我有一个数据库，我存储两种不同的数据。一个表用于收藏路由，另一个表存储从服务器检索到的路由。
我可以检索路线等就好了。但是在检索到第一条路线后，按回或HOME，然后检索另一条路线，路线表将填满所有旧路线和新路线。
所以我的问题是：如何删除路由表而不是整个数据库，因为我不想删除添加的收藏夹....？！
我在android docs中找到了以下功能：
public int delete (String table, String whereClause, String[] whereArgs)
我试图实现它，但我必须传递一个SQLiteDataBase作为参数。但是如何？
我实施了：
public void deleteTableRoutes(SQLiteDataBase db){
db.delete("routes", null, null);
}
但我想从另一个类中调用此函数，我没有引用数据库..所以我必须作为参数传递什么？或者我如何获得对数据库的引用？
我根据dev docs中的NotePadExample的代码示例构建我的数据库。
如何解决这个问题？
这是我想要调用的类的摘录deleteTableRoutes（...）
package mjb.project.AVV;
/**
* Parse the request and fill the ListView with the results
* @author MJB
*
*/
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import net.htmlparser.jericho.Element;
import net.htmlparser.jericho.Source;
import android.app.ListActivity;
import android.database.Cursor;
import android.os.Bundle;
import android.util.Log;
import android.widget.SimpleCursorAdapter;
public class ParseHTML extends ListActivity {
private String start_from;
private String destination;
private int hour;
private int min;
private int day;
private int month;
private Source source;
private List<Element> tempList;
private List<Element> routes;
private String startT = "";
private String arrivalT = "";
private String duration = "";
private String line = "";
private boolean first=true;
private List<ResultElement> finalResult;
private List<Element> results;
private DbAdapter mDbHelper;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.parsehtml_list);
mDbHelper = new DbAdapter(this);
mDbHelper.open();
// Used to store the final results + data
finalResult = new ArrayList<ResultElement>();
// Get applicationsContext
Context context = getApplicationContext();
// Receive extras from Input.class
...
try {
// Set up the URL with the above parameters
source = new Source(new URL("...."));
// Get the requested routes
getResultElements();
// Get the data from the Elements
getResultData();
// Print Result's data
printRes();
// Delete old entries
deleteTableRoutes(context);
//错误类型DbAdapter中的方法deleteRoutesTable（SQLiteDatabase）不适用于参数（Context）....当然，但是如何获取对我的db的引用??
// Insert data to database
insertRoutesToDb();
// Fill data into the ListView
fillData();
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
mDbHelper.close();
mDbHelper=null;
}
}
}
这是我的DbAdapter
package mjb.project.AVV;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
public class DbAdapter {
public static final String KEY_FROM = "title";
public static final String KEY_TO = "body";
public static final String KEY_ROWID = "_id";
public static final String KEY_START = "start";
public static final String KEY_ARRIVAL = "arrival";
public static final String KEY_LINE = "line";
public static final String KEY_DURATION = "duration";
private static final String DATABASE_NAME = "data";
private static final String DATABASE_NOTESTABLE = "notes";
private static final String DATABASE_ROUTESTABLE = "routes";
private static final String TAG = "DbAdapter";
private DatabaseHelper mDbHelper;
private SQLiteDatabase mDb;
/**
* Database creation sql statement
*/
private static final String DATABASE_CREATE_NOTES =
"create table notes (_id integer primary key autoincrement, "
+ "title text not null, body text not null)";
private static final String DATABASE_CREATE_ROUTES =
"create table routes (_id integer primary key autoincrement, "
+ "start , arrival , "
+ "line , duration );";
private static final int DATABASE_VERSION = 3;
private final Context mCtx;
private static class DatabaseHelper extends SQLiteOpenHelper {
DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(DATABASE_CREATE_NOTES);
Log.d(TAG, "created notes table");
db.execSQL(DATABASE_CREATE_ROUTES);
Log.d(TAG, "created routes table");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
+ newVersion + ", which will destroy all old data");
db.execSQL("DROP TABLE IF EXISTS notes");
db.execSQL("DROP TABLE IF EXISTS routes");
onCreate(db);
}
}
/**
* Constructor - takes the context to allow the database to be
* opened/created
*
* @param ctx the Context within which to work
*/
public DbAdapter(Context ctx) {
this.mCtx = ctx;
}
/**
* Open the notes database. If it cannot be opened, try to create a new
* instance of the database. If it cannot be created, throw an exception to
* signal the failure
*
* @return this (self reference, allowing this to be chained in an
* initialization call)
* @throws SQLException if the database could be neither opened or created
*/
public DbAdapter open() throws SQLException {
mDbHelper = new DatabaseHelper(mCtx);
mDb = mDbHelper.getWritableDatabase();
return this;
}
public void close() {
mDbHelper.close();
}
/**
* Create a new note using the title and body provided. If the note is
* successfully created return the new rowId for that note, otherwise return
* a -1 to indicate failure.
