我正在尝试将数据保存到SQLite数据库。
AddBook.java
public class AddBook extends Activity {
String myUrl = "";
DownloadBook obj_Downloadbook;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
obj_Downloadbook = new DownloadBook(AddBook.this);//line26
...
obj_Downloadbook.execute(myUrl);
此活动正在调用 DownloadBook.java 类,该类扩展了AsyncTask。在DownloadBook.java类中,我正在检索html作为字符串并保存到文件。保存后,我正在尝试将标题和作者记录到数据库中
public class DownloadBook extends AsyncTask<String, Void, String> {
private Context context;
public DownloadBook(Context context) {
this.context = context;
}
MyDatabase myDatabase=new MyDatabase(context);//line27
...
myDatabase.addBookInformation(title, author);
为了管理SQLite数据库,我是using android-sqlite-asset-helper库。在库的github中,它写成:SQLiteAssetHelper旨在作为框架的SQLiteOpenHelper的替代品。的 MyDatabase.java
public class MyDatabase extends SQLiteAssetHelper {
MyDatabase(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);//line21
setForcedUpgradeVersion(3);
}
...
public void addBookInformation(String title, String author){
...
这会产生以下错误:
03-24 03:13:37.248: E/AndroidRuntime(21237): Caused by:
java.lang.NullPointerException 03-24 03:13:37.248:
E/AndroidRuntime(21237): at
com.readystatesoftware.sqliteasset.SQLiteAssetHelper.<init>(SQLiteAssetHelper.java:110)
03-24 03:13:37.248: E/AndroidRuntime(21237): at
com.readystatesoftware.sqliteasset.SQLiteAssetHelper.<init>(SQLiteAssetHelper.java:130)
03-24 03:13:37.248: E/AndroidRuntime(21237): at
com.joericharduz.akita.MyDatabase.<init>(MyDatabase.java:21) 03-24
03:13:37.248: E/AndroidRuntime(21237): at
com.joericharduz.akita.DownloadBook.<init>(DownloadBook.java:27) 03-24
03:13:37.248: E/AndroidRuntime(21237): at
com.joericharduz.akita.AddBook.onCreate(AddBook.java:26)
如何解决这个问题?
答案 0 :(得分:2)
在构造函数运行之前初始化成员变量。所以在这里,
public class DownloadBook extends AsyncTask<String, Void, String> {
private Context context;
public DownloadBook(Context context) {
this.context = context;
}
MyDatabase myDatabase=new MyDatabase(context);//line27
在构造函数中初始化myDatabase
之前, ... null
初始化为context
context
。
将new MyDatabase(context)
initiazlization移动到构造函数。