java - Android: What's the best hierarchy for this app? -

i have imagebutton , want image change every time onresume() or oncreate() called.

i have method changes image, called mupdatebackground, i'm having trouble working out how structure hierarchy mupdatebackground, (containing imagebutton) can called onresume() , oncreate().

if backgroundpic true, , image selected randomly, if false r.drawable.bg0 used instead.

there separate settings activity manages whether backgroundpic variable set true or false.

here's code far:

1 public class mainactivity extends activity { 2   public static imagebutton mgetclicktime; 3 @override 4   protected void onresume() { 5        super.onresume(); 6       //get shared preferences 7               msharedpreferences = getsharedpreferences(preferences_name, mode_private); 8           dp = msharedpreferences.getint("decimalplaces", 0); 9               length_setting = msharedpreferences.getint("msselector", 1); 10              backgroundpic = msharedpreferences.getboolean("backgroundpic", true); 11              //mupdatebackground();   12          } 12  @override 13  protected void oncreate(bundle savedinstancestate) { 14      super.oncreate(savedinstancestate); 15      setcontentview(r.layout.activity_main); 16 17      //get shared preferences 18      msharedpreferences = getsharedpreferences(preferences_name, mode_private); 19      dp = msharedpreferences.getint("decimalplaces", 0); 20      length_setting = msharedpreferences.getint("msselector", 5); 21      mupdatebackground(); 22          mgetclicktime.setonclicklistener(new view.onclicklistener() { 23      mupdatebackground(); 24          } 25     } 26 } 27  public void mupdatebackground() { 28      if (backgroundpic) { 29          int[] imageids = {  30                  r.drawable.bg1, 31                  r.drawable.bg2, 32                  r.drawable.bg3, 33                  r.drawable.bg4, 34 35          }; 36          random generator = new random(); 37          randomimageid = imageids[generator.nextint(imageids.length)]; 38          log.d("1", "backgroundpic: "+randomimageid); 39      } 40      else { 41          randomimageid = r.drawable.bg0; 42          log.d("1", "backgroundpic: "+randomimageid); 43      } 44      mgetclicktime = (imagebutton) findviewbyid(r.id.clicker); 45      mgetclicktime.setimageresource(randomimageid); 46  } 

the problem have if uncomment line 11 nullpointerexception. there better way should organise code?

you should check code if null before using it. don't check if imageids[] null before use , (its been awhile) want make sure mgetclicktime assigned findviewbyid. in onresume seems possible stuff can have gone out of memory. typically check see if need re-initialize variable in on resume when i've done android in past. i'm suggesting way debug , more robust.