Android/Java - Communication from classes return java.lang.nullpointerexception

Question

I have created in an Android project the main class that extend the activity, and another one that extend the main class. I want to use in the main class a method declared in the second class. But i am receiving always a nullpointerexception. I put here the two simple classes. The main class:

package com.example.androidserverapp;

import android.os.Bundle;
import android.app.Activity;
import android.content.pm.ActivityInfo;
import android.view.Menu;
import android.widget.ImageView;
public class AndroidServer extends Activity {

  public ImageView main_image;


  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_android_server);
    this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    Utilities utilities = new Utilities();
    utilities.initialize();
  }


}

The second class:

package com.example.androidserverapp;

import android.widget.ImageView;; 

public class Utilities extends AndroidServer{

  public ImageView main_image;

  public void initialize(){
      main_image.findViewById(R.id.main_image);
      main_image.setImageResource(R.drawable.image3);
  }

}

I saw that the error is referring to the line main_image = (ImageView) findViewById(R.id.main_image); but i'm not understanding what i'm doing wrong. Any help? Here there is the log cat:

07-12 11:35:04.473: E/AndroidRuntime(14954): FATAL EXCEPTION: main
07-12 11:35:04.473: E/AndroidRuntime(14954): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidserverapp/com.example.androidserverapp.AndroidServer}: java.lang.NullPointerException
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2351)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.access$600(ActivityThread.java:151)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1331)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.os.Handler.dispatchMessage(Handler.java:99)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.os.Looper.loop(Looper.java:155)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.main(ActivityThread.java:5454)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at java.lang.reflect.Method.invokeNative(Native Method)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at java.lang.reflect.Method.invoke(Method.java:511)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1029)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:796)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at dalvik.system.NativeStart.main(Native Method)
07-12 11:35:04.473: E/AndroidRuntime(14954): Caused by: java.lang.NullPointerException
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.Activity.findViewById(Activity.java:1870)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at com.example.androidserverapp.Utilities.initialize(Utilities.java:8)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at com.example.androidserverapp.AndroidServer.onCreate(AndroidServer.java:20)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.Activity.performCreate(Activity.java:5066)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1101)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307)
07-12 11:35:04.473: E/AndroidRuntime(14954):    ... 11 more

Thanks

Answer 1

Your main_image is null, you need to declare your imageView in the same class where you declare setContentView(R.layout.activity_android_server);

Caused by: java.lang.NullPointerException 07-12 11:35:04.473: E/AndroidRuntime(14954): at android.app.Activity.findViewById(Activity.java:1870) 07-12 11:35:04.473: E/AndroidRuntime(14954): at com.example.androidserverapp.Utilities.initialize(Utilities.java:8) 07-12 11:35:04.473:

Answer 2

The reason why it is crashing is that you create object of class Utilities which extends AndroidServer but onCreate() method is not called because it is just new java object and activity is not started. So in initialize() method you are finding view but you can't do that until you start that activity. Hope you understand what I mean.

EDIT: so you don't need to extend Activity in Utilities class and you can make something like this:

public class Utilities {

   public void initialize(View mainHolder){
      ImageView imageView = mainHolder.findViewById(R.id.main_image);
      imageView.setImageResource(R.drawable.image3);
   }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   LayoutInflater inflater = LayoutInflater.from(this);
   View contentView = infler.inflate(R.layout.activity_android_server, null);
   setContentView(contentView);
   this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

   Utilities utilities = new Utilities();
   utilities.initialize(contentView);
}