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java - 用户使用 SQLite 数据库登录 java.lang.NullPointerException

转载 作者：行者123 更新时间：2023-12-01 11:41:17 25 4
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我在使用数据库检查用户的用户名和密码时遇到问题。我使用查询来选择特定行，然后根据用户输入的内容检查密码。当我调用

时，我收到的错误是登录页面中的 java.lang.NullPointerException

用户 userlogin = db.userlogin(usernameinput);

查看该方法后，我认为这是导致其失败的第一个光标

cursor.getInt(0);

我想知道我的想法是否正确以及我能做些什么来改变它？我尝试将 if 语句更改为

If(cursor.getcount() > 0)

但仍然没有运气。

public class LoginPage extends ActionBarActivity {
        Button loginbutton;
        EditText usernameuser, passworduser;
        DatabaseHandler db;
        String usernameinput, passwordinput;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_login_page);




        loginbutton = (Button) findViewById(R.id.loginbtn);
        usernameuser = (EditText) findViewById(R.id.usernameInsert);
        passworduser = (EditText) findViewById(R.id.passwordInsert);





        loginbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                usernameinput = usernameuser.getText().toString();
                passwordinput = passworduser.getText().toString();
                Users userlogin = db.userLogin(usernameinput);
                if (usernameinput.equals(userlogin.get_username()) && passwordinput.equals(userlogin.get_password())) {
                    startActivity(new Intent(getApplicationContext(), Home_Page.class));
                }
                else {
                    Toast.makeText(getApplicationContext(), "Error", Toast.LENGTH_SHORT).show();
                }
            }

        });

用于检查登录的数据库处理程序查询:

public Users userLogin(String username) {

    SQLiteDatabase db = this.getReadableDatabase();
    String[] projection = {KEY_USER_ID, KEY_USERNAME, KEY_PASSWORD};
    String selection = KEY_USERNAME + " =?";
    String[] selectionargs = {username};
   Cursor cursor = db.query(TABLE_USERS, projection, selection, selectionargs, null, null,null );
    if (cursor != null)
        cursor.moveToFirst();
            Users users = new Users(
                    cursor.getInt(0),
                    cursor.getString(1),
                    cursor.getString(2),
                    cursor.getInt(3),
                    cursor.getString(4),
                    cursor.getString(5),
                    cursor.getDouble(6),
                    cursor.getDouble(7),
                    cursor.getDouble(8),
                    cursor.getDouble(9),
                    cursor.getDouble(10),
                    cursor.getDouble(11),
                    cursor.getDouble(12),
                    cursor.getDouble(13),
                    cursor.getDouble(14),
                    cursor.getDouble(15),
                    cursor.getDouble(16),
                    cursor.getDouble(17),
                    cursor.getDouble(18),
                    cursor.getDouble(19));
    cursor.close();
    return users;

}

04-08 13:05:33.194    2565-2565/com.example.john.fitnessapp E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.john.fitnessapp, PID: 2565
java.lang.NullPointerException
        at com.example.john.fitnessapp.LoginPage$1.onClick(LoginPage.java:42)
        at android.view.View.performClick(View.java:4569)
        at android.view.View$PerformClick.run(View.java:18553)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:212)
        at android.app.ActivityThread.main(ActivityThread.java:5137)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:902)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:718)
        at dalvik.system.NativeStart.main(Native Method)

用户类别:

public class Users {
int _id, _age;
String _username, _password, _email, _gender;
double _startWeight, _currentWeight, _weightChange, _height, _BMI, _BMR, _reqCal, _monCal, _tuesCal, _wedCal, _thurCal, _friCal, _satCal, _sunCal;

public Users(){}
public Users(int _id, String _username, String _password, int _age, String _email, String _gender, double _height, double _startWeight,
             double _currentWeight, double _weightChange, double _BMI, double _BMR, double _reqCal, double _monCal, double _tuesCal, double _wedCal,
             double _thurCal, double _friCal, double _satCal, double _sunCal){




