GridView中的异常

Question

我正在尝试一个示例程序来理解动态GridView的行为。当我尝试执行该程序时，我在.java文件中的texts.setText(str[position]);处获得了运行时异常。

的.java

package GridView_dynamic.grid;
import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;
import android.widget.GridView;
import android.widget.TextView;
import android.widget.Toast;

public class GridView_dynamic extends Activity {
    /** Called when the activity is first created. */
 TextView select; 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        select = (TextView)findViewById(R.id.text);
        GridView grid = (GridView)findViewById(R.id.grid);
        grid.setAdapter(new my_Adapter(this));
    }
    public class my_Adapter extends BaseAdapter{
     String[] str = {"A","B","C","D","E","F","G","H","I","J","K","L","M","N"};
     public Context context;
     public my_Adapter(Context context) {
      // TODO Auto-generated constructor stub
      this.context = context;
     }
     @Override
     public int getCount() {
      // TODO Auto-generated method stub
      return str.length;
     }
     @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 0;
     }
     @Override
     public View getView(int position, View convertView, ViewGroup parent) {
      // TODO Auto-generated method stub
      if(convertView == null){
       Toast.makeText(context,"convertView is NULL", Toast.LENGTH_LONG).show();
      }
      TextView texts;
      texts = (TextView)convertView;
      //texts = (TextView)findViewById(R.id.text);
      texts.setText(str[position]);
      return texts;
     }

    }
}

main.xml中

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<TextView 
android:id="@+id/text"
android:layout_width="fill_parent" 
android:layout_height="wrap_content" 
android:background="#0000aa"
android:textColor="#00aa00"
/>
<GridView
android:id="@+id/grid"
android:layout_width="fill_parent" 
android:layout_height="match_parent"
android:background="#aaaaaa"
/>
</LinearLayout>

Answer 1

你得到NullPointerException，因为当你得到一个空convertView（列表是第一次建立的）时，你不会创建一个新的TextView。您的getView()方法应为：

    @Override
         public View getView(int position, View convertView, ViewGroup parent) {
          // TODO Auto-generated method stub
          if(convertView == null){
           Toast.makeText(context,"convertView is NULL", Toast.LENGTH_LONG).show();
           //if convertView is null is your job to make a new View 
               convertView = new TextView(context);
          }
          ((TextView)convertView).setText(str[position]);
          return convertView;         

}

Answer 2

从外观上看，你实际上并没有在getView中实例化TextView。如果convertView为null，则以下代码将创建一个新的textView。

public View getView(int position, View convertView, ViewGroup parent) {
    if(convertView == null)
        convertView = new TextView(GridView_dynamic.this);

    ((TextView)convertView).setText(str[position]);
    return convertView;
}

希望这有帮助！