*
* @param title the title of the note
* @param body the body of the note
* @return rowId or -1 if failed
*/
public long createNote(String title, String body) {
ContentValues initialValues = new ContentValues();
initialValues.put(KEY_FROM, title);
initialValues.put(KEY_TO, body);
return mDb.insert(DATABASE_NOTESTABLE, null, initialValues);
}
/**
* Create a new route using the title and body provided. If the route is
* successfully created return the new rowId for that route, otherwise return
* a -1 to indicate failure.
*
* @param start the start time of the route
* @param arrival the arrival time of the route
* @param line the line number of the route
* @param duration the routes duration
* @return rowId or -1 if failed
*/
public long createRoute(String start, String arrival, String line, String duration){
ContentValues initialValues = new ContentValues();
initialValues.put(KEY_START, start);
initialValues.put(KEY_ARRIVAL, arrival);
initialValues.put(KEY_LINE, line);
initialValues.put(KEY_DURATION, duration);
return mDb.insert(DATABASE_ROUTESTABLE, null, initialValues);
}
/**
* Delete the note with the given rowId
*
* @param rowId id of note to delete
* @return true if deleted, false otherwise
*/
public boolean deleteNote(long rowId) {
return mDb.delete(DATABASE_NOTESTABLE, KEY_ROWID + "=" + rowId, null) > 0;
}
/**
* Return a Cursor over the list of all notes in the database
*
* @return Cursor over all notes
*/
public Cursor fetchAllNotes() {
return mDb.query(DATABASE_NOTESTABLE, new String[] {KEY_ROWID, KEY_FROM,
KEY_TO}, null, null, null, null, null);
}
/**
* Return a Cursor over the list of all routes in the database
*
* @return Cursor over all routes
*/
public Cursor fetchAllRoutes() {
return mDb.query(DATABASE_ROUTESTABLE, new String[] {KEY_ROWID, KEY_START,
KEY_ARRIVAL, KEY_LINE, KEY_DURATION}, null, null, null, null, null);
}
/**
* Return a Cursor positioned at the note that matches the given rowId
*
* @param rowId id of note to retrieve
* @return Cursor positioned to matching note, if found
* @throws SQLException if note could not be found/retrieved
*/
public Cursor fetchNote(long rowId) throws SQLException {
Cursor mCursor =
mDb.query(true, DATABASE_NOTESTABLE, new String[] {KEY_ROWID,
KEY_FROM, KEY_TO}, KEY_ROWID + "=" + rowId, null,
null, null, null, null);
if (mCursor != null) {
mCursor.moveToFirst();
}
return mCursor;
}
/**
* Return a Cursor positioned at the route that matches the given rowId
*
* @param rowId id of route to retrieve
* @return Cursor positioned to matching route
* @throws SQLException if note could not be found/retrieved
*/
public Cursor fetchRoute(long rowId) throws SQLException {
Cursor mCursor =
mDb.query(true, DATABASE_ROUTESTABLE, new String[] {KEY_ROWID,
KEY_START, KEY_ARRIVAL, KEY_LINE, KEY_DURATION}, KEY_ROWID + "=" + rowId, null,
null, null, null, null);
if (mCursor != null) {
mCursor.moveToFirst();
}
return mCursor;
}
/**
* Update the note using the details provided. The note to be updated is
* specified using the rowId, and it is altered to use the title and body
* values passed in
*
* @param rowId id of note to update
* @param title value to set note title to
* @param body value to set note body to
* @return true if the note was successfully updated, false otherwise
*/
public boolean updateNote(long rowId, String title, String body) {
ContentValues args = new ContentValues();
args.put(KEY_FROM, title);
args.put(KEY_TO, body);
return mDb.update(DATABASE_NOTESTABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
}
public void deleteRoutesTable(SQLiteDatabase db){
db.delete("routes", null, null);
}
}
答案 0 :(得分：2)
在我的DbAdapter类中：
public void deleteRoutesTable(){
mDb.delete("routes", null, null);
}
我称之为：
public void onCreate(){
.......
try {
mDbHelper.deleteRoutesTable();
} catch (NullPointerException e) {
// TODO Auto-generated catch block
Log.d(TAG_AVV, "ERROR!");
e.printStackTrace();
}
我解决了。我只是用错误的方法放置了try / catch调用。调用onResume（）时，没有要删除的表。现在我将它放入我的onCreate（）方法，它工作得很好！
感谢所有帮助者！
答案 1 :(得分：1)
您可以使用DbAdapter课程访问数据库。构建此内容需要Context。这可能是放置deleteTableRoutes方法的合适位置。实际上，您已经有了类似的deleteNote方法（尽管删除了一行）。请注意，您无需向其传递参数，因为DbAdapter具有mDb实例字段。
答案 2 :(得分：1)
参考你最近的帖子（顺便说一句，这应该是对你的问题的编辑，而不是'答案'）：使用mDbHelper来访问你的数据库是应该工作的方式。但是，你获得空指针的原因可能是你的DbAdapter中没有正确的构造函数。
如果您不熟悉该术语，构造函数就是代码的一部分：
public DbAdapter(Context ctx) {
this.mCtx = ctx;
}
它缺少您实例化DatabaseHelper类的行。您只需要添加一行：
mDbHelper = new DatabaseHelper(ctx);