尝试从SQLite数据库获取数据时发生NullPointerException / CursorOutOfBoundsException

Question

[编辑] logcat已添加

按照标题。 当我尝试从数据库中恢复数据并且没有任何内容时，我曾预料会发生这种情况，因此我尝试避免这种情况。

目前，我有一个User类，它带有三个参数（String，String，int）。 在扩展SQLiteOpenHelper的类中，我还具有以下方法：

public User retrieveUserInfo() {
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_USER, null);


        String name = null;
        String surname = null;
        String age = null;

        if (cursor == null) {
            Log.d("NULL", "CURSOR IS NULL");
            user = new User("Please connect to server", " ", 0);

        }

        if (cursor.moveToFirst()) { 

            Log.d("NOT NULL", "CURSOR IS NOT NULL");

            do {
                name = cursor.getString(cursor.getColumnIndex(COLUMN_NAME));
                surname = cursor.getString(cursor
                        .getColumnIndex(COLUMN_SURNAME));
                age = cursor.getString(cursor.getColumnIndex(COLUMN_AGE));
            } while (cursor.moveToNext());
            Log.d("NAME", name);
            int age_p = Integer.parseInt(age);
            user = new User(name, surname, age_p);

        }



        return user;
    }

然后，我从另一个活动中调用此方法：

DatabaseTask dt = new DatabaseTask(this);
        User user = dt.retrieveUserInfo();
        String name = user.getName();
        String surname = user.getSurname();

        TextView textName = (TextView) findViewById(R.id.textView1);
        TextView textSurname = (TextView) findViewById(R.id.textView2);

        textName.setText(name);
        textSurname.setText(surname);

因此，当前我正在尝试通过在游标为null时向后发送带有一些占位符字段的User实例来避免崩溃（此方法中的“ user”字段是User类型的公共静态成员变量，该变量在该类的前面定义） ），并在游标不为null时返回存储在数据库中的普通数据。 不过，它仍然不起作用：我卸载应用程序并重新安装后，该应用程序便崩溃了。 现在，如果我在数据库中插入一些东西，然后重新安装该应用程序而没有先将其删除（我想更新它并保留旧数据库），则该应用程序不会崩溃。

关于为什么我的游标检查是否为null的任何想法都行不通？

[编辑] logcat如下：

> 03-17 02:48:35.036: E/AndroidRuntime(1225): FATAL EXCEPTION: main
> 03-17 02:48:35.036: E/AndroidRuntime(1225): Process:
> com.example.jsonbourne, PID: 1225 03-17 02:48:35.036:
> E/AndroidRuntime(1225): java.lang.RuntimeException: Unable to start
> activity
> ComponentInfo{com.example.jsonbourne/com.example.jsonbourne.MainActivity}:
> java.lang.NullPointerException 03-17 02:48:35.036:
> E/AndroidRuntime(1225):   at
> android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
> 03-17 02:48:35.036: E/AndroidRuntime(1225):   at
> android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
> 03-17 02:48:35.036: E/AndroidRuntime(1225):   at
> android.app.ActivityThread.access$800(ActivityThread.java:135) 03-17
> 02:48:35.036: E/AndroidRuntime(1225):     at
> android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
> 03-17 02:48:35.036: E/AndroidRuntime(1225):   at
> android.os.Handler.dispatchMessage(Handler.java:102) 03-17
> 02:48:35.036: E/AndroidRuntime(1225):     at
> android.os.Looper.loop(Looper.java:136) 03-17 02:48:35.036:
> E/AndroidRuntime(1225):   at
> android.app.ActivityThread.main(ActivityThread.java:5017) 03-17
> 02:48:35.036: E/AndroidRuntime(1225):     at
> java.lang.reflect.Method.invokeNative(Native Method) 03-17
> 02:48:35.036: E/AndroidRuntime(1225):     at
> java.lang.reflect.Method.invoke(Method.java:515) 03-17 02:48:35.036:
> E/AndroidRuntime(1225):   at
> com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
> 03-17 02:48:35.036: E/AndroidRuntime(1225):   at
> com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 03-17
> 02:48:35.036: E/AndroidRuntime(1225):     at
> dalvik.system.NativeStart.main(Native Method) 03-17 02:48:35.036:
> E/AndroidRuntime(1225): Caused by: java.lang.NullPointerException
> 03-17 02:48:35.036: E/AndroidRuntime(1225):   at
> com.example.jsonbourne.MainActivity.onCreate(MainActivity.java:31)
> 03-17 02:48:35.036: E/AndroidRuntime(1225):   at
> android.app.Activity.performCreate(Activity.java:5231) 03-17
> 02:48:35.036: E/AndroidRuntime(1225):     at
> android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
> 03-17 02:48:35.036: E/AndroidRuntime(1225):   at
> android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
> 03-17 02:48:35.036: E/AndroidRuntime(1225):   ... 11 more

[edit 2] MainActivity和activity_mail.xml如下：

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //GetJson gj = new GetJson(this);
        //gj.getData();

        DatabaseTask dt = new DatabaseTask(this);

        User user = dt.retrieveUserInfo(); // **<------- LINE 31**



        String name = user.getName();
        String surname = user.getSurname();

        TextView textName = (TextView) findViewById(R.id.textView1);
        TextView textSurname = (TextView) findViewById(R.id.textView2);

        textName.setText(name);
        textSurname.setText(surname); 

        Button buttonconnect = (Button) findViewById(R.id.button_connect);

activity_mail.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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/button_connect"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="188dp"
        android:background="#F0FF00"
        android:text="@string/pressconnect" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/button_connect"
        android:layout_alignParentTop="true"
        android:layout_marginTop="100dp"
        android:text="Medium Text"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/textView1"
        android:layout_alignRight="@+id/button_connect"
        android:layout_marginRight="17dp"
        android:text="Medium Text"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView1"
        android:layout_alignParentTop="true"
        android:layout_marginTop="22dp"
        android:text="Welcome,"
        android:textAppearance="?android:attr/textAppearanceLarge" />

</RelativeLayout>

Answer 1

尝试：

    if (cursor == null) {
        Log.d("NULL", "CURSOR IS NULL");
        user = new User("Please connect to server", " ", 0);

    } else if (cursor.moveToFirst()) { 
        Log.d("NOT NULL", "CURSOR IS NOT NULL");
        //...
    }

代替：

    if (cursor == null) {
        Log.d("NULL", "CURSOR IS NULL");
        user = new User("Please connect to server", " ", 0);

    }

    if (cursor.moveToFirst()) { 

好像您忘记了else语句

Answer 2

有问题的代码

DatabaseTask dt = new DatabaseTask(this);

User user = dt.retrieveUserInfo(); // **<------- LINE 31**

String name = user.getName();

不能真正产生stacktrace，因此我们假设NPE确实发生在下一行， getName()对null user调用getName() 。 可以通过修复retrieveUserInfo()来解决此问题，例如：

Cursor cursor = ...;

if (cursor.moveToFirst()) {
  // create user from cursor data
} else {
  // create default user
}
return user;

cursor null检查不是必需的，如注释中所述。

同样，您的查询从表中获取所有行，但仅使用最后一行中的数据。 稍后，您可能需要考虑优化，即仅获取您实际使用的数据。