java.lang.NullPointerException：在可扩展列表视图中显示数据库中的记录

正如问题所示，我需要显示数据库中的记录，每个孩子的记录不同。即， 子1=记录1个数据。 child 2 =记录2数据.so等等。 我想出了以下显示代码：

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();
    } 
}   

尽管我的数据库有数据，但使用命令提示符 adb shell 确认，这会以文本形式显示“无数据”。 所以我假设它无法访问 onClick 方法。 现在，如果我取消注释 onCreate 方法中注释的行，它将获得访问权限，但会导致 NullPointerException viewbutton.setOnClickListener(new Buttonx());//onCreate方法

如何在不生成此异常的情况下访问 onClick 并根据需要显示记录？ 我想我在某个地方犯了根本性的错误。如果有人能指出这个缺陷，我将不胜感激。你看，这件事让我发疯。：/

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

应该像下面这样。另请发布堆栈跟踪，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
       }   

});
}

Edit:

<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
            }

        });
    }
}