【发布时间】：2022-03-17 23:01:00
【问题描述】：
我正在创建一个将查询数据库的应用程序，但是无法检测到表。
我已按照此博客中指示的步骤操作：http://www.reigndesign.com/blog/using-your-own-sqlite-database-in-android-applications/
我制作了android_metadata表，复制到assets文件夹，创建了DataBaseHelper类，如下图：
public class DataBaseHelper extends SQLiteOpenHelper{
private static final String MedicalStaff = null;
private static final String Patients = null;
//The Android's default system path of your application database.
private static String DB_PATH = "/data/data/com.angelo.thesis/databases/";
private static String DB_NAME = "androidHospital";
private SQLiteDatabase myDataBase;
private final Context myContext;
/**
* Constructor
* Takes and keeps a reference of the passed context in order to access to the application assets and resources.
* @param context
*/
public DataBaseHelper(Context context) {
super(context, DB_NAME, null, 1);
this.myContext = context;
}
/**
* Creates a empty database on the system and rewrites it with your own database.
* */
public void createDataBase() throws IOException{
boolean dbExist = checkDataBase();
if(dbExist){
//do nothing - database already exist
}else{
//By calling this method and empty database will be created into the default system path
//of your application so we are gonna be able to overwrite that database with our database.
this.getReadableDatabase();
try {
copyDataBase();
} catch (IOException e) {
//throw new Error("Error copying database");
System.out.println("Error copying database");
}
}
}
/**
* Check if the database already exist to avoid re-copying the file each time you open the application.
* @return true if it exists, false if it doesn't
*/
private boolean checkDataBase(){
SQLiteDatabase checkDB = null;
try{
String myPath = DB_PATH + DB_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}catch(SQLiteException e){
//database does't exist yet.
}
if(checkDB != null){
checkDB.close();
}
return checkDB != null ? true : false;
}
/**
* Copies your database from your local assets-folder to the just created empty database in the
* system folder, from where it can be accessed and handled.
* This is done by transfering bytestream.
* */
private void copyDataBase() throws IOException{
//Open your local db as the input stream
InputStream myInput = myContext.getAssets().open(DB_NAME);
// Path to the just created empty db
String outFileName = DB_PATH + DB_NAME;
//Open the empty db as the output stream
OutputStream myOutput = new FileOutputStream(outFileName);
//transfer bytes from the inputfile to the outputfile
byte[] buffer = new byte[1024];
int length;
while ((length = myInput.read(buffer))>0){
myOutput.write(buffer, 0, length);
}
//Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
public void openDataBase() throws SQLException{
//Open the database
String myPath = DB_PATH + DB_NAME;
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}
@Override
public synchronized void close() {
if(myDataBase != null)
myDataBase.close();
super.close();
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}
然后使用以下方法初始化此数据库助手：
DataBaseHelper myDbHelper = new DataBaseHelper(null);
在onCreate 方法中：
myDbHelper = new DataBaseHelper(this);
try {
myDbHelper.createDataBase();
System.out.println("Database Created!");
}
catch (IOException e) {
// TODO Auto-generated catch block
System.out.println("Unable to create database");
}
try {
myDbHelper.openDataBase();
System.out.println("Database Opened!");
}
catch (SQLException e) {
// TODO Auto-generated catch block
System.out.println("Unable to open database");
}
try-catch 语句起作用，LogCat 显示 Created 和 Opened 消息。
然后通过这个函数进行查询： 字符串输入是用户输入，一个数字，用来寻找那个数字对应的人。
public String searchStaff(DataBaseHelper myDB, String input){
System.out.println("searchStaff entered");
String data = null;
Cursor cursor = null;
SQLiteDatabase db = myDB.getReadableDatabase();
//int myNum = Integer.parseInt(input);
try{
cursor = db.rawQuery("SELECT LastName, FirstName FROM MedicalStaff WHERE _id =" + input, null);
}
catch(Exception e){
}
if( cursor != null ){
cursor.moveToFirst();
data = cursor.getString(cursor.getColumnIndexOrThrow("LastName")) + " " +
cursor.getString(cursor.getColumnIndex("FirstName"));
cursor.close();
}
System.out.println("Data: " + data);
return data;
}
但是，显示的数据变成了null，Log Cat 告诉我表MedicalStaff 不存在。
我觉得我在这里错过了一些非常基本的东西。 谢谢大家的帮助。
【问题讨论】：
标签： android sqlite android-cursor