Android Sqlite select语句不工作
我正在制作一个示例应用程序,其中我为sqlite创建了一个数据库,我还在表中插入了值。但是当我使用select查询时,我得到了nullpointer异常 这是我的班级:Android Sqlite select语句不工作,android,database,sqlite,Android,Database,Sqlite,我正在制作一个示例应用程序,其中我为sqlite创建了一个数据库,我还在表中插入了值。但是当我使用select查询时,我得到了nullpointer异常 这是我的班级: public class MainActivity extends Activity { Button insert; Button show; EditText name; EditText mobile; SQLiteDatabase db; String name_str;
public class MainActivity extends Activity {
Button insert;
Button show;
EditText name;
EditText mobile;
SQLiteDatabase db;
String name_str;
String mob_str;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
name = (EditText)findViewById(R.id.name_editText);
insert = (Button) findViewById(R.id.insert_button);
insert.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
//Toast.makeText(getApplicationContext(), "Onclick works", Toast.LENGTH_LONG).show();
name_str=name.getText().toString();
Toast.makeText(getApplicationContext(), name_str, Toast.LENGTH_LONG).show();
// create or open database file
db = openOrCreateDatabase("Test.db" , SQLiteDatabase.CREATE_IF_NECESSARY,
null);
db.setVersion(1);
// db.setLocale(Locale.getDefault());
db.setLockingEnabled(true);
//System.out.println("Database Created");
// creating table in database
db.execSQL("CREATE TABLE IF NOT EXISTS "+"contacts"+" " +
"( id INTEGER PRIMARY KEY AUTOINCREMENT," +
" name varchar, " +
" mobile varchar ); ");
db.execSQL("INSERT INTO contacts(name, mobile)VALUES ('Swift','9830098300');");
db.execSQL("drop table contacts");
//Database Exists check
File database=getApplicationContext().getDatabasePath("Test.db");
if (!database.exists()) {
// Database does not exist so copy it from assets here
Log.i("Database", "Not Found");
} else {
Log.i("Database", "Found");
}
}
});
show = (Button) findViewById(R.id.show_button);
show.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// //Toast.makeText(getApplicationContext(), "Onclick works", Toast.LENGTH_LONG).show();
Cursor c =db.rawQuery("SELECT COUNT() FROM sqlite_master WHERE name ='contacts';",null);
if (c != null ) {
if (c.moveToFirst()) {
do {
String firstName = c.getString(c.getColumnIndex("name"));
}while (c.moveToNext());
}
}
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
这是main.xml文件:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<TextView
android:id="@+id/name_textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginTop="60dp"
android:text="Name:"
android:textAppearance="?android:attr/textAppearanceMedium" />
<EditText
android:id="@+id/name_editText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/name_textView"
android:layout_alignBottom="@+id/name_textView"
android:layout_marginLeft="14dp"
android:layout_toRightOf="@+id/name_textView"
android:ems="10" >
<requestFocus />
</EditText>
<TextView
android:id="@+id/mobile_textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_below="@+id/name_editText"
android:layout_marginTop="53dp"
android:text="Mobile: "
android:textAppearance="?android:attr/textAppearanceMedium" />
<EditText
android:id="@+id/mobile_editText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/mobile_textView"
android:layout_alignBottom="@+id/mobile_textView"
android:layout_alignLeft="@+id/name_editText"
android:ems="10"
android:inputType="phone" />
<Button
android:id="@+id/insert_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/mobile_editText"
android:layout_centerVertical="true"
android:text="Insert" />
<Button
android:id="@+id/show_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/insert_button"
android:layout_alignBottom="@+id/insert_button"
android:layout_alignRight="@+id/mobile_editText"
android:text="Show" />
</RelativeLayout>
我到底哪里出错了………..您没有在查询中选择
name
列,因此当循环光标(String firstName=c.getString(c.getColumnIndex(“name”);
)时,它找不到它
此外,还应在SQL查询中指定要计数或全部计数的列:
db.rawQuery("SELECT name, COUNT(*) AS "+ "\""+ "count"+ "\""+ " FROM sqlite_master WHERE name ='contacts'",null);
然后可以使用
stringfirstname=c.getString(c.getColumnIndex(“name”)
和count
将具有所需的值,因为穆罕默德说您没有在查询中选择name列
正如我所知,在Android中,要计算行数,您应该选择所需的任何列,然后获取cursor.size()作为计数。(最佳实践)
如果要从游标中获取数据,首先应验证
如果(c!=null)。从数据库获取数据的示例:
if (cursor != null && cursor.size() > 0)
{
// get count
int count = cursor.size();
// to get data
cursor.moveToFirst();
while(!cursor.isAfterLast())
{
// do get data from cursor
}
}
它用于计算数据发生率ejava.lang.NullPointerException某些内容为NULL!!是的,我知道,但我不能确定是什么。如果可以的话,请告诉我。代码中的第97行是什么?检查光标值的If条件是否仍然存在?我仍然得到相同的例外情况第97行是什么(MainActivity.java:97)?我无法理解。我不确定它是从哪里来的。在Eclipse中,从首选项中,启用ShowLineNumber。哪一行是MainActivity类中的第97行(您甚至可以计算Eclipse中的行:)
if (cursor != null && cursor.size() > 0)
{
// get count
int count = cursor.size();
// to get data
cursor.moveToFirst();
while(!cursor.isAfterLast())
{
// do get data from cursor
}
}