Question

我正在尝试创建一个自定义列表，它从数据库中获取数据，并且有两个文本视图，但它给出了NPE，我不知道为什么会这样，并且太过头了。请帮忙这是我的代码：

public class NotesList extends ListActivity{
    private DataSource datasource;
     ArrayAdapter<Message> adapter;
     CustomList adap;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.list);

       //  List<Message> values = datasource.getAllMessages();
     //  adapter = new ArrayAdapter<Message>(this, android.R.layout.simple_list_item_1, values);
    //  setListAdapter(adapter);
      adap = new CustomList();

      setListAdapter(adap);



    }
    @Override
    public void onResume() {
        datasource.open();

        super.onResume();
      }


      @Override
    public void onPause() {
        datasource.close();

        super.onPause();

      }
      public class CustomList extends BaseAdapter {

            List<Message> DataList = datasource.getAllMessages();
            @Override
            public int getCount() {
                // TODO Auto-generated method stub
                return DataList.size();
            }

            @Override
            public Message getItem(int arg0) {
                // TODO Auto-generated method stub
                return DataList.get(arg0);
            }

            @Override
            public long getItemId(int arg0) {
                // TODO Auto-generated method stub
                return arg0;
            }

            @Override
            public View getView(int arg0, View arg1, ViewGroup arg2) {

                if(arg1==null)
                {
                    LayoutInflater inflater = (LayoutInflater) NotesList.this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                    arg1 = inflater.inflate(R.layout.listitem, arg2,false);
                }

                TextView heading = (TextView)arg1.findViewById(R.id.textView1);
                TextView notes = (TextView)arg1.findViewById(R.id.textView2);

                Message MessageObj = DataList.get(arg0);

                heading.setText(MessageObj.getmessage());
                notes.setText(MessageObj.getmessage());

                return arg1;
            }

            public Message getCodeLearnChapter(int position)
            {
                return DataList.get(position);
            }

        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
         //   getMenuInflater().inflate(R.menu.list_view_with_simple_adapter, menu);
            return true;
        }

}

Logcat是：

03-19 07:24:00.936: E/AndroidRuntime(28816): FATAL EXCEPTION: main
03-19 07:24:00.936: E/AndroidRuntime(28816): java.lang.RuntimeException: Unable to start activity ComponentInfo{soft.b.notes/soft.b.notes.NotesList}: java.lang.NullPointerException
03-19 07:24:00.936: E/AndroidRuntime(28816):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at android.app.ActivityThread.access$600(ActivityThread.java:122)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at android.os.Looper.loop(Looper.java:137)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at android.app.ActivityThread.main(ActivityThread.java:4340)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at java.lang.reflect.Method.invokeNative(Native Method)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at java.lang.reflect.Method.invoke(Method.java:511)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
03-19 07:24:00.936: E/AndroidRuntime(28816):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)

问候！

Answer 1

您永远不会初始化datasource并调用datasource.getAllMessages()导致NPE

Answer 2

可以在logcat中找到NPE，并使用有用的堆栈跟踪来诊断它。

解释NPE的一种方法是

List<Message> DataList = datasource.getAllMessages();

调用datasource上的方法，当您拨打new CustomList()时，该方法未初始化。

自定义列表中的NPE

2 个答案: