java - App crashes when trying to send SMS -


this question has answer here:

app crashes when when try send sms application. have app sends sms onclick of button. crashes null pointer exception.

i think has context. don't know how achieve it. i'm new android appreciated.

logcat:

09-12 00:32:25.895 17629-17629/com.nitsystem e/androidruntime: fatal exception: main      process: com.nitsystem, pid: 17629      java.lang.nullpointerexception: attempt invoke virtual method 'java.lang.string android.content.context.getpackagename()' on null object reference          @ android.preference.preferencemanager.getdefaultsharedpreferencesname(preferencemanager.java:499)          @ android.preference.preferencemanager.getdefaultsharedpreferences(preferencemanager.java:488)          @ com.nitsystem.activity.mainactivity.sendsms(mainactivity.java:208)          @ com.nitsystem.activity.mainactivity$1.onclick(mainactivity.java:77)          @ android.view.view.performclick(view.java:6213)          @ android.widget.textview.performclick(textview.java:11074)          @ android.view.view$performclick.run(view.java:23645)          @ android.os.handler.handlecallback(handler.java:751)          @ android.os.handler.dispatchmessage(handler.java:95)          @ android.os.looper.loop(looper.java:154)          @ android.app.activitythread.main(activitythread.java:6692)          @ java.lang.reflect.method.invoke(native method)          @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:1468)          @ com.android.internal.os.zygoteinit.main(zygoteinit.java:1358)

code:

private static final string sms_phone_number="sms"; public static long lastsmssentat = 0; public static string smscontent = "hello world"; public  static mainactivity ref= null;  public class mainactivity extends appcompatactivity {     @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);             msendsms = (button) findviewbyid(r.id.start_stop_detection);                  msendsms.setonclicklistener(new view.onclicklistener() {                     @override                     public void onclick(view v) {                              sendsms(ref);                         }                 });     }      public static void sendsms(mainactivity context) {         sharedpreferences sharedpref = preferencemanager.getdefaultsharedpreferences(context);         string phonenumber = sharedpref.getstring(sms_phone_number, "");         long smsnow = system.currenttimemillis();         long diff = smsnow - lastsmssentat;         log.d(tag, "diff : " + diff);         log.d(tag, "pre: " + lastsmssentat + " now: " + smsnow);         if(diff> 30000){             lastsmssentat = smsnow;             calendar calendar = calendar.getinstance();             simpledateformat simpledf =  new simpledateformat("yyyy-mm-dd hh:mm:ss");             smsmanager sms = smsmanager.getdefault();             string destinationaddress = phonenumber;             string message = smscontent +"\n";             message += "\ntest sms time: " + simpledf.format(calendar.gettime()) + "";             sms.sendtextmessage(destinationaddress, null, message, null,null);             log.d(tag,"sms sent");         }     } }

try folowing

msendsms.setonclicklistener(new view.onclicklistener() {             @override             public void onclick(view v) {                      sendsms(youractivity.this);                 }         });

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