标签:database android sqlite cursor
我被卡住了.我无法找出我做错了什么,所以请帮助任何人.
虽然这是一项非常简单的任务,但我不明白数据库的实际情况.
这是我用来将数据插入数据库中的表的代码:
EditText title = (EditText)findViewById(R.id.Title);
String Title = title.getText().toString();
EditText d = (EditText)findViewById(R.id.director);
String Director = d.getText().toString();
SQLiteDatabase db = openOrCreateDatabase("Films", MODE_PRIVATE, null);
db.execSQL("CREATE TABLE IF NOT EXISTS MyFilms (idFilm INTEGER PRIMARY KEY AUTOINCREMENT, Title VARCHAR, Director VARCHAR, Year INTEGER, Rating INTEGER);");
db.execSQL("INSERT INTO MyFilms (Title) VALUES('" + Title + "');");
db.execSQL("INSERT INTO MyFilms (Director) VALUES('" + Director + "');");
db.close();
这段代码是否为我提供了一个包含idfilm,Title,Director,Year,Rating的列表?
如果是这样,当我尝试使用以下代码从Title列检索所有标题时仍然存在问题:
SQLiteDatabase db = openOrCreateDatabase("Films", MODE_PRIVATE, null);
Cursor c = db.rawQuery("SELECT * FROM MyFilms",null);
c.moveToFirst();
while (c.isAfterLast() == false) {
String Title = c.getString(c.getColumnIndex("Director"));
stringList.add(Title); //This I use to create listlayout dynamically and show all the Titles in it
c.moveToNext();
}
错误日志:
05-21 17:50:14.666: E/AndroidRuntime(27045): FATAL EXCEPTION: main
05-21 17:50:14.666: E/AndroidRuntime(27045): java.lang.NullPointerException
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:355)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.ArrayAdapter.getView(ArrayAdapter.java:323)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.AbsListView.obtainView(AbsListView.java:1409)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.ListView.measureHeightOfChildren(ListView.java:1216)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.ListView.onMeasure(ListView.java:1127)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.View.measure(View.java:8313)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.RelativeLayout.measureChild(RelativeLayout.java:566)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:381)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.View.measure(View.java:8313)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3138)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.FrameLayout.onMeasure(FrameLayout.java:250)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.View.measure(View.java:8313)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.LinearLayout.measureVertical(LinearLayout.java:531)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.LinearLayout.onMeasure(LinearLayout.java:309)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.View.measure(View.java:8313)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3138)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.widget.FrameLayout.onMeasure(FrameLayout.java:250)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.View.measure(View.java:8313)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.ViewRoot.performTraversals(ViewRoot.java:839)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.view.ViewRoot.handleMessage(ViewRoot.java:1859)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.os.Handler.dispatchMessage(Handler.java:99)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.os.Looper.loop(Looper.java:123)
05-21 17:50:14.666: E/AndroidRuntime(27045): at android.app.ActivityThread.main(ActivityThread.java:3683)
05-21 17:50:14.666: E/AndroidRuntime(27045): at java.lang.reflect.Method.invokeNative(Native Method)
05-21 17:50:14.666: E/AndroidRuntime(27045): at java.lang.reflect.Method.invoke(Method.java:507)
05-21 17:50:14.666: E/AndroidRuntime(27045): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
05-21 17:50:14.666: E/AndroidRuntime(27045): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
05-21 17:50:14.666: E/AndroidRuntime(27045): at dalvik.system.NativeStart.main(Native Method)
为什么DDMS会抛出NullPointerException?我该如何从“标题”获取数据?我试图使用c.getString(),但是当我要求c.getString(1)时,仍然有NullPointerException但是当我设置c.getString(2)时它返回“Director”.请帮忙!
解决方法:
填充数据库(第一个代码段)时会出现问题.
您需要组合INSERT语句:
db.execSQL("INSERT INTO MyFilms (Title, Director, Year) VALUES('"
+ title + "','" + director + "'," + year + ");");
否则,您最终会得到包含Title或Director空值的行.
(注意:不要使用上面的SQL表示法,因为它可能使您容易受到SQL注入.我只是为了简单起见而这样写.在生产代码中,您应该使用参数或清理您的值.不要成为Little Bobby Tables的受害者).
编辑
以下是有关准备语句的一些主题:
> How do I use prepared statements in SQlite in Android?
> Queries with prepared statements in Android?
标签:database,android,sqlite,cursor 来源: https://codeday.me/bug/20190714/1461632.html
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