android - Unable to start activity ComponentInfo java.lang.NullPointerException File Browser -

i have copied class, opens dialog in order pick file, tutorial , don't know why throws me exception, can me?

this class :

import android.app.activity; import android.app.alertdialog; import android.app.dialog; import android.content.dialoginterface; import android.os.environment; import android.util.log;  import java.io.file; import java.io.filenamefilter; import java.util.arraylist; import java.util.list;  public class filedialog {     private static final string parent_dir = "..";     private final string tag = getclass().getname();     private string[] filelist;     private file currentpath;     public interface fileselectedlistener {         void fileselected(file file);     }     public interface directoryselectedlistener {         void directoryselected(file directory);     }     private listenerlist<fileselectedlistener> filelistenerlist = new listenerlist<filedialog.fileselectedlistener>();     private listenerlist<directoryselectedlistener> dirlistenerlist = new listenerlist<filedialog.directoryselectedlistener>();     private final activity activity;     private boolean selectdirectoryoption;     private string fileendswith;      public filedialog(activity activity, file path) {         this.activity = activity;         if (!path.exists()) path = environment.getexternalstoragedirectory();         loadfilelist(path);     }      public dialog createfiledialog() {         dialog dialog = null;         alertdialog.builder builder = new alertdialog.builder(activity);          builder.settitle(currentpath.getpath());         if (selectdirectoryoption) {             builder.setpositivebutton("select directory", new dialoginterface.onclicklistener() {                 public void onclick(dialoginterface dialog, int which) {                     log.d(tag, currentpath.getpath());                     firedirectoryselectedevent(currentpath);                 }             });         }          builder.setitems(filelist, new dialoginterface.onclicklistener() {             public void onclick(dialoginterface dialog, int which) {                 string filechosen = filelist[which];                 file chosenfile = getchosenfile(filechosen);                 if (chosenfile.isdirectory()) {                     loadfilelist(chosenfile);                     dialog.cancel();                     dialog.dismiss();                     showdialog();                 } else firefileselectedevent(chosenfile);             }         });          dialog = builder.show();         return dialog;     }       public void addfilelistener(fileselectedlistener listener) {         filelistenerlist.add(listener);     }      public void removefilelistener(fileselectedlistener listener) {         filelistenerlist.remove(listener);     }      public void setselectdirectoryoption(boolean selectdirectoryoption) {         this.selectdirectoryoption = selectdirectoryoption;     }      public void adddirectorylistener(directoryselectedlistener listener) {         dirlistenerlist.add(listener);     }      public void removedirectorylistener(directoryselectedlistener listener) {         dirlistenerlist.remove(listener);     }      /**      * show file dialog      */     public void showdialog() {         createfiledialog().show();     }      private void firefileselectedevent(final file file) {         filelistenerlist.fireevent(new listenerlist.firehandler<fileselectedlistener>() {             public void fireevent(fileselectedlistener listener) {                 listener.fileselected(file);             }         });     }      private void firedirectoryselectedevent(final file directory) {         dirlistenerlist.fireevent(new listenerlist.firehandler<directoryselectedlistener>() {             public void fireevent(directoryselectedlistener listener) {                 listener.directoryselected(directory);             }         });     }      private void loadfilelist(file path) {         this.currentpath = path;         list<string> r = new arraylist<string>();         if (path.exists()) {             if (path.getparentfile() != null) r.add(parent_dir);             filenamefilter filter = new filenamefilter() {                 public boolean accept(file dir, string filename) {                     file sel = new file(dir, filename);                     if (!sel.canread()) return false;                     if (selectdirectoryoption) return sel.isdirectory();                     else {                         boolean endswith = fileendswith != null ? filename.tolowercase().endswith(fileendswith) : true;                         return endswith || sel.isdirectory();                     }                 }             };             string[] filelist1 = path.list(filter);             (string file : filelist1) {                 r.add(file);             }         }         filelist = (string[]) r.toarray(new string[]{});     }      private file getchosenfile(string filechosen) {         if (filechosen.equals(parent_dir)) return currentpath.getparentfile();         else return new file(currentpath, filechosen);     }      public void setfileendswith(string fileendswith) {         this.fileendswith = fileendswith != null ? fileendswith.tolowercase() : fileendswith;     } }  class listenerlist<l> {     private list<l> listenerlist = new arraylist<l>();      public interface firehandler<l> {         void fireevent(l listener);     }      public void add(l listener) {         listenerlist.add(listener);     }      public void fireevent(firehandler<l> firehandler) {         list<l> copy = new arraylist<l>(listenerlist);         (l l : copy) {             firehandler.fireevent(l);         }     }      public void remove(l listener) {         listenerlist.remove(listener);     }      public list<l> getlistenerlist() {         return listenerlist;     } } 

this call in activity:

@override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_main);     file mpath = new file(environment.getexternalstoragedirectory()+ "//dir//");     filedialog = new filedialog(this, mpath);     filedialog.setfileendswith(".txt");     filedialog.addfilelistener(new filedialog.fileselectedlistener() {         public void fileselected(file file) {             log.d(getclass().getname(), "selected file " + file.tostring());         }     });     filedialog.showdialog(); } 

and stack trace:

java.lang.runtimeexception: unable start activity componentinfo{mainactivity}                                                                                                             java.lang.nullpointerexception         @ android.app.activitythread.performlaunchactivity(activitythread.java:2200)         @ android.app.activitythread.handlelaunchactivity(activitythread.java:2250)         @ android.app.activitythread.access$800(activitythread.java:139)         @ android.app.activitythread$h.handlemessage(activitythread.java:1200)         @ android.os.handler.dispatchmessage(handler.java:102)         @ android.os.looper.loop(looper.java:136)         @ android.app.activitythread.main(activitythread.java:5105)         @ java.lang.reflect.method.invokenative(native method)         @ java.lang.reflect.method.invoke(method.java:515)         @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:792)         @ com.android.internal.os.zygoteinit.main(zygoteinit.java:608)         @ dalvik.system.nativestart.main(native method)  caused by: java.lang.nullpointerexception         @ rpstudios.com.filedialog.loadfilelist(filedialog.java:130)         @ rpstudios.com.filedialog.<init>(filedialog.java:35)         @ rpstudios.com.mainactivity.oncreate(mainactivity.java:22) 

any suggestion welcomed

edit: nullpointerexception in line 130: for(string file : filelist1) in loadfilelist() method.

i think in public dialog createfiledialog() missing create()

dialog = builder.show(); 

should be

dialog = builder.create().show(); 

let's try ;)