我想用复选框制作一个简单的列表视图。问题是，每当我点击一个复选框时，我的应用程序会因NullPointerException而崩溃，我真的无法弄清楚问题是什么。
这是我的java代码：
public class ShowTypes extends Activity {
MyCustomAdapter dataAdapter = null;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_show_types);
//Generate list View from ArrayList
displayListView();
Button btn1 = (Button)findViewById(R.id.nextmap);
btn1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(ShowTypes.this, map.class));
}
});
}
private void displayListView() {
//Array list of countries
ArrayList<TypePoi> typeList = new ArrayList<TypePoi>();
TypePoi country = new TypePoi("All of them",false);
typeList.add(country);
country = new TypePoi("tourist attractions",false);
typeList.add(country);
country = new TypePoi("parks",false);
typeList.add(country);
country = new TypePoi("restaurants",false);
typeList.add(country);
country = new TypePoi("hotels",false);
typeList.add(country);
country = new TypePoi("tube stations",false);
typeList.add(country);
//create an ArrayAdaptar from the String Array
dataAdapter = new MyCustomAdapter(this,
R.layout.typepoi_info, typeList);
ListView listView = (ListView) findViewById(R.id.list_types);
// Assign adapter to ListView
listView.setAdapter(dataAdapter);
listView.setOnItemClickListener(new OnItemClickListener() {
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
// When clicked, show a toast with the TextView text
TypePoi country = (TypePoi) parent.getItemAtPosition(position);
// Toast.makeText(getApplicationContext(),
// "Clicked on Row: " + country.getName(),
// Toast.LENGTH_LONG).show();
}
});
}
private class MyCustomAdapter extends ArrayAdapter<TypePoi> {
private ArrayList<TypePoi> typeList;
public MyCustomAdapter(Context context, int textViewResourceId,
ArrayList<TypePoi> typeList) {
super(context, textViewResourceId, typeList);
this.typeList = new ArrayList<TypePoi>();
this.typeList.addAll(typeList);
}
private class ViewHolder {
CheckBox name;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
Log.v("ConvertView", String.valueOf(position));
if (convertView == null) {
LayoutInflater vi = (LayoutInflater)getSystemService(
Context.LAYOUT_INFLATER_SERVICE);
convertView = vi.inflate(R.layout.typepoi_info, null);
holder = new ViewHolder();
holder.name = (CheckBox) convertView.findViewById(R.id.checkBox2);
convertView.setTag(holder);
holder.name.setOnClickListener( new View.OnClickListener() {
public void onClick(View v) {
CheckBox cb = (CheckBox) v ;
TypePoi country = (TypePoi) cb.getTag();
Toast.makeText(getApplicationContext(),
"Clicked on : " + cb.getText(),
Toast.LENGTH_LONG).show();
country.setSelected(cb.isChecked());
}
});
}
else {
holder = (ViewHolder) convertView.getTag();
}
TypePoi country = typeList.get(position);
holder.name.setText(country.getName());
holder.name.setChecked(country.isSelected());
return convertView;
}
}
}
activity_show_types.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
android:background="#FFE5B4">
<TextView android:layout_width="fill_parent"
android:layout_height="wrap_content" android:padding="10dp"
android:layout_gravity="center"
android:text="Pick type:" android:textSize="20sp" />
<Button
android:id="@+id/nextmap"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="#EBD1A0"
android:text="Show map" />
<ListView android:id="@+id/list_types" android:layout_width="fill_parent"
android:layout_height="fill_parent" />
</LinearLayout>
typepoi_info.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:padding="6dip" >
<CheckBox
android:id="@+id/checkBox2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:focusable="false"
android:focusableInTouchMode="false"
android:text="CheckBox" />
</RelativeLayout>
和我的logcat
FATAL EXCEPTION: main
java.lang.NullPointerException
at com.example.diana.track.ShowTypes$MyCustomAdapter$1.onClick(ShowTypes.java:120)
at android.view.View.performClick(View.java:4421)
at android.widget.CompoundButton.performClick(CompoundButton.java:100)
at android.view.View$PerformClick.run(View.java:17903)
at android.os.Handler.handleCallback(Handler.java:730)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5214)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
at dalvik.system.NativeStart.main(Native Method)
知道可能是什么问题吗？
答案 0 :(得分：3)
您可以通过使用简单代码实现此目的，Listview已经具有属性ChoiceMode。您可以将其设置为CHOICE_MODE_MULTIPLE。
以下是来自android示例项目的代码示例 -
完整代码为 -
public class List11 extends ListActivity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setListAdapter(new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_multiple_choice, GENRES));
final ListView listView = getListView();
listView.setItemsCanFocus(false);
listView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
}
private static final String[] GENRES = new String[] {
"Action", "Adventure", "Animation", "Children", "Comedy", "Documentary", "Drama",
"Foreign", "History", "Independent", "Romance", "Sci-Fi", "Television", "Thriller"
};
}
答案 1 :(得分：2)
您忘记了代码中的一个重要行，
...
@Override
public View getView(int position, View convertView, ViewGroup parent) {
...
TypePoi country = typeList.get(position);
holder.name.setText(country.getName());
holder.name.setChecked(country.isSelected());
// Try inserting this line
holder.name.setTag(country);
return convertView;
}
它会引发NullPointerException，因为在onClick中，
holder.name.setOnClickListener( new View.OnClickListener() {
public void onClick(View v) {
CheckBox cb = (CheckBox) v ;
TypePoi country = (TypePoi) cb.getTag();
Toast.makeText(getApplicationContext(),
"Clicked on : " + cb.getText(),
Toast.LENGTH_LONG).show();
country.setSelected(cb.isChecked()); // throws NullPointerException!
}
});
此处的cb.getTag()将导致始终返回null。
因此，country为null，country.setSelected(cb.isChecked())抛出异常。
答案 2 :(得分：1)
在onClick()中进行更改，它会正常工作。
CheckBox cb = (CheckBox) v.findById(R.id.checkBox2);