Android Array List uncaught exception -

i'm trying pull in list of locations db array list can later use create dialog box when call below method can errors below. can see i'm doing wrong , why uncaught exception when catching it? appreciate help!

public void alllocations() {      try{         cursor mcursor = mdb.rawquery("select * " + table_location, null);         arraylist<string> marraylist = new arraylist<string>();         mcursor.movetofirst();         while(!mcursor.isafterlast()) {              marraylist.add(mcursor.getstring(mcursor.getcolumnindex(location)));              mcursor.movetonext();         }     }catch (sqliteexception e){         log.e("all locations", "error getting locations: " + e.tostring());     } } 

// errors

06-05 03:36:29.345: d/dalvikvm(2110): gc_external_alloc freed 655 objects / 51272 bytes in 77ms 06-05 03:36:30.185: v/one(2110): locationselection 06-05 03:36:31.805: d/androidruntime(2110): shutting down vm 06-05 03:36:31.805: w/dalvikvm(2110): threadid=1: thread exiting uncaught exception (group=0x4001d800) 06-05 03:36:31.825: e/androidruntime(2110): fatal exception: main 06-05 03:36:31.825: e/androidruntime(2110): java.lang.nullpointerexception 06-05 03:36:31.825: e/androidruntime(2110):     @ com.myapp.sqlite.location.alllocations(location.java:137) 06-05 03:36:31.825: e/androidruntime(2110):     @ com.myapp.options.displaylocations(options.java:178) 06-05 03:36:31.825: e/androidruntime(2110):     @ com.myapp.options$1.onclick(options.java:69) 06-05 03:36:31.825: e/androidruntime(2110):     @ android.view.view.performclick(view.java:2408) 06-05 03:36:31.825: e/androidruntime(2110):     @ android.view.view$performclick.run(view.java:8816) 06-05 03:36:31.825: e/androidruntime(2110):     @ android.os.handler.handlecallback(handler.java:587) 06-05 03:36:31.825: e/androidruntime(2110):     @ android.os.handler.dispatchmessage(handler.java:92) 06-05 03:36:31.825: e/androidruntime(2110):     @ android.os.looper.loop(looper.java:123) 06-05 03:36:31.825: e/androidruntime(2110):     @ android.app.activitythread.main(activitythread.java:4627) 06-05 03:36:31.825: e/androidruntime(2110):     @ java.lang.reflect.method.invokenative(native method) 06-05 03:36:31.825: e/androidruntime(2110):     @ java.lang.reflect.method.invoke(method.java:521) 06-05 03:36:31.825: e/androidruntime(2110):     @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:868) 06-05 03:36:31.825: e/androidruntime(2110):     @ com.android.internal.os.zygoteinit.main(zygoteinit.java:626) 06-05 03:36:31.825: e/androidruntime(2110):     @ dalvik.system.nativestart.main(native method) 

//the whole class

public class location {      private databasehelper mdbhelper;     private sqlitedatabase mdb;     private final context mctx;      private static final string location = ("create table " + table_location             + " (" + location + " text," + location_id + " text " + ");");      private static class databasehelper extends sqliteopenhelper {          databasehelper(context context) {             super(context, database_name, null, database_version);         }          @override         public void oncreate(sqlitedatabase db) {             db.execsql(location);         }          @override         public void onupgrade(sqlitedatabase db, int oldversion, int newversion) {             db.execsql("drop table if exists " + database_name);             oncreate(db);          }     }      public location(context ctx) {         this.mctx = ctx;     }      public location open() throws sqlexception {         this.mdbhelper = new databasehelper(this.mctx);         this.mdb = this.mdbhelper.getwritabledatabase();         return this;     }      public void close() {         this.mdbhelper.close();     }      public long addlocation(string locations, string location_id) {          contentvalues values = new contentvalues();          values.put(location, locations);         values.put(location_id, location_id);          return this.mdb.insert(table_location, null, values);     }      public string getlocation(long l) throws sqlexception {          string[] columns = new string[] { location, location_id };         cursor c = mdb.query(table_location, columns, null, null, null, null,                 null);         string result = "";          int glocation = c.getcolumnindex(location);         int glocationid = c.getcolumnindex(location_id);          (c.movetofirst(); !c.isafterlast(); c.movetonext()) {             result = result + c.getstring(glocation) + " "                     + c.getstring(glocationid) + " " + "\n";         }         return result;     }      /** delete location table */     public void deletealllocations() {         mdb.delete(table_location, null, null);     }      /** check there locations stored */     public boolean countlocation() {          cursor count = mdb.rawquery("select count(*) " + table_location,                 null);          if (count == null) {             return true;         }         return false;     }      public void alllocations() {          try{             cursor mcursor = mdb.rawquery("select * " + table_location, null);             arraylist<string> marraylist = new arraylist<string>();             mcursor.movetofirst();             while(!mcursor.isafterlast()) {                  marraylist.add(mcursor.getstring(mcursor.getcolumnindex(location)));                  mcursor.movetonext();             }         }catch (sqliteexception e){             log.e("all locations", "error getting locations: " + e.tostring());         }     } } 

mdb null @ least in code shown here.

do have other code initializing mdb reference?

edit :]

the change needs made in activity class using class.

you need make sure in activity calling location.open(), before other methods.