    this._id = _id;
    this._username = _username;
    this._password = _password;
    this._age = _age;
    this._email = _email;
    this._gender = _gender;
    this._height = _height;
    this._startWeight = _startWeight;
    this._currentWeight = _currentWeight;
    this._weightChange = _weightChange;
    this._BMI = _BMI;
    this._BMR = _BMR;
    this._reqCal = _reqCal;
    this._monCal = _monCal;
    this._tuesCal = _tuesCal;
    this._wedCal = _wedCal;
    this._thurCal = _thurCal;
    this._friCal = _friCal;
    this._satCal = _satCal;
    this._sunCal = _sunCal;
}



public int get_id(){
    return this._id;
}
public void set_id(int id){
    this._id = id;
}
public String get_username(){
    return this._username;
}
public void set_username(String username){
    this._username = username;
}
public String get_password(){
    return this._password;
}
public void set_password(String password){
    this._password = password;
}
public int get_age(){
    return this._age;
}
public void set_age(int age){
    this._age = age;
}
public String get_email(){
    return this._email;
}
public void set_email(String email){
    this._email = email;
}
public String get_gender(){
    return this._gender;
}
public void set_gender(String gender){
    this._gender = gender;
}
public double get_height(){
    return this._height;
}
public void set_height(double height){
    this._height = height;
}
public double get_startWeight(){
    return this._startWeight;
}
public void set_startWeight(double startWeight){
    this._startWeight = startWeight;
}
public double get_currentWeight(){
    return this._currentWeight;
}
public void set_currentWeight(double currentWeight){
    this._currentWeight = currentWeight;
}
public double get_weightChange(){
    return this._weightChange;
}
public void set_weightChange(){
    this._weightChange = _currentWeight - _startWeight;
}
public double get_BMI(){
    return this._BMI;
}
public void set_BMI(double BMI){
    this._BMI = BMI;
}
public double get_BMR(){
    return this._BMR;
}
public void set_BMR(double BMR){
    this._BMR = BMR;
}
public double get_reqCal(){
    return this._reqCal;
}
public void set_reqCal(double reqCal){
    this._reqCal = reqCal;
}
public double get_monCal(){
    return this._monCal;
}
public void set_monCal(double monCal){
    this._monCal = monCal;
}
public double get_tuesCal(){
    return this._tuesCal;
}
public void set_tuesCal(double tuesCal){
    this._tuesCal = tuesCal;
}
public double get_wedCal(){
    return this._wedCal;
}
public void set_wedCal(double wedCal){
    this._wedCal = wedCal;
}
public double get_thurCal(){
    return this._thurCal;
}
public void set_thurCal(double thurCal){
    this._thurCal = thurCal;
}
public double get_friCal(){
    return this._friCal;
}
public void set_friCal(double friCal){
    this._friCal = friCal;
}
public double get_satCal(){
    return this._satCal;
}
public void set_satCal(double satCal){
    this._satCal = satCal;
}
public double get_sunCal(){
    return this._sunCal;
}
public void set_sunCal(double sunCal){
    this._sunCal = sunCal;
}

}

数据库处理程序:

