i'm trying add several markers sqlite mapsactivity. created do-while cycle getting cities in sqlite table , convert cities in coordinates (lat, long), there's in error. can me?

---mapsactivity.java---

public class mapsactivity extends fragmentactivity implements onmapreadycallback {  private googlemap mmap; databasehelper mydb;  @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_maps);     // obtain supportmapfragment , notified when map ready used.     supportmapfragment mapfragment = (supportmapfragment) getsupportfragmentmanager()             .findfragmentbyid(r.id.map);     mapfragment.getmapasync(this); }  @override public void onmapready(googlemap googlemap) {     mmap = googlemap;      // add marker in sydney , move camera     latlng sydney = new latlng(-34, 151);     latlng ruvo = new latlng(-45, 123);     mmap.addmarker(new markeroptions().position(sydney).title("marker in sydney"));     mmap.addmarker(new markeroptions().position(ruvo).title("marker in sydney"));     mmap.movecamera(cameraupdatefactory.newlatlng(ruvo));     displaycities(); }  public void onsearch(view view) {     edittext textcity = (edittext) findviewbyid(r.id.textcity);     string location = textcity.gettext().tostring();     list<android.location.address> addresslist = null;      if (location != null) {         geocoder geocoder = new geocoder(this);         try {             addresslist = geocoder.getfromlocationname(location, 1);         } catch (ioexception e) {             e.printstacktrace();         }         android.location.address address = addresslist.get(0);         latlng latlng = new latlng(address.getlatitude(), address.getlongitude());         mmap.addmarker(new markeroptions().position(latlng).title("marker pointed"));         mmap.animatecamera(cameraupdatefactory.newlatlng(latlng));     }  }  public void displaycities() {     list<android.location.address> addresslist = null;     cursor database = mydb.display();      if (database.getcount()== 0) {         toast.maketext(this, "nothing found", toast.length_long).show();         return;     }     //string array of cities     database.movetofirst();     string[] cities = new string[database.getcount()];     int n = 0;     {         cities[n] = database.getstring(3).tostring();         n=n+1;     } while (database.movetonext());          geocoder geocoder = new geocoder(this);         try {             addresslist = geocoder.getfromlocationname(cities[n], 1);         } catch (ioexception e) {             e.printstacktrace();         }         android.location.address address = addresslist.get(0);         latlng latlng = new latlng(address.getlatitude(), address.getlongitude());         mmap.addmarker(new markeroptions().position(latlng).title("marker pointed"));     }  } 

---databasehelper.java---

public class databasehelper extends sqliteopenhelper { private static final int database_version = 1; private static final string database_name = "student.db"; public static final string table_name = "student_table"; //very important: cursoradapter needs "_id" column, written here public static final string col_1 = "_id"; public static final string col_2 = "name"; public static final string col_3 = "surname"; public static final string col_4 = "city";  public arraylist<string> results = new arraylist<>();  public databasehelper(context context, string name, sqlitedatabase.cursorfactory factory, int version) {     super(context, database_name, factory, database_version); }  @override public void oncreate(sqlitedatabase db) {     db.execsql("create table " + table_name + " ( " + col_1 + " integer primary key autoincrement , "             + col_2 + " text , " + col_3 + " text , " + col_4 + " text );"); }  @override public void onupgrade(sqlitedatabase db, int oldversion, int newversion) {     db.execsql("drop table if exists" + table_name);     oncreate(db); }  public cursor selected(long id) {     sqlitedatabase db = this.getwritabledatabase();     string[] columns = new string[]{databasehelper.col_1, databasehelper.col_2, databasehelper.col_3, databasehelper.col_4};     cursor c = db.query(databasehelper.table_name, columns, databasehelper.col_1 + "=" + id, null, null, null, null);     if (c != null) {         c.movetofirst();     }     return c; }  public boolean insertdata(string name, string surname, string city) {     sqlitedatabase db = this.getwritabledatabase();     contentvalues contentvalues = new contentvalues();     contentvalues.put(col_2, name);     contentvalues.put(col_3, surname);     contentvalues.put(col_4, city);     long result = db.insert(table_name, null, contentvalues);     if (result == -1)         return false;     else         return true; }  //method select query public cursor display() {     sqlitedatabase db = this.getreadabledatabase();     cursor res = db.rawquery("select * " + table_name, null);     return res; }   public boolean updatedata(string id, string name, string surname, string city) {     sqlitedatabase db = this.getwritabledatabase();     contentvalues contentvalues = new contentvalues();     contentvalues.put(col_1, id);     contentvalues.put(col_2, name);     contentvalues.put(col_3, surname);     contentvalues.put(col_4, city);     db.update(table_name, contentvalues, "_id = ?", new string[] {id});     return true; }  public integer deletedata (string id) {     sqlitedatabase db = this.getwritabledatabase();     return db.delete(table_name, "_id = ?", new string[] {id}); }   } 

edit: add error in logcat

beginning of crash

06-28 12:17:13.332 2915-2915/com.example.android.sqlite e/androidruntime: fatal exception: main process: com.example.android.sqlite, pid: 2915 java.lang.nullpointerexception: attempt invoke virtual method 'android.database.cursor com.example.android.sqlite.databasehelper.display()' on null object reference

at com.example.android.sqlite.mapsactivity.displaycities(mapsactivity.java:76)
@ com.example.android.sqlite.mapsactivity.onmapready(mapsactivity.java:51)
@ com.google.android.gms.maps.supportmapfragment$zza$1.zza(unknown source)
@ com.google.android.gms.maps.internal.zzo$zza.ontransact(unknown source)
@ android.os.binder.transact(binder.java:387)
@ com.google.android.gms.maps.internal.be.a(sourcefile:82)
@ com.google.maps.api.android.lib6.e.fb.run(unknown source)
@ android.os.handler.handlecallback(handler.java:739)
@ android.os.handler.dispatchmessage(handler.java:95)
@ android.os.looper.loop(looper.java:148)
@ android.app.activitythread.main(activitythread.java:5417)
@ java.lang.reflect.method.invoke(native method)
@ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:726)
@ com.android.internal.os.zygoteinit.main(zygoteinit.java:616)
06-28 12:17:15.410 2915-2915/com.example.android.sqlite i/process: sending signal. pid: 2915 sig: 9