爲什麼全局變量變爲null如果擴展抽象BaseActivity

我有一個BaseActivity像bellow ...在超類的我的接口全局對象beame null。爲什麼全局變量變爲null如果擴展抽象BaseActivity

public abstract class BaseActivity extends BaseTabActivity { 
      //did some implementation 
} 


public abstract class BaseTabActivity extends BasePickerActivity { 
    //did some implementation 
} 

public abstract class BasePickerActivity extends Activity { 
    // This is an interface 
     private IOnImagSelected iOnImagSelected; 

     public void imagePicker(IOnImagSelected iOnImagSelected){ 
      this.iOnImagSelected = iOnImagSelected; 
     } 

     @Override 
     protected void onActivityResult(int requestCode, int resultCode, Intent data) { 
      // process data and called iOnImagSelected 

        // Here I get null point which mean iOnImagSelected is null 
          iOnImagSelected.onImageSelected(imagepath) 
     } 
}

現在我有一個像

public class MyActivity extends BaseActivity implements IOnImagSelected { 

     public void onPickImageClick(View v){ 
      imagePicker(this); 
     } 

    @Override 
    poublic void onImageSelected(){ 

     } 

}

一個活動爲什麼我的接口變量成爲空。請幫我解決這個問題

來源

2014-10-20 user3782810

是onPickImageClick（視圖V）onActivityResult（..） – 2014-10-20 10:55:06

onPickImageClick之前調用調用時，我按下一個BTN。在那個活動 – user3782810 2014-10-20 11:01:29