无法启动活动ComponentInfo：java.lang.NullPointerException

Question

我正在为我的学位构建一个android应用程序。我需要从手机获取所有联系人 开始申请（稍后我还需要获得电子邮件etcetc）。所以我用了两个类

Degree_thesisActivity

public class Degree_thesisActivity extends Activity { 
/** Called when the activity is first created. */ 
@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 

    GetFromPhone gf = new GetFromPhone(); 
    gf.getContacts(); 

} 

}

GetFromPhone.java

class GetFromPhone extends Activity { 

public void getContacts() { 
    // TODO Auto-generated method stub 


    /**Declare a Contacts Cursor which have a list of all contacts*/ 
    Cursor cursor = getContentResolver().query(ContactsContract.Contacts.CONTENT_URI, null, null, null, null); 
    /**Move while cursor has another istance to process*/ 
    while(cursor.moveToNext()){ 
     /**Get the contact name pointed by cursor at that time*/ 
     cursor.getString(cursor.getColumnIndex(ContactsContract.Contacts.DISPLAY_NAME)); 
     String contactID = cursor.getString(cursor.getColumnIndex(ContactsContract.Contacts.DISPLAY_NAME)); 
     /**show the contact pointed.THIS WILL BE MODIFIED FURTHER!!!!*/ 
       new AlertDialog.Builder(this) 
        .setMessage(contactID) 
        .setNeutralButton("ok", new OnClickListener() { 
         public void onClick(DialogInterface dialog, int which) { 
          // TODO Auto-generated method stub 

         } 
        }) 
        .show(); 

    } 

} 

}

，但我得到这个错误：

05-05 15：49：20.060：E/AndroidRuntime（824）：java.lang.RuntimeException：无法启动活动ComponentInfo {it.paki.degreethesis/it.paki.degreethesis.Degree_thesisActivity}：java.lang.NullPointerException

为什么？有人可以帮助我吗？

Answer 1

由于类GetFromPhone扩展Activity，你不能使用new关键词创建类的实例。作为Activity，它需要遵循activity lifecycle。我认为你需要阅读application fundamentals。