Question

我想从我的数据库中读取一个文本列，然后更新它。我在下面创建了两种方法：

public boolean editNote(Lesson lesson) {

    ContentValues values = new ContentValues();
    values.put(LessonsDBOpenHelper.COLUMN_NOTE, lesson.getNote());
    String where = LessonsDBOpenHelper.COLUMN_ID + "=" + lesson.getId();
    int result = database.update(LessonsDBOpenHelper.TABLE_LESSONS, values, where, null);
    return (result == 1);

}

和

public String readNote(Lesson lesson) {

        String query = "SELECT * FROM lessons " +
                "WHERE lessonId =" +lesson.getId();
        Cursor cursor = database.rawQuery(query, null);

        String returnString = "";   // default value if nothing is found

        if (cursor.moveToFirst()){
            returnString = cursor.getString(cursor.getColumnIndex(LessonsDBOpenHelper.COLUMN_NOTE));
        }
        cursor.close();
        return returnString;
        }

然后我从另一个活动中调用editNote（）和readNote（）方法：

public class Note extends Activity {

    private static final String LOGTAG = "MYAPP";
    Lesson lesson;
    LessonsDataSource datasource;

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

        Bundle b = getIntent().getExtras();
        lesson = b.getParcelable(".model.Lesson");

        Button noteSave = (Button) findViewById(R.id.note_save);
        Button noteClear = (Button) findViewById(R.id.note_clear);
        final TextView noteText = (TextView) findViewById(R.id.noteText);

        String dbNote = datasource.readNote(lesson);
        noteText.setText(dbNote);
        Log.i(LOGTAG, "Note loaded from DB is "+dbNote);


        noteClear.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                noteText.setText("");
                lesson.setNote("");
                datasource.editNote(lesson);
            }
        });

        noteSave.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                String note = noteText.getText().toString();
                lesson.setNote(note);
                datasource.editNote(lesson);
                Log.i(LOGTAG, "Note text changed to "+note);

                String dbNote = datasource.readNote(lesson);
                Log.i(LOGTAG, "Note text in DB is "+dbNote);


            }
        });
    }
}

但是当我的应用程序获得readNote或editNote方法时崩溃了。我做错了什么？

我只想阅读并更新我的数据库中的文本。

修改这是LogCat：

01-30 02:47:29.809: D/AndroidRuntime(30850): Shutting down VM
01-30 02:47:29.809: W/dalvikvm(30850): threadid=1: thread exiting with uncaught exception (group=0x41744ba8)
01-30 02:47:29.809: E/AndroidRuntime(30850): FATAL EXCEPTION: main
01-30 02:47:29.809: E/AndroidRuntime(30850): Process: com.me.linux, PID: 30850
01-30 02:47:29.809: E/AndroidRuntime(30850): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.me.linux/com.me.linux.Note}: java.lang.NullPointerException
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.os.Handler.dispatchMessage(Handler.java:102)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.os.Looper.loop(Looper.java:136)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.app.ActivityThread.main(ActivityThread.java:5017)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at java.lang.reflect.Method.invokeNative(Native Method)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at java.lang.reflect.Method.invoke(Method.java:515)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at dalvik.system.NativeStart.main(Native Method)
01-30 02:47:29.809: E/AndroidRuntime(30850): Caused by: java.lang.NullPointerException
01-30 02:47:29.809: E/AndroidRuntime(30850):    at com.me.linux.Note.onCreate(Note.java:31)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.app.Activity.performCreate(Activity.java:5231)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
01-30 02:47:29.809: E/AndroidRuntime(30850):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
01-30 02:47:29.809: E/AndroidRuntime(30850):    ... 11 more
01-30 02:59:51.589: I/MYAPP(31030): Database closed

编辑2： 以下是启动此活动的意图的putExtra代码：

case R.id.action_edit_note:
            Intent intent = new Intent(this, Note.class);

            intent.putExtra(".model.Lesson", lesson);
            startActivityForResult(intent, 1002);
            break;

Answer 1

您必须使用readNote()方法重新打开数据库，以便创建查询。

在SQLite android中读写单个列

1 个答案: