列表视图不显示

我用自定义的listview制作项目。当我运行项目时，我只是得到一个空白的活动。我编写了BaseAdapter类，以将数据放入列表视图中。我将一个String ArrayList传递给baseadapter来填充listview。有什么帮助吗？谢谢

  public class MainActivity extends Activity {

  ListView L;
 ArrayList<String> data= new ArrayList<String>();
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    data.add("two");
    data.add("Three");
    data.add("Four");
    L=(ListView) findViewById(R.id.listView1);

    MyAdapter Ada = new MyAdapter(MainActivity.this,data);
    L.setAdapter(Ada);


}

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

}

公共类MyAdapter扩展BaseAdapter {

Context context;
private ArrayList<String> data=  new ArrayList<String>();

 public MyAdapter(Context context ,ArrayList<String> data )
 {
     this.context=context;
     this.data=data;
 }


@Override
public int getCount() {
    // TODO Auto-generated method stub
    return 0;
}

@Override
public Object getItem(int position) {
    // TODO Auto-generated method stub
    return null;
}

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

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    // TODO Auto-generated method stub
    ViewHolder holder; 
    convertView = null;
    if (convertView == null) {
        convertView = LayoutInflater.from(context)
          .inflate(R.layout.listlayout, parent, false);

        holder = new ViewHolder(); 

        holder.mainpic = (ImageView)convertView.findViewById(R.id.imageView1);
        holder.text= (TextView)convertView.findViewById(R.id.textView1);
        convertView.setTag(holder); 
    } 
    else
    {
        holder = (ViewHolder) convertView.getTag();

    }

    holder.text.setText(this.data.get(position));


    return convertView;
}







static class ViewHolder {            

    ImageView mainpic; 
    TextView text;
} 

}

    <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:id="@+id/listView1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true" >

 </ListView>

</RelativeLayout>