Android - Changing background color after a specific time

Question

I have a problem with changing background color. I want to change it three seconds after the start of the activity. I have created a timer and a handler+runnable.It works fine when I want to change a special EditText after three seconds. but when I add the line layout_interact.setBackgroundColor(Color.GREEN); and start the app in the virtual device, it crashes when it is starting the activity which contains the timer and handler. I have also tried to start a new activity after three seconds, but that ended up with the same error. Here's my code:

public class ActivityInteractProcess extends Activity{
final Handler handler_interact=new Handler();//not defined as final variable. may cause problem
View layout_interact=(View) findViewById(R.id.layoutintprocess);

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.page_interact_process);


    //creating timer
    Timer timer_interact=new Timer();
    timer_interact.schedule(new TimerTask() {
          @Override
         public void run() {UpdateGUI();}
      }, 3000);
    }
   private void UpdateGUI() {
     handler_interact.post(runnable_interact);           
   }
    //creating runnable
   final Runnable runnable_interact = new Runnable() {
      public void run() {
      layout_interact.setBackgroundColor(Color.GREEN); //this is the line which makes the app Force close.
      }
   };
}

I haven't forgot to add the following line to xml layout of the activity:

android:id="@+id/layoutintprocess"

Here are the error I got in LogCat:

AndroidRuntime(571): FATAL EXCEPTION: main
AndroidRuntime(571): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{aiLab.tests.autism/aiLab.tests.autism.ActivityInteractProcess}: java.lang.NullPointerException
AndroidRuntime(571):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1739)
AndroidRuntime(571):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831)
AndroidRuntime(571):    at android.app.ActivityThread.access$500(ActivityThread.java:122)  
AndroidRuntime(571):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024)
AndroidRuntime(571):    at android.os.Handler.dispatchMessage(Handler.java:99)
...

Answer 1

change your code to this:

public class ActivityInteractProcess extends Activity{
final Handler handler_interact=new Handler();//not defined as final variable. may cause        problem
View layout_interact;

@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.page_interact_process);
layout_interact =(View) findViewById(R.id.layoutintprocess);


}


@Override
public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);

    //creating timer
    Timer timer_interact=new Timer();
    timer_interact.schedule(new TimerTask() {
        @Override
        public void run() {UpdateGUI();}
    }, 3000);
}
private void UpdateGUI() {
    handler_interact.post(runnable_interact);
}
//creating runnable
final Runnable runnable_interact = new Runnable() {
    public void run() {
        layout_interact.setBackgroundColor(Color.GREEN);
    }
};
}