nullpointerexception:在可扩展的列表视图中显示数据库中的记录

Question

正如问题所指示的，我需要显示数据库中的记录，每个孩子的不同记录。子1=记录1个数据。子2 =记录2个数据。某某。我编写了下面的显示代码:

package com.example.moolah;
import android.app.ListActivity;
import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v4.widget.SimpleCursorAdapter;
import android.view.View;
import android.widget.Button;
public class DisplayActivity extends ListActivity {
DBAdapter db=new DBAdapter(this);

@Override
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_display_cursor); 
    Button viewbutton=(Button)findViewById(R.id.button2);
 //    viewbutton.setOnClickListener(new Buttonx());

}
public class Buttonx extends DisplayActivity implements Button.OnClickListener{
 @SuppressWarnings("deprecation")
  @Override
    public void onClick(View v)
    { 

Button viewbutton=(Button)v.findViewById(R.id.button2);
        Object viewid=viewbutton.getTag();
            int i=Integer.parseInt(viewid.toString());          
MyCustomAdapter c=new MyCustomAdapter();
            long id=c.getChildId(0,i);
        //DisplayCursor d;
            //d.filldata();                
         db.open();
           // View view;
           Cursor cursor = db.getRecord(id);
           startManagingCursor(cursor);
            String[] columns = new String[] {DBAdapter.invest_type,DBAdapter.curr_per_share_price, DBAdapter.share_name,
                    DBAdapter.no_of_shares,DBAdapter.share_identity,DBAdapter.purchase_price,
                    DBAdapter.purchase_from,DBAdapter.purchase_date,DBAdapter.purchase_contact};
            int[] to = new int[] { R.id.investmenttype,R.id.currpershareprice,R.id.sharename,R.id.shareno,R.id.shareid, R.id.purprice,
                    R.id.purfrom,R.id.purdate,R.id.purcon};
            SimpleCursorAdapter mAdapter = new SimpleCursorAdapter( this, R.layout.row, cursor, columns, to);
            this.setListAdapter(mAdapter);

           db.close();
    } 
}   

在文本中显示"no data"，尽管我的数据库有数据，正如使用命令提示符adb shell确认的那样。所以我假设它没有访问onClick方法。现在，如果我取消onCreate方法中注释的行，它可以访问，但会导致NullPointerExceptionviewbutton。setOnClickListener(new Buttonx());//onCreate方法

我如何获得访问onClick而不产生此异常，并显示所需的记录?我觉得我从根本上就错了。如果有人能指出这个缺陷，我会很感激。你看，这事快把我逼疯了。:/

LOGCAT:

     06-17 17:33:32.444: E/AndroidRuntime(569):
     FATAL EXCEPTION: main
     06-17 17:33:32.444: E/AndroidRuntime(569): java.lang.RuntimeException: Unable to start activity      ComponentInfo{com.example.moolah/com.example.moolah.DisplayActivity}: java.lang.NullPointerException
   06-17 17:33:32.444: E/AndroidRuntime(569):   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at android.app.ActivityThread.access$600(ActivityThread.java:122)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at android.os.Handler.dispatchMessage(Handler.java:99)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at android.os.Looper.loop(Looper.java:137)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at android.app.ActivityThread.main(ActivityThread.java:4340)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at java.lang.reflect.Method.invokeNative(Native Method)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at java.lang.reflect.Method.invoke(Method.java:511)
   06-17 17:33:32.444: E/AndroidRuntime(569):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
    06-17 17:33:32.444: E/AndroidRuntime(569):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
    06-17 17:33:32.444: E/AndroidRuntime(569):  at dalvik.system.NativeStart.main(Native Method)        
    06-17 17:33:32.444: E/AndroidRuntime(569): Caused by: java.lang.NullPointerException
    06-17 17:33:32.444: E/AndroidRuntime(569):  at com.example.moolah.DisplayActivity.onCreate(DisplayActivity.java:21)
    06-17 17:33:32.444: E/AndroidRuntime(569):  at android.app.Activity.performCreate(Activity.java:4465)
    06-17 17:33:32.444: E/AndroidRuntime(569):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
    06-17 17:33:32.444: E/AndroidRuntime(569):  at             android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919)
    06-17 17:33:32.444: E/AndroidRuntime(569):  ... 11 more

Answer 1

应该如下所示:还可以发布堆栈跟踪，以便我们可以进一步帮助您

public class DisplayActivity extends ListActivity {
DBAdapter db;
@Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_display_cursor); 
 db=new DBAdapter(this);  
Button viewbutton=(Button)findViewById(R.id.button2);
viewbutton.setOnClickListener(new OnClickListener()
{
       @override
       public void onClick(View v)
       {
             // do something
       }   
});
}

编辑:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >
  <ListView 
android:layout_height="wrap_content" 
android:layout_above="@+id/button1"
android:layout_width="fill_parent" 
android:id="@+id/lv" />
  <Button
      android:id="@+id/button1"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_alignParentBottom="true"
      android:layout_centerHorizontal="true"
      android:text="Button" />
</RelativeLayout>

MainActivity.java

public class MainActivity extends Activity {
    ListView lv;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button b = (Button) findViewById(R.id.button1);
        lv= (ListView) findViewById(R.id.lv);
        b.setOnClickListener(new OnClickListener()
        {
            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                // set your adapter to listview here
            }
        });
    }
}