java - cursor.moveToFirst 似乎被跳过

Question

我是 Java 新手，只是尝试创建一个数据库。我设法创建了一个数据库，但是当我想读取这些值时，它似乎出现了错误。

这是我的设置 Activity 代码(要求设置值并将它们添加到特定 ID 的数据库中)

public class Settings extends Activity{

    Button Save;
    static Switch SwitchCalculations;
    public static String bool;
    public static List<Integer> list_id = new ArrayList<Integer>();
    public static List<String> list_idname = new ArrayList<String>();
    public static List<String> list_kind = new ArrayList<String>();
    public static List<String> list_value = new ArrayList<String>();
    static Integer[] arr_id;
    static String[] arr_idname;
    static String[] arr_kind;
    static String[] arr_value;
    public static final String TAG = "Settings";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.settings);

        Save = (Button) findViewById(R.id.btnSave);
        SwitchCalculations = (Switch) findViewById(R.id.switchCalcOnOff);

        readData();

        Save.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                        writeData();
                        //Toast.makeText(this, "Data has been saved.", Toast.LENGTH_SHORT).show();
                        readData();

                    Save.setText("Opgeslagen");
            }
        });
    }
    public void writeData() {
            int id = 1;
            String idname = "switchCalcOnOff";
            String kind = "switch";
            boolean val = SwitchCalculations.isChecked();
            String value = new Boolean(val).toString();

            dbHelper_Settings dbh = new dbHelper_Settings(this);
            SQLiteDatabase db = dbh.getWritableDatabase();
            ContentValues cv = new ContentValues();
            cv.put(dbh.C_ID, id);
            cv.put(dbh.C_IDNAME, idname);
            cv.put(dbh.C_KIND, kind);
            cv.put(dbh.C_VALUE, value);

            if (dbh.C_ID.isEmpty() == true) {
                db.insert(dbh.TABLE, null, cv);
                Log.d(TAG, "Insert: Data has been saved.");
            } else if (dbh.C_ID.isEmpty() == false) {
                db.update(dbh.TABLE, cv, "n_id='1'", null);
                Log.d(TAG, "Update: Data has been saved.");
            } else {
                Log.d(TAG, "gefaald");
            }
            db.close();
    }
    public void readData() {
        dbHelper_Settings dbh = new dbHelper_Settings(this);
        SQLiteDatabase db = dbh.getWritableDatabase();
        List<String> list_value = new ArrayList<String>();
        String[] arr_value;
        list_value.clear();

        Cursor cursor = db.rawQuery("SELECT " + dbh.C_VALUE + " FROM " + dbh.TABLE + ";", null);

        if (cursor.moveToFirst()) {
        do {
            list_value.add(cursor.getString(0));
        } while (cursor.moveToNext());
        }

        if (cursor != null && !cursor.isClosed()){

            cursor.close();
        }
        db.close();
        arr_value = new String[list_value.size()];

        for (int i = 0; i < list_value.size(); i++){
            arr_value[i] = list_value.get(i);
        }
    }
}

然后我的 dbHelper Activity 如下所示:

package com.amd.nutrixilium;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class dbHelper_Settings extends SQLiteOpenHelper{

    private static final String TAG="dbHelper_Settings";

    public static final String DB_NAME = "settings.db";
    public static final int DB_VERSION = 10;
    public final String TABLE = "settings";
    public final String C_ID = "n_id";  // Special for id
    public final String C_IDNAME = "n_idname";
    public final String C_KIND = "n_kind";
    public final String C_VALUE = "n_value";
    Context context;

    public dbHelper_Settings(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
        this.context = context;
    }

    // oncreate wordt maar 1malig uitgevoerd per user voor aanmaken van database
    @Override
    public void onCreate(SQLiteDatabase db) {
        String sql = String.format("create table %s (%s int primary key, %s TEXT, %s TEXT, %s TEXT)", TABLE, C_ID, C_IDNAME, C_KIND, C_VALUE);

        Log.d(TAG, "onCreate sql: " + sql);

        db.execSQL(sql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("drop table if exists " + TABLE); // wist een oudere database versie
        Log.d(TAG, "onUpgrate dropped table " + TABLE);
        this.onCreate(db);
    }
}

奇怪的是我在这里没有收到任何错误消息。但我使用 Log.d(TAG, text) 来检查脚本被跳过的位置，即 cursor.moveToFirst()。

有人可以帮我解决这个问题吗？

Answer 1

在这里，与您似乎期望的相反，您实际上检查文本常量是否不为空:

if (dbh.C_ID.isEmpty() == true) {

事实并非如此:它总是包含“n_id”

我认为您的 Intent 是查找具有该 id 的记录，并根据结果插入或更新。

您应该这样做:尝试通过助手进行选择，然后按照上面的代码插入或更新。

编辑:

向你的助手添加如下内容:

public boolean someRowsExist(SQLiteDatabase db) {
    Cursor cursor = db.rawQuery("select EXISTS ( select 1 from " + TABLE + " )", new String[] {});
    cursor.moveToFirst();
    boolean exists = (cursor.getInt(0) == 1);
    cursor.close();
    return exists;
}

并用它来检查数据库中是否有任何行:

if (dbh.someRowsExist(db)) {  // instead of (dbh.C_ID.isEmpty() == true) {