我正在创建一个示例应用程序，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;
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：
<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>
logcat的：
10-16 17:16:28.938: D/AndroidRuntime(413): Shutting down VM
10-16 17:16:28.938: W/dalvikvm(413): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
10-16 17:16:28.958: E/AndroidRuntime(413): FATAL EXCEPTION: main
10-16 17:16:28.958: E/AndroidRuntime(413): java.lang.NullPointerException
10-16 17:16:28.958: E/AndroidRuntime(413): at com.cyberswift.database.example.MainActivity$2.onClick(MainActivity.java:97)
10-16 17:16:28.958: E/AndroidRuntime(413): at android.view.View.performClick(View.java:2408)
10-16 17:16:28.958: E/AndroidRuntime(413): at android.view.View$PerformClick.run(View.java:8816)
10-16 17:16:28.958: E/AndroidRuntime(413): at android.os.Handler.handleCallback(Handler.java:587)
10-16 17:16:28.958: E/AndroidRuntime(413): at android.os.Handler.dispatchMessage(Handler.java:92)
10-16 17:16:28.958: E/AndroidRuntime(413): at android.os.Looper.loop(Looper.java:123)
10-16 17:16:28.958: E/AndroidRuntime(413): at android.app.ActivityThread.main(ActivityThread.java:4627)
10-16 17:16:28.958: E/AndroidRuntime(413): at java.lang.reflect.Method.invokeNative(Native Method)
10-16 17:16:28.958: E/AndroidRuntime(413): at java.lang.reflect.Method.invoke(Method.java:521)
10-16 17:16:28.958: E/AndroidRuntime(413): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
10-16 17:16:28.958: E/AndroidRuntime(413): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
10-16 17:16:28.958: E/AndroidRuntime(413): at dalvik.system.NativeStart.main(Native Method)
10-16 17:16:30.998: I/Process(413): Sending signal. PID: 413 SIG: 9
我到底错在哪里...............
答案 0 :(得分：1)
您没有在查询中选择name列，因此在循环游标（String firstName = c.getString(c.getColumnIndex("name"));）时，找不到它。
此外，您应该在SQL查询中指定您计算或计算所有列的列：
db.rawQuery("SELECT name, COUNT(*) AS "+ "\""+ "count"+ "\""+ " FROM sqlite_master WHERE name ='contacts'",null);
然后您可以使用String firstName = c.getString(c.getColumnIndex("name"));，count将拥有所需的值
答案 1 :(得分：0)
正如Mohamed所说，你没有在查询中选择名称列。
据我所知，在Android中要计算行数，你应该选择你想要的任何列，然后将cursor.size（）作为计数。（最佳实践）
如果要从光标获取数据，首先应验证cursor.size（）＆gt;在你的0 if（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
}
}