android - How to use java classes within one activity? -

how can use java class within 1 activity, mean having different components of activity spread out in bunch of java classes. i'm little new android , have tried far:

mainactivity.java

package com.example.alex.myapplication;     import android.support.v7.app.actionbaractivity;  import android.os.bundle;  import android.view.menu;  import android.view.menuitem;     public class mainactivity extends actionbaractivity {  @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_main);      new something(this);   }      @override public boolean oncreateoptionsmenu(menu menu) {     // inflate menu; adds items action bar if present.     getmenuinflater().inflate(r.menu.menu_main, menu);     return true; }  @override public boolean onoptionsitemselected(menuitem item) {     // handle action bar item clicks here. action bar     // automatically handle clicks on home/up button, long     // specify parent activity in androidmanifest.xml.     int id = item.getitemid();      //noinspection simplifiableifstatement     if (id == r.id.action_settings) {         return true;     }      return super.onoptionsitemselected(item); }   } 

something.java

import android.view.view; import android.widget.edittext; import android.widget.button; import android.app.activity;         public class  {    private activity activity;   private button add,subtract,multiply,devide;   private edittext edita, editb, editc;   private double doublea,doubleb,doublec;            public something(activity a){                activity=a;               click();         }          public void click(){            edita = (edittext) activity.findviewbyid(r.id.edittext);            editb = (edittext) activity.findviewbyid(r.id.edittext2);            editc = (edittext) activity.findviewbyid(r.id.edittext3);           doublea =double.parsedouble(edita.gettext().tostring());           doubleb =double.parsedouble(editb.gettext().tostring());                 add = (button) activity.findviewbyid(r.id.add);               subtract = (button) activity.findviewbyid(r.id.subtract);               multiply = (button) activity.findviewbyid(r.id.multiply);               devide = (button) activity.findviewbyid(r.id.devide);                 add.setonclicklistener(new view.onclicklistener() {                    @override                    public void onclick(view v) {                       doublec = doublea+doubleb;                      string s = "" + doublec;                       editc.settext(s);                      }                });                 subtract.setonclicklistener(new view.onclicklistener() {                @override                public void onclick(view v) {                     doublec = doublea-doubleb;                    string s = "" + doublec;                    editc.settext(s);                  }            });                 multiply.setonclicklistener(new view.onclicklistener() {                @override                public void onclick(view v) {                     doublec = doublea*doubleb;                    string s = "" + doublec;                    editc.settext(s);                  }            });               devide.setonclicklistener(new view.onclicklistener() {                @override                public void onclick(view v) {                     doublec = doublea/doubleb;                    string s = "" + doublec;                    editc.settext(s);                  }            });           }           } 

so wasn't sure why listeners weren't working on buttons tried passing activity class has listeners added buttons didn't work in fact application won't start in emulator. wanted have "mainactivity" handle "gui" , have "something" class handle listeners no matter can't seem make them communicate 1 form 1 activity.

logcat

caused by: java.lang.nullpointerexception: attempt invoke virtual method 'android.view.view android.view.window.findviewbyid(int)' on null object reference         @ android.app.activity.findviewbyid(activity.java:2071)         @ com.example.alex.myapplication.something.click(something.java:32)         @ com.example.alex.myapplication.mainactivity.oncreate(mainactivity.java:19)         @ android.app.activity.performcreate(activity.java:5937)         @ android.app.instrumentation.callactivityoncreate(instrumentation.java:1105)         @ android.app.activitythread.performlaunchactivity (activitythread.java:2251)         @ android.app.activitythread.handlelaunchactivity(activitythread.java:2360)         @ android.app.activitythread.access$800(activitythread.java:144)         @ android.app.activitythread$h.handlemessage(activitythread.java:1278)         @ android.os.handler.dispatchmessage(handler.java:102)         @ android.os.looper.loop(looper.java:135)         @ android.app.activitythread.main(activitythread.java:5221)         @ java.lang.reflect.method.invoke(native method)         @ java.lang.reflect.method.invoke(method.java:372)         @   com.android.internal.os.zygoteinit$methodandargscaller.run           (zygoteinit.java:899)         @ com.android.internal.os.zygoteinit.main(zygoteinit.java:694) 

like before post xd 1 java class, call again after pressing buttons, example, can have invisible buttons, radiobuttons whatever want invisible , click turns visible , useful you, here go:

first variable controls activity going do

string num =""; 

then have buttons, use 2 of them , others invisible

button bn1; button bn2; button bn3; button bn4; bn3.setvisibility(view.invisible); bn4.setvisibility(view.invisible); 

then button code, depends on how many buttons want

button.setonclicklistener(new button onclicklistener(){         public void onclick(){             //get default variable in case string num     intent intent = new intent(mainactivity.this, mainactivity.class); num="cero"; intent.putextra("po", num);                         codigopeticion=2; startactivityforresult (intent,codigopeticion); finish(); break;                 }             }         }); 

this 1 string num:

bundle extras = getintent().getextras();         if (extras!= null) {             num =extras.getstring("po");         } 

and @ last not least 1 someting depending string:

    if (num.matches("cero")){                 //do something, enable more buttons, disable radiobuttons, bn3.setvisibility(view.visible);             }else if(num.matches("one")){//this string button                 //do else in same activity, spected enable radiobuttons, show image, etc bn4.setvisibility(view.visible);             }else{                 //some textview specific title                 textview.settext("something's wrong");             } 

don't forget bn3 , bn4 listeners!

hope helps you, see ya!