java - What exactly is causing this NullPointerException? -


this question has answer here:

i'm pretty new android development , working on app supposed save current time , date when button pressed, store sharedpreferences , present string when different button pressed. got working did tinkering today , i'm getting nullpointerexception can't solve, after having spent 45 minutes on google , stackoverflow trying different code fragments. can better @ me tell me i'm missing?

here relevant code: 

public class gettimestamp extends appcompatactivity {      private string date;     public void setdate (view view){         dateformat df = new simpledateformat("dd.mm.yyy 'om' hh:mm");         date = df.format(calendar.getinstance().gettime());     }     public string getdate(){         return date;     }         sharedpreferences prefs = this.getsharedpreferences("sharedprefs",       context.mode_private);     sharedpreferences.editor editor = prefs.edit();     protected void oncreate(bundle savedinstancestate) {          super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_get_time_stamp);         toolbar toolbar = (toolbar) findviewbyid(r.id.toolbar);         setsupportactionbar(toolbar);          floatingactionbutton fab = (floatingactionbutton) findviewbyid(r.id.fabsavetimestamp);         fab.setonclicklistener(new view.onclicklistener() {             @override             public void onclick(view view) {                 setdate(view);                 editor.putstring("storeddate", date);                 editor.commit();                 toast.maketext(getbasecontext(), "blablabla", toast.length_short).show();             }         }) floatingactionbutton fab3 = (floatingactionbutton) findviewbyid(r.id.fabshowtimestamp);         fab3.setonclicklistener(new view.onclicklistener() {             @override             public void onclick(view view) {                 //retrieve date sharedpreferences , store local string                 string retrieveddate = prefs.getstring("storeddate", null);                 if (retrieveddate!=null){                     alertdialog alertdialog = new alertdialog.builder(gettimestamp.this).create(); //read update                     alertdialog.settitle("");                     alertdialog.setmessage("somemessage" + retrieveddate);                     alertdialog.setbutton("ok", new dialoginterface.onclicklistener() {                         public void onclick(dialoginterface dialog, int which) {                         }                     });                     alertdialog.show();                 }else {                     //create alert dialog explaining user no date has been saved yet                     alertdialog alertdialog = new alertdialog.builder(gettimestamp.this).create(); //read update                     alertdialog.settitle("error");                     alertdialog.setmessage("errorexplanation");                     alertdialog.setbutton("ok", new dialoginterface.onclicklistener() {                         public void onclick(dialoginterface dialog, int which) {                         }                     });                     alertdialog.show();                }             }         });

and here errors: 

02-08 22:49:36.142 19735-19735/? w/dalvikvm: threadid=1: thread exiting uncaught exception (group=0x41ed7ce0) 02-08 22:49:36.152 19735-19735/? e/androidruntime: fatal exception: main    process: com.example.user.appname, pid: 19735    java.lang.runtimeexception: unable instantiate activity componentinfo{com.example.user.appname/com.example.user.appname.gettimestamp}: java.lang.nullpointerexception        @ android.app.activitythread.performlaunchactivity(activitythread.java:2131)        @ android.app.activitythread.handlelaunchactivity(activitythread.java:2264)        @ android.app.activitythread.access$800(activitythread.java:144)        @ android.app.activitythread$h.handlemessage(activitythread.java:1205)        @ android.os.handler.dispatchmessage(handler.java:102)        @ android.os.looper.loop(looper.java:136)        @ android.app.activitythread.main(activitythread.java:5139)        @ java.lang.reflect.method.invokenative(native method)        @ java.lang.reflect.method.invoke(method.java:515)        @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:796)        @ com.android.internal.os.zygoteinit.main(zygoteinit.java:612)        @ dalvik.system.nativestart.main(native method)     caused by: java.lang.nullpointerexception        @ android.content.contextwrapper.getsharedpreferences(contextwrapper.java:173)        @ com.example.user.appname.gettimestamp.<init>(gettimestamp.java:36)        @ java.lang.class.newinstanceimpl(native method)        @ java.lang.class.newinstance(class.java:1208)        @ android.app.instrumentation.newactivity(instrumentation.java:1061)        @ android.app.activitythread.performlaunchactivity(activitythread.java:2122)        @ android.app.activitythread.handlelaunchactivity(activitythread.java:2264)         @ android.app.activitythread.access$800(activitythread.java:144)         @ android.app.activitythread$h.handlemessage(activitythread.java:1205)         @ android.os.handler.dispatchmessage(handler.java:102)         @ android.os.looper.loop(looper.java:136)         @ android.app.activitythread.main(activitythread.java:5139)         @ java.lang.reflect.method.invokenative(native method)         @ java.lang.reflect.method.invoke(method.java:515)         @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:796)         @ com.android.internal.os.zygoteinit.main(zygoteinit.java:612)         @ dalvik.system.nativestart.main(native method) 

you trying initialize following class field before allowed to:

sharedpreferences prefs = this.getsharedpreferences("sharedprefs",       context.mode_private);

only declare field:

sharedpreferences prefs;

and assign in oncreate() method:

protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     prefs = getsharedpreferences("sharedprefs", context.mode_private);     ... }

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