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Question

我的問題是listview.adapter（適配器）就可以返回我一個錯誤日誌，它是：

09-27 12:30:02.550: E/AndroidRuntime(2151): FATAL EXCEPTION: main 
09-27 12:30:02.550: E/AndroidRuntime(2151): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.prueba_listas/com.example.prueba_listas.Lists}: java.lang.NullPointerException 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.app.ActivityThread.access$600(ActivityThread.java:141) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.os.Handler.dispatchMessage(Handler.java:99) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.os.Looper.loop(Looper.java:137) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.app.ActivityThread.main(ActivityThread.java:5103) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at java.lang.reflect.Method.invokeNative(Native Method) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at java.lang.reflect.Method.invoke(Method.java:525) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at dalvik.system.NativeStart.main(Native Method) 
09-27 12:30:02.550: E/AndroidRuntime(2151): Caused by: java.lang.NullPointerException 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at com.example.prueba_listas.Lists.onCreate(Lists.java:43) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.app.Activity.performCreate(Activity.java:5133) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175) 
09-27 12:30:02.550: E/AndroidRuntime(2151):  ... 11 more 

代碼：

import android.content.Context; 
import android.util.Log; 
import android.view.Menu; 
import android.widget.ArrayAdapter; 
import android.widget.ListView; 

public class Lists extends Activity { 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     ListView listview = (ListView) findViewById(R.layout.listview); 

     // String[] array = getResources().getStringArray(R.array.lista); 
     String[] array = new String[] { "Android", "iPhone", "WindowsMobile", 
       "Blackberry", "WebOS", "Ubuntu", "Windows7", "Max OS X", 
       "Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", 
       "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2", 
       "Android", "iPhone", "WindowsMobile" }; 
     final ArrayList<String> lista = new ArrayList<String>(); 
     for (int i = 0; i < array.length; ++i) { 
      lista.add(array[i]); 
     } 

     Log.i("OnCreate", "Listview adpt "); 

     final StableArrayAdapter adapter = new StableArrayAdapter(
       getApplicationContext(), android.R.layout.simple_list_item_1, 
       (List<String>) lista); 
     for (int i = 0; i < lista.size(); i++) 
      Log.i("OnCreate", adapter.getItem(i)); 

     Log.i("OnCreateadaptercount", String.valueOf(adapter.getCount())); 

       //here is the problem! 
      listview.setAdapter(adapter); 

//  listview.getAdapter(); 
     Log.i("OnCreate", "set adpater final "); 

    } 
} 

Answer 1

您需要在查找列表視圖之前充氣您的佈局，否則listView將是null。因此：

listview.setAdapter(adapter);拋出NullPointerException。

super.onCreate(savedInstanceState); 
//inflateLayout here 
setContentView(R.layout.your_layout); 
ListView listview = (ListView) findViewById(R.id.listview_id_in_layout);