我必须建立一个问题及其答案的数据库。我使用 sqlitedatabase 浏览器制作并放入 Assets 文件夹中。这是我检索特定问题 ID 行的代码。
一些重要信息 我的数据库名称是 extenalDB。我的表名是问题；
有coloum : id,question,ans
我必须搜索 ID 为 201 的问题。
当我编写有问题的查询时。java 显示未声明的错误 ID。
我是stackoverflow的新手，所以如果我不能说清楚，请询问
这是我的 DatabaseHelper 类代码
public class DatabaseHelper extends SQLiteOpenHelper{
//The Android's default system path of your application database.
String DB_PATH =null;
private static String DB_NAME = "extenalDB";
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;
DB_PATH="/data/data/"+context.getPackageName()+"/"+"databases/";
} // Creates a empty database on the system and rewrites it with your owndatabase.*/
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");
}
}}
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) {
}
public Cursor query(String table,String[] columns, String selection,String[] selectionArgs,String groupBy,String having,String orderBy){
return myDataBase.query("question", null, null, null, null, null, null);
}
}
还有另一个类 question.java，通过单击开始按钮问题答案，他们的 id 显示在 toast 消息中。question.java 的代码是。
public class question extends Activity{
Cursor c=null;
int rid=201;
@Override
public void onCreate(Bundle savedInstanceState) {
Intent mintent = getIntent();
int intValue = mintent.getIntExtra("qtype",0);
super.onCreate(savedInstanceState);
setContentView(R.layout.question);
((Button)findViewById(R.id.button1)).setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v) {
DatabasseHelper myDbHelper = new DatabasseHelper(question.this);
try {
myDbHelper.createDataBase();
} catch (IOException ioe) {
throw new Error("Unable to create database");
}
try {
myDbHelper.openDataBase();
}catch(SQLException sqle){
throw sqle;
}
Toast.makeText(question.this, "Success", Toast.LENGTH_SHORT).show();
c=myDbHelper.query("question", null, id + "=" + rid, null, null,null, null);
Toast.makeText(question.this,
"id: " + c.getString(0) + "\n" +
"question: " + c.getString(1) + "\n" +
"ans: " + c.getString(2) + "\n" +
Toast.LENGTH_LONG).show();
}
});
}}