为什么全局变量变为null如果扩展抽象BaseActivity

我有一个BaseActivity像bellow ...在超类的我的接口全局对象beame null。为什么全局变量变为null如果扩展抽象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) 
     } 
}

现在我有一个像

public class MyActivity extends BaseActivity implements IOnImagSelected { 

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

    @Override 
    poublic void onImageSelected(){ 

     } 

}

一个活动为什么我的接口变量成为空。请帮我解决这个问题

来源

2014-10-20 user3782810

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

onPickImageClick之前调用调用时，我按下一个BTN。在那个活动 – user3782810 2014-10-20 11:01:29