问题描述

ListAdapter 的空指针异常

我想在List View中显示数据，但是在运行时获得Null Pointer Exception错误。

fList.setAdapter(la);

下面是我用的代码。list 在 Log 中全部打印出来了。

public class Forfattere extends Activity {/** Called when the activity is first created. */ArrayList<String> aList = new ArrayList<String>();@Overridepublic void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.forfattere);    ListView fList = (ListView)findViewById(R.id.Author_list);    ArrayList<String> a = DBlistbookfestival.getAuthorName();    for(int i=0;i<a.size();i++){        aList.add(a.get(i));        Log.v(""all_data""a.get(i));    }    ListAdapter1 la = new ListAdapter1(this R.id.Author_list aList);    fList.setAdapter(la); }private class ListAdapter1 extends ArrayAdapter<String> {  // --CloneChangeRequired    private ArrayList<String> mList;  // --CloneChangeRequired    public ListAdapter1(Context context int textViewResourceIdArrayList<String> list) { // --CloneChangeRequired        super(context textViewResourceId list);        this.mList = list;    }    public View getView(int position View convertView ViewGroup parent){        View view = convertView;        try{        if (view == null) {            LayoutInflater vi = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);            view = vi.inflate(R.layout.custom_radio null);     // --CloneChangeRequired(list_item)        }        final String listItem = mList.get(position);    // --CloneChangeRequired                        if (listItem != null) {            ( (TextView) view.findViewById(R.id.textView111) ).setText(listItem);        }}catch(Exception e){        }        return view;    }}   

如何出现的这个异常啊？

解决方案

使用这个方法：

public class Forfattere extends Activity {/** Called when the activity is first created. */ArrayList<String> aList = new ArrayList<String>();ListAdapter1 la = null;ListView fList = null;@Overridepublic void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.forfattere);    fList = (ListView)findViewById(R.id.Author_list);    ArrayList<String> a = DBlistbookfestival.getAuthorName();    for(int i=0;i<a.size();i++){        aList.add(a.get(i));        Log.v(""all_data""a.get(i));    }    la = new ListAdapter1(thisandroid.R.layout.simple_list_item_1 aList);    fList.setAdapter(la); }private class ListAdapter1 extends ArrayAdapter<String> {  // --CloneChangeRequired    private ArrayList<String> mList;  // --CloneChangeRequired    public ListAdapter1(Context context int textViewResourceIdArrayList<String> list) { // --CloneChangeRequired        super(context textViewResourceId list);        this.mList = list;    }    public View getView(int position View convertView ViewGroup parent){        View view = convertView;        try{        if (view == null) {            LayoutInflater vi = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);            view = vi.inflate(R.layout.custom_radio null);     // --CloneChangeRequired(list_item)        }        final String listItem = mList.get(position);    // --CloneChangeRequired                        if (listItem != null) {            ( (TextView) view.findViewById(R.id.textView111) ).setText(listItem);        }}catch(Exception e){        }        return view;    }}   }

如果没有申明 list item，请使用android 中默认的。

la = new ListAdapter1(thisandroid.R.layout.simple_list_item_1 aList);

解决方案二：
你的问题出自这一行：

ListView fList = (ListView)findViewById(R.id.Author_list);

findviewbyid 返回 null，意思就是ID是 R.id.Author_list 的布局 R.layout.forfattere 里是没有元素的，查看布局来看listview 是否有这个ID.