我正在创建导航抽屉列表视图，根据列表视图的位置在特定文本视图中进行更改，但总是多次出现该异常并且我不知道如何解决它
MainActivity Class
public class DBHelper extends SQLiteOpenHelper {
private static String DB_NAME = "duaa.sqlite";
private SQLiteDatabase db;
private final Context context;
private String DB_PATH;
public DBHelper(Context context) {
super(context, DB_NAME, null, 1);
this.context = context;
DB_PATH = "/data/data/" + context.getPackageName() + "/" + "databases/";
}
public void createDataBase() throws IOException {
boolean dbExist = checkDataBase();
if (dbExist) {
} else {
this.getReadableDatabase();
try {
copyDataBase();
} catch (IOException e) {
throw new Error("Error copying database");
}
}
}
private boolean checkDataBase() {
File dbFile = new File(DB_PATH + DB_NAME);
return dbFile.exists();
}
private void copyDataBase() throws IOException {
InputStream myInput = context.getAssets().open(DB_NAME);
String outFileName = DB_PATH + DB_NAME;
OutputStream myOutput = new FileOutputStream(outFileName);
byte[] buffer = new byte[1024];
int length;
while ((length = myInput.read(buffer)) > 0) {
myOutput.write(buffer, 0, length);
}
// Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
public Cursor getData() {
String myPath = DB_PATH + DB_NAME;
db = SQLiteDatabase.openDatabase(myPath, null,
SQLiteDatabase.OPEN_READONLY);
Cursor c = db.query("main", null, null, null, null,null,null);
// Note: Master is the one table in External db. Here we trying to access the records of table from external db.
return c;
}
@Override
public void onCreate(SQLiteDatabase arg0) {
// TODO Auto-generated method stub
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
}
}
扩展SQLiteOpenHelper的DbHelper类
public class MainActivity extends ActionBarActivity {
ListView lv;
DrawerList adapter;
DrawerLayout drawer;
String[] duaaList;
ActionBarDrawerToggle drawerToggle;
TextView tv;
ArrayList<String>data;
DBHelper helper;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
duaaList = getResources().getStringArray(R.array.duaa);
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
tv = (TextView) findViewById(R.id.tvDesc);
lv = (ListView) findViewById(R.id.left_drawer);
adapter = new DrawerList();
lv.setAdapter(adapter);
drawerToggle = new ActionBarDrawerToggle(this,drawer,0,0);
lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
selectItem(position);
changeText(position);
}
public void selectItem(int position){
lv.setItemChecked(position, true);
setTitle(duaaList[position]);
}
public void setTitle(String title){
getSupportActionBar().setTitle(title);
}
//set data into a textview according to the position of the list
public void changeText(int position){
try{
lv.setItemChecked(position, true);
lv.setSelection(position);
drawer.closeDrawer(lv);
tv.setText(data.get(position));
}catch(Exception e){
e.printStackTrace();
}
}
});
}
public void changeTextView(){
helper = new DBHelper(getBaseContext());
try {
helper.createDataBase();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Cursor c = helper.getData();
if(c.moveToFirst() && c != null)
{
do {
data.add(c.getString(c.getColumnIndex("desc")));
} while (c.moveToNext());
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_settings) {
finish();
}
return super.onOptionsItemSelected(item);
}
class DrawerList extends BaseAdapter{
@Override
public int getCount() {
return duaaList.length;
}
@Override
public Object getItem(int position) {
return duaaList[position];
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View v = convertView;
if (v == null){
v = getLayoutInflater().inflate(R.layout.duaa_list_item , parent , false);
}
TextView tv = (TextView) v.findViewById(R.id.tv2);
tv.setText(duaaList[position]);
return v;
}
}
}
这里是xml文件
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- The main content view -->
<FrameLayout
android:id="@+id/content_frame"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<ScrollView
android:layout_width="match_parent"
android:layout_height="480dp" >
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="Medium Text"
android:id="@+id/tvDesc"
android:layout_gravity="center"
android:paddingTop="10dp"
/>
</LinearLayout>
</ScrollView>
</FrameLayout>
<!-- The navigation drawer -->
<ListView android:id="@+id/left_drawer"
android:layout_width="290dp"
android:layout_height="match_parent"
android:layout_gravity="start"
android:choiceMode="singleChoice"
android:splitMotionEvents="true"
android:soundEffectsEnabled="true"
android:divider="@android:color/darker_gray"
android:dividerHeight="1dp"
android:background="#ffbababa"/>
</android.support.v4.widget.DrawerLayout>
以下是例外：
04-13 14：26：00.573：W / System.err（18010）： java.lang.NullPointerException 04-13 14：26：00.573： W / System.err（18010）：at com.anabil.duaaapp.MainActivity $ 1.changeText（MainActivity.java:98） 04-13 14：26：00.573：W / System.err（18010）：at com.anabil.duaaapp.MainActivity $ 1.onItemClick（MainActivity.java:78） 04-13 14：26：00.573：W / System.err（18010）：at android.widget.AdapterView.performItemClick（AdapterView.java:299） 04-13 14：26：00.573：W / System.err（18010）：at android.widget.AbsListView.performItemClick（AbsListView.java:1113） 04-13 14：26：00.573：W / System.err（18010）：at android.widget.AbsListView $ PerformClick.run（AbsListView.java:2911） 04-13 14：26：00.573：W / System.err（18010）：at android.widget.AbsListView $ 3.run（AbsListView.java:3645)04-13 14：26：00.573：W / System.err（18010）：at android.os.Handler.handleCallback（Handler.java:733)04-13 14：26：00.573：W / System.err（18010）：at android.os.Handler.dispatchMessage（Handler.java:95）04-13 14：26：00.573：W / System.err（18010）：at android.os.Looper.loop（Looper.java:136)04-13 14：26：00.573： W / System.err（18010）：at android.app.ActivityThread.main（ActivityThread.java:5001)04-13 14：26：00.573：W / System.err（18010）：at java.lang.reflect.Method.invokeNative（Native Method）04-13 14：26：00.573：W / System.err（18010）：at java.lang.reflect.Method.invoke（Method.java:515)04-13 14：26：00.573： W / System.err（18010）：at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:785） 04-13 14：26：00.573：W / System.err（18010）：at com.android.internal.os.ZygoteInit.main（ZygoteInit.java:601)04-13 14：26：00.573：W / System.err（18010）：at dalvik.system.NativeStart.main（原生方法）
答案 0 :(得分：0)
我看不到您初始化ArrayList<String> data的位置，我只看到您尝试在importDB()中添加一些内容。我怀疑你能够看到这个日志：Log.d("TAG" , "the database has been successfully readed :)" );。导致NullPointerException的行很可能就是这一行：tv.setText(data.get(position));
无论如何，几个笔记：
readOrCopyFile()调用onCreate()）。这是一个非常糟糕的主意，因为您的应用程序可能会在很长一段时间内无法响应。至少将其提取到AsyncTask。 NullPointerException发生在changeText()。假设您已经在某处初始化了data数组，为了避免出现进一步的问题，您应该在调用size()之前完全检查它data.get(position)。