java - How to clear user data in android programmatically? -

hello new in android , followed tutorial explain how clear app data in android, when ever trying clear data getting error of null object reference.

main.java

public class mainactivity extends actionbaractivity {         button b;         @override         protected void oncreate(bundle savedinstancestate) {             super.oncreate(savedinstancestate);             setcontentview(r.layout.activity_main);             b = (button)findviewbyid(r.id.start);               adddatainappdir();              b.setonclicklistener(new onclicklistener() {                 @override                 public void onclick(view v) {                      cleardataapplication.getinstance().clearapplicationdata(getbasecontext());                   }               });         }          private void adddatainappdir() {             sharedpreferences settings = getsharedpreferences("sample", 0);             sharedpreferences.editor editor = settings.edit();             editor.putboolean("key1", true);             editor.putstring("key2", "some strings in prefs");             editor.commit();              try {                  final string filecontent = "this string in file samplefile.txt";                 fileoutputstream fout = openfileoutput("samplefile.txt", mode_private);                 outputstreamwriter osw = new outputstreamwriter(fout);                 osw.write(filecontent);                 osw.flush();                 osw.close();             } catch (ioexception e) {                 e.printstacktrace();             }         }       } 

cleardataapplication.java

  public class cleardataapplication extends application      {         private static cleardataapplication instance;             @override            public void oncreate() {                super.oncreate();                instance = this;            }             public static cleardataapplication getinstance() {                return instance;            }             public void clearapplicationdata(context mcontext) {                 file cache = mcontext.getcachedir();                  file appdir = new file(cache.getparent());                 if(appdir.exists()) {                    string[] children = appdir.list();                     for(string s : children) {                        if(!s.equals("lib")) {                            deletedir(new file(appdir, s));                        }                    }                }            }              public static boolean deletedir(file dir) {                if (dir != null && dir.isdirectory()) {                    string[] children = dir.list();                    (int = 0; < children.length; i++) {                        boolean success = deletedir(new file(dir, children[i]));                        if (!success) {                            return false;                        }                    }                }                 return dir.delete();            }     } 

logcat

10-10 16:25:55.589: e/androidruntime(6121): fatal exception: main 10-10 16:25:55.589: e/androidruntime(6121): process: com.example.fetchdata, pid: 6121 10-10 16:25:55.589: e/androidruntime(6121): java.lang.nullpointerexception: attempt invoke virtual method 'void com.example.fetchdata.cleardataapplication.clearapplicationdata(android.content.context)' on null object reference 10-10 16:25:55.589: e/androidruntime(6121):     @ com.example.fetchdata.mainactivity$1.onclick(mainactivity.java:29) 10-10 16:25:55.589: e/androidruntime(6121):     @ android.view.view.performclick(view.java:4756) 10-10 16:25:55.589: e/androidruntime(6121):     @ android.view.view$performclick.run(view.java:19749) 10-10 16:25:55.589: e/androidruntime(6121):     @ android.os.handler.handlecallback(handler.java:739) 10-10 16:25:55.589: e/androidruntime(6121):     @ android.os.handler.dispatchmessage(handler.java:95) 10-10 16:25:55.589: e/androidruntime(6121):     @ android.os.looper.loop(looper.java:135) 10-10 16:25:55.589: e/androidruntime(6121):     @ android.app.activitythread.main(activitythread.java:5221) 10-10 16:25:55.589: e/androidruntime(6121):     @ java.lang.reflect.method.invoke(native method) 10-10 16:25:55.589: e/androidruntime(6121):     @ java.lang.reflect.method.invoke(method.java:372) 10-10 16:25:55.589: e/androidruntime(6121):     @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:899) 10-10 16:25:55.589: e/androidruntime(6121):     @ com.android.internal.os.zygoteinit.main(zygoteinit.java:694) 

the npe saying cleardataapplication.getinstance() returning null.

cleardataapplication extends application means it's intended representation of application lifetime of running process. don't need getinstance() method runtime guarantees there 1 instance. instead, activity, call getapplication() , cast cleardataapplicatiom.

in order above work, need make change in manifest file specify cleardataapplication should used representation of application.

<application android:name=".cleardataapplication" ...> 

reference: http://developer.android.com/guide/topics/manifest/application-element.html#nm