创建sqlite数据库android时出现空指针异常

Question

这是代码。

public class DatabaseManager extends android.app.Activity
{
    public SQLiteDatabase mydatabase ;
    private static final Object MODE_PRIVATE = null;
    public static final String TABLE_NAME = "Fests";

    // this function is called first.
    public  void createDatabase()
    {
        try{

    mydatabase = openOrCreateDatabase("fests",SQLiteDatabase.CREATE_IF_NECESSARY,null);
    System.out.println("no probs with open function.");
    mydatabase.execSQL("CREATE TABLE IF NOT EXISTS Fests (FestId VARCHAR,Festname VARCHAR);");
    }catch(Exception e)
    {
        Log.d("creating db","exception caught."+e);
    }
}

我在第一行本身即openOrCreateDatabase函数中得到空指针异常，无法弄清原因。 我是android编程的新手，请帮助我。

Answer 1

我认为您正在使用public class DatabaseManager extends android.app.Activity ，将其更改为public class DatabaseManager extends SQLiteOpenHelper ，这是错误的

Answer 2

如上所述，更改您的课程扩展范围。

您也不需要变量mydatabase 。 该类中已经有一个名为database的类。 如果您提供名称和版本，它将为您处理它的创建。

例如

private static final String DATABASE_NAME = "Name.db";
private static final int DATABASE_VERSION = 2;

/**
 * this gets called automatically on opening the database
 * 
 * @param context
 */
public DatabaseManager(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);

}

// if the database does not exist this method is called to create it
@Override
public void onCreate(SQLiteDatabase database) {
    for (String CREATE_TABLE : CREATE_TABLES)
    {
        database.execSQL(CREATE_TABLE);
    }
}

// when the version changes this method updates the version number and
// creates a log entry
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    Log.w(SQLiteHelper.class.getName(), "Upgrading database from version "
            + oldVersion + " to " + newVersion
            + ", which will destroy all old data");
    for (String TABLE: TABLES)
    {
        db.execSQL("DROP TABLE IF EXISTS " + TABLE);
    }
    onCreate(db);
}