java - Execute a method from a class in mainActivity (Android) -

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i m trying execute method class called qrscann in mainactivity class when user click on button everytime app crash click button , gives me log crash output 

 06-02 17:00:24.890    7067-7084/t.n.app w/egl_emulation﹕ eglsurfaceattrib not implemented 06-02 17:00:24.890    7067-7084/t.n.app w/openglrenderer﹕ failed set egl_swap_behavior on surface 0xb3ee6da0, error=egl_success 06-02 17:00:27.473    7067-7067/t.n.app d/androidruntime﹕ shutting down vm 06-02 17:00:27.474    7067-7067/t.n.app e/androidruntime﹕ fatal exception: main     process: snet.tuberlin.app, pid: 7067     java.lang.nullpointerexception: attempt invoke virtual method 'android.app.activitythread$applicationthread android.app.activitythread.getapplicationthread()' on null object reference             @ android.app.activity.startactivityforresult(activity.java:3745)             @ android.app.activity.startactivityforresult(activity.java:3706)             @ snet.tuberlin.app.qrcode.scanqr(qrcode.java:23)             @ snet.tuberlin.app.mainactivity.onclick(mainactivity.java:48)             @ android.view.view.performclick(view.java:4780)             @ android.view.view$performclick.run(view.java:19866)             @ android.os.handler.handlecallback(handler.java:739)             @ android.os.handler.dispatchmessage(handler.java:95)             @ android.os.looper.loop(looper.java:135)             @ android.app.activitythread.main(activitythread.java:5257)             @ java.lang.reflect.method.invoke(native method)             @ java.lang.reflect.method.invoke(method.java:372)             @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:903)             @ com.android.internal.os.zygoteinit.main(zygoteinit.java:698)

and class has method(scanner) want execute : 

public class qrscann extends activity {  static final string action_scan = "com.google.zxing.client.android.scan";  public void scanner(view v) {     try {         intent intent = new intent(action_scan);         intent.putextra("scan_mode", "qr_code_mode");         startactivityforresult(intent, 0);     } catch (activitynotfoundexception anfe) {         showdialog(qrscann.this, "no scanner found", "download scanner code activity?", "yes", "no").show();     } }

the class has other method well. , finelly have done in mainactivity class :

 public class mainactivity extends activity implements view.onclicklistener {    public button scanner;     @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);         scanner =(button)findviewbyid(r.id.buttonscanqr);         scanner.setonclicklistener(this);      }       @override     public void onclick(view v) {         //intent qrscann = new intent(this, qrscann.class);         //startactivity(qrscann );         qrscann qrscanner =new qrscann();         qrscanner.scanner(v);      } }

i appreciate explanation of may missing. 

public class qrscann {  static final string action_scan = "com.google.zxing.client.android.scan"; private mainactivity mcontext; public qrscann(mainactivity context) {    mcontext = context; }  public void scanner(view v) {     try {         intent intent = new intent(action_scan);         intent.putextra("scan_mode", "qr_code_mode");         mcontext.startactivityforresult(intent, 0);     } catch (activitynotfoundexception anfe) {         //show dialog deprecated , try dialogfragments         mcontext.showdialog(qrscann.this, "no scanner found", "download scanner code activity?", "yes", "no").show();     } }

from activity

@override     public void onclick(view v) {          qrscann qrscanner =new qrscann(this);         qrscanner.scanner(v);      }

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