java - What's this Error means in Android studio -

this question has answer here:

• what nullpointerexception, , how fix it? 12 answers

i change can me see wrong because it's same error log

enter code here   public class mainactivity extends activity {      webview mwebview; public imagebutton button01; public imagebutton button02; public class mainactivity extends activity  {  webview mwebview; public imagebutton button01;           `` public imagebutton button02;  @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(mylayout);     button01 = (imagebutton) findviewbyid(r.id.button01);     button02 = (imagebutton) findviewbyid(r.id.button02);`enter code here`     button01.setonclicklistener(new imagebutton.onclicklistener(){     public void onclick(view v){  jumptolayout03();    intent intent = new intent();        intent.setclass(mainactivity.this, web.class);        startactivity(intent); }}      public void jumptolayout03() {        setcontentview(activity_main01);    }     });}} enter code here 

i change can me see wrong because it's same error log

it means have null pointer on line 37 of program. in main activity.