Question

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...............

Answer 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

Answer 2

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
}
}

Sqlite select语句不起作用

2 个答案: