Question

大家好久以后我在这里发布任何查询。我在以下代码中收到NullPointer异常错误： -

public class DBAdapter {

int id = 0;
public static final String KEY_ROWID = "_id";
public static final String KEY_FNAME = "FirstName";
public static final String KEY_LNAME = "LastName";
public static final String KEY_EMAILID = "Email";
public static final String KEY_PASSWORD = "Password";
private static final String TAG = "DBAdapter";

private static final String DATABASE_NAME = "UserDetail";
private static final String DATABASE_TABLE = "tblUserDetail";

private static final int DATABASE_VERSION = 1;

private static final String DATABASE_CREATE =  "create table tblUserDetail (_id integer primary key autoincrement, "
        + "FirstName text not null, LastName text, Email text not null, Password text not null );";

private final Context context;
private DatabaseHelper DBHelper;
private SQLiteDatabase db;

public DBAdapter(Context ctx){
    this.context = ctx;
    DBHelper = new DatabaseHelper(ctx);
}

private static class DatabaseHelper extends SQLiteOpenHelper{

    public DatabaseHelper(Context context) {
            super(context,DATABASE_NAME,null,DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
            db.execSQL(DATABASE_CREATE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
            Log.w(TAG, "Upgrading database version from"+ oldVersion +
                    "to"+ newVersion +", which will destroy all old data");
            db.execSQL("DROP TABLE IF EXIST tblUserDetail");
            onCreate(db);
    }
}

    public DBAdapter open() throws SQLException{

        db = DBHelper.getWritableDatabase();
        return this;

    }

    public void close(){
        DBHelper.close();
    }

    public long insertData(String fName,String lName, String emailId, String password){
        ContentValues intialValues = new ContentValues();
        intialValues.put(KEY_FNAME, fName);
        intialValues.put(KEY_LNAME, lName);
        intialValues.put(KEY_EMAILID, emailId);
        intialValues.put(KEY_PASSWORD, password);
        return db.insert(DATABASE_TABLE, null, intialValues);
    }

    public String getAllEntries()
    {
        SQLiteDatabase database = DBHelper.getReadableDatabase();

        Cursor cursor = database.rawQuery(
                    "SELECT FirstName FROM tblUserDetail WHERE _id = 1", null);
                if(cursor.moveToFirst()) {
                    return cursor.getString(0);
                }
                return cursor.getString(0);

    }

}

我点击按钮时调用insertData和getAllEntries： -

Submit.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
    //      Toast toast = Toast.makeText(getApplicationContext(), "cliecked on submit button", Toast.LENGTH_LONG);
    //      toast.show();

            String str;
            try{
                db.insertData(etFN.getText().toString(), etLN.getText().toString(), etEI.getText().toString(), etPS.getText().toString());

                str = db.getAllEntries();

                Toast toast = Toast.makeText(getApplicationContext(), str, Toast.LENGTH_LONG);
                toast.show();
            }

            catch(Exception ex){
                 Context context = getApplicationContext();
                 CharSequence text = ex.toString();
                 int duration = Toast.LENGTH_LONG;

                 Toast toast = Toast.makeText(context, text, duration);
                 toast.show();
            }

        } 
    });

请帮我摆脱这个NullPointer Exception Error.Every help将受到高度赞赏。

我收到了db.insertData()行的错误。

Answer 1

这可能有两个原因。

1）如果您尚未初始化 DBADapter 对象。你刚宣布了它。

所以初始化

 db = new DBAdapter(this);

2）在将数据插入数据库之前，您尚未打开或关闭数据库。因此，首先打开数据库，然后再将数据插入其中。

谢谢！

获取NullPointer异常在android中从数据库创建和获取数据时出错

1 个答案: