我在将数据保存到使用 SQLITE 数据库浏览器预先创建的 sqlite 数据库时遇到问题。我将数据库存储在资产文件夹中。
这是我使用的代码：
ApplicationContextProvider 类：
import android.app.Application;
import android.content.Context;
public class ApplicationContextProvider extends Application {
private static Context sContext;
public void onCreate() {
super.onCreate();
sContext = getApplicationContext();
}
public static Context getContext() {
return sContext;
}
}
我的 DatabaseHelper 类：
public class DataBaseHelper extends SQLiteOpenHelper {
// The Android's default system path of your application database.
//data/data/ and /databases remain the same always. The one that must be changed is com.example which represents
//the MAIN package of your project
private static String DB_PATH = "/data/data/com.fps/databases/";
//the name of your database
private static String DB_NAME = "bmiDB";
private static SQLiteDatabase mDataBase;
private static DataBaseHelper sInstance = null;
// database version
private static final int DATABASE_VERSION = 1;
/**
* Constructor Takes and keeps a reference of the passed context in order to
* access to the application assets and resources.
*/
private DataBaseHelper() {
super( ApplicationContextProvider.getContext(), DB_NAME, null, DATABASE_VERSION );
try {
createDataBase();
openDataBase();
} catch (IOException e) {
e.printStackTrace();
}
}
/**
* Singleton for DataBase
*
* @return singleton instance
*/
public static DataBaseHelper instance() {
if ( sInstance == null ) {
sInstance = new DataBaseHelper();
}
return sInstance;
}
/**
* Creates a empty database on the system and rewrites it with your own
* database.
*
* @throws java.io.IOException io exception
*/
private void createDataBase() throws IOException {
boolean dbExist = checkDataBase();
if ( dbExist ) {
// do nothing - database already exist
} else {
// By calling this method an 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" );
}
}
}
/**
* 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 doesn't exist yet.
}
if ( checkDB != null ) {
checkDB.close();
}
return checkDB != null;
}
/**
* 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.
*
* @throws java.io.IOException io exception
*/
public void copyDataBase() throws IOException {
// Open your local db as the input stream
InputStream myInput = ApplicationContextProvider.getContext().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();
}
private void openDataBase() throws SQLException {
// Open the database
String myPath = DB_PATH + DB_NAME;
mDataBase = SQLiteDatabase.openDatabase( myPath, null,
SQLiteDatabase.OPEN_READWRITE );
}
/**
* Select method
*
* @param query select query
* @return - Cursor with the results
* @throws android.database.SQLException sql exception
*/
public Cursor select( String query ) throws SQLException {
return mDataBase.rawQuery( query, null );
}
/**
* Insert method
*
* @param table - name of the table
* @param values values to insert
* @throws android.database.SQLException sql exception
*/
public void insert( String table, ContentValues values ) throws SQLException {
mDataBase.insert( table, null, values );
}
/**
* Delete method
*
* @param table - table name
* @param where WHERE clause, if pass null, all the rows will be deleted
* @throws android.database.SQLException sql exception
*/
public void delete( String table, String where ) throws SQLException {
mDataBase.delete( table, where, null );
}
/**
* Update method
*
* @param table - table name
* @param values - values to update
* @param where - WHERE clause, if pass null, all rows will be updated
*/
public void update( String table, ContentValues values, String where ) {
mDataBase.update( table, values, where, null );
}
/**
* Let you make a raw query
*
* @param command - the sql comand you want to run
*/
public void sqlCommand( String command ) {
mDataBase.execSQL( command );
}
@Override
public synchronized void close() {
if (mDataBase != null)
mDataBase.close();
super.close();
}
@Override
public void onCreate( SQLiteDatabase db ) {
}
@Override
public void onUpgrade( SQLiteDatabase db, int oldVersion, int newVersion ) {
}
}
以及方法 viewSavedToast() 中的一段代码：
AlertDialog ad = new AlertDialog.Builder( this ).create();
//ad.setCancelable( true );
ad.setTitle( "Successful" );
ad.setMessage( "You're result is saved in the database!" );
ad.setButton( "OK", new DialogInterface.OnClickListener() {
public void onClick( DialogInterface dialog, int which ) {
//with ContentValues put the data we want into the database
ContentValues values = new ContentValues();
String column_name = etanswer.getText().toString();
values.put( COLUMN_NAME, column_name );
//here we insert the data we have put in values
dataBase.insert( TABLE_NAME, values );
dialog.dismiss();
}
});
ad.show();
当我运行应用程序，将数据保存到数据库时，onClick() 方法出现错误？问题是什么？我相信我遵循了我在这里找到的教程中的所有步骤：http: //myandroidsolutions.blogspot.com/2012/07/android-database-tutorial.html但不知何故我得到了一个错误。
日志：
10-17 20:50:11.118: E/AndroidRuntime(368): FATAL EXCEPTION: main
10-17 20:50:11.118: E/AndroidRuntime(368): java.lang.NullPointerException
10-17 20:50:11.118: E/AndroidRuntime(368): at com.fps.iHealthFirst.BMICalculator.viewBMISavedToast(BMICalculator.java:121)
10-17 20:50:11.118: E/AndroidRuntime(368): at com.fps.iHealthFirst.BMICalculator.calculateMen(BMICalculator.java:194)
10-17 20:50:11.118: E/AndroidRuntime(368): at com.fps.iHealthFirst.BMICalculator.onClick(BMICalculator.java:143)
10-17 20:50:11.118: E/AndroidRuntime(368): at android.view.View.performClick(View.java:2485)
10-17 20:50:11.118: E/AndroidRuntime(368): at android.view.View$PerformClick.run(View.java:9080)
10-17 20:50:11.118: E/AndroidRuntime(368): at android.os.Handler.handleCallback(Handler.java:587)