了java.lang.RuntimeException：無法啓動活動ComponentInfo錯誤

Question

我想請教一些關於Android編程.. 我創建從RSS XML文件到Android返回顯示器的一類，但我得到了一些錯誤

04 -08 14：37：19.162：E/AndroidRuntime（381）： java.lang.RuntimeException：無法啓動活動 ComponentInfo {com.example.xmlreader/com.example.xmlreader.MainActivity}： java.lang.NullPointerException

代碼：

static final String URL = "http://api.androidhive.info/pizza/?format=xml"; 
// XML node keys 
static final String KEY_ITEM = "item"; // parent node 
static final String KEY_ID = "id"; 
static final String KEY_NAME = "name"; 
static final String KEY_COST = "cost"; 
static final String KEY_DESC = "description"; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    //setContentView(R.layout.activity_main); 

    ArrayList<HashMap<String, String>> menuItems = new ArrayList<HashMap<String, String>>(); 

    XMLParser parser = new XMLParser(); 
    String xml = parser.getXMLFromURL(URL); // getting XML 
    Document doc = parser.getDomElem(xml); // getting DOM element 

(*) NodeList nl = doc.getElementsByTagName(KEY_ITEM); 
    // looping through all item nodes <item> 
    for (int i = 0; i < nl.getLength(); i++) { 
     // creating new HashMap 
     HashMap<String, String> map = new HashMap<String, String>(); 
     Element e = (Element) nl.item(i); 
     // adding each child node to HashMap key => value 
     map.put(KEY_ID, parser.getValue(e, KEY_ID)); 
     map.put(KEY_NAME, parser.getValue(e, KEY_NAME)); 
     map.put(KEY_COST, "Rs." + parser.getValue(e, KEY_COST)); 
     map.put(KEY_DESC, parser.getValue(e, KEY_DESC)); 

     // adding HashList to ArrayList 
     menuItems.add(map); 
    } 

    // Adding menuItems to ListView 
    ListAdapter adapter = new SimpleAdapter(this, menuItems, 
      R.layout.list_item, 
      new String[] { KEY_NAME, KEY_DESC, KEY_COST }, new int[] { 
        R.id.name, R.id.desciption, R.id.cost }); 

    setListAdapter(adapter); 

    // selecting single ListView item 
    ListView lv = getListView(); 

    lv.setOnItemClickListener(new OnItemClickListener() { 

     @Override 
     public void onItemClick(AdapterView<?> parent, View view, 
       int position, long id) { 
      // getting values from selected ListItem 
      String name = ((TextView) view.findViewById(R.id.name)).getText().toString(); 
      String cost = ((TextView) view.findViewById(R.id.cost)).getText().toString(); 
      String description = ((TextView) view.findViewById(R.id.desciption)).getText().toString(); 

      // Starting new intent 
      Intent in = new Intent(getApplicationContext(), SingleActivity.class); 
      in.putExtra(KEY_NAME, name); 
      in.putExtra(KEY_COST, cost); 
      in.putExtra(KEY_DESC, description); 
      startActivity(in); 

     } 
    }); 
} 

在第38行或行的錯誤我是給（*）這個標誌... 懇求幫助我...

Answer 1

如果空指針就要到了下面行（帶*號）：

(*) NodeList nl = doc.getElementsByTagName(KEY_ITEM); //i reckon the nullpointer is coming up on the doc object... 

然後我想檢查哪個正在從getXMLFromURL方法 檢索的XML是否正確？：

XMLParser parser = new XMLParser(); 
String xml = parser.getXMLFromURL(URL); // getting XML (Is this correctly retreived? 
Document doc = parser.getDomElem(xml); // getting DOM element (This is returning null) 

如需更多幫助，分享您XMLParser的類

Answer 2

1. 你需要使用一個線程或的AsyncTask執行「網絡操作」。否則，你會得到NetworkOnMainThreadException。 read this

2. 爲AndroidManifest.xml添加權限。使用線程

   menuItems = new ArrayList<HashMap<String, String>>(); 
   final XMLParser parser = new XMLParser(); 
   
   Thread th = new Thread(new Runnable() { 
       @Override 
       public void run() { 
        xml = parser.getXmlFromUrl(URL); 
   
        handler.post(new Runnable() { //create an object of Handler class in onCreate() - (android.os.Handler) 
         @Override 
         public void run() { 
          Document doc = parser.getDomElement(xml); 
          nl =doc.getElementsByTagName(KEY_ITEM); 
          //other code inside onCreate() - for loop 
         } 
        }); 
       } 
   } 
   th.start(); 
   
   SimpleAdapter adapter = new SimpleAdapter(this, menuItems, R.layout.list_item, 
         new String[]{KEY_NAME, KEY_COST, KEY_DESCRIPTION}, new int[]{R.id.name, R.id.cost, R.id.description}); 
   
   setListAdapter(adapter);