public class DatabaseHandler extends SQLiteOpenHelper {
public static final String TAG = "DBHelper";
//DATABASE VERSION
private static int DATABASE_VERSION = 1;
//DATABASE NAME
private static final String DATABASE_NAME = "AppDatabase";
//TABLE NAMES
private static final String TABLE_USERS = "Users_Table";
private static final String TABLE_PRODUCTS = "Products_Table";
//COMMON COLUMN NAMES
private static final String KEY_USER_ID = "User_ID";
private static final String KEY_PRODUCT_ID = "Product_ID";
//USER TABLE
private static final String KEY_USERNAME = "Username";
private static final String KEY_PASSWORD = "Password";
private static final String KEY_AGE = "Age";
private static final String KEY_EMAIL = "Email";
private static final String KEY_GENDER = "Gender";
private static final String KEY_HEIGHT = "Height";
private static final String KEY_CURRENT_WEIGHT = "Current_Weight";
private static final String KEY_START_WEIGHT = "Start_Weight";
private static final String KEY_WEIGHT_CHANGE = "Weight_Change";
private static final String KEY_BMI = "BMI";
private static final String KEY_BMR = "BMR";
private static final String KEY_REQ_CAL = "Required_Calories";
private static final String KEY_MON_CAL = "Monday_Calories";
private static final String KEY_TUES_CAL = "Tuesday_Calories";
private static final String KEY_WED_CAL = "Wednesday_Calories";
private static final String KEY_THUR_CAL = "Thursday_Calories";
private static final String KEY_FRI_CAL = "Friday_Calories";
private static final String KEY_SAT_CAL = "Saturday_Calories";
private static final String KEY_SUN_CAL = "Sunday_Calories";
//PRODUCT TABLE
private static final String KEY_ITEMNAME = "Item_name";
private static final String KEY_ITEMCALORIES = "Item_Calories";
//
private static final String CREATE_USER_TABLE = "CREATE TABLE " + TABLE_USERS + "( "
        + KEY_USER_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
        + KEY_USERNAME + " TEXT, "
        + KEY_PASSWORD + " TEXT, "
        + KEY_AGE + " INTEGER, "
        + KEY_EMAIL + " TEXT, "
        + KEY_GENDER + " TEXT, "
        + KEY_HEIGHT + " DOUBLE, "
        + KEY_START_WEIGHT + " DOUBLE, "
        + KEY_CURRENT_WEIGHT + " DOUBLE, "
        + KEY_WEIGHT_CHANGE + " DOUBLE, "
        + KEY_BMI + " DOUBLE, "
        + KEY_BMR + " DOUBLE, "
        + KEY_REQ_CAL + " DOUBLE, "
        + KEY_MON_CAL + " DOUBLE, "
        + KEY_TUES_CAL + " DOUBLE, "
        + KEY_WED_CAL + " DOUBLE, "
        + KEY_THUR_CAL + " DOUBLE, "
        + KEY_FRI_CAL + " DOUBLE, "
        + KEY_SAT_CAL + " DOUBLE, "
        + KEY_SUN_CAL + " DOUBLE ); ";

private static final String CREATE_PRODUCT_TABLE = "CREATE TABLE " + TABLE_PRODUCTS + "( " + KEY_PRODUCT_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
        + KEY_ITEMNAME + " TEXT, "
        + KEY_ITEMCALORIES + " DOUBLE );";

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

}

public void onCreate(SQLiteDatabase db) {
    db.execSQL(CREATE_USER_TABLE);
    db.execSQL(CREATE_PRODUCT_TABLE);
}

public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    Log.w(TAG,
            "Upgrading the database from version " + oldVersion + " to " + newVersion);
    DATABASE_VERSION = 2;
    db.execSQL("DROP TABLE IF IT EXISTS " + TABLE_USERS);
    db.execSQL("DROP TABLE IF IT EXISTS " + TABLE_PRODUCTS);
    onCreate(db);

}

最佳答案

编辑:确保初始化扩展SQLiteOpenHelper的类。

请确保您调用:

 db = new DatabaseHandler(this);

如果你没有初始化db，那么当你调用db.userLogin(usernameinput);时，它将为空，这可能是的原因>NullPointerException 你得到的。

你可以像这样在`onCreate()'中调用它:

public class LoginPage extends ActionBarActivity {
        Button loginbutton;
        EditText usernameuser, passworduser;
        DatabaseHandler db;
        String usernameinput, passwordinput;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_login_page);

            db = new DatabaseHandler(this); //initialize the DatabaseHandler
            //........

此外，您似乎只查询数据库的三列，并尝试访问游标中的二十列。

此外，您应该检查 cursor.moveToFirst() 的返回值。

一般来说，光标永远不会为空。

这可能会使其在短期内工作，但您可能需要考虑重构代码，这样您就不必查询所有行来使其工作:

public Users userLogin(String username) {
    Users users = null;
    SQLiteDatabase db = this.getReadableDatabase();
    //String[] projection = {KEY_USER_ID, KEY_USERNAME, KEY_PASSWORD};
    String selection = KEY_USERNAME + " =?";
    String[] selectionargs = {username};
    Cursor cursor = db.query(TABLE_USERS, null, selection, selectionargs, null, null,null );
    if (cursor.moveToFirst()){

            users = new Users(
                    cursor.getInt(0),
                    cursor.getString(1),
                    cursor.getString(2),
                    cursor.getInt(3),
                    cursor.getString(4),
                    cursor.getString(5),
                    cursor.getDouble(6),
                    cursor.getDouble(7),
                    cursor.getDouble(8),
                    cursor.getDouble(9),
                    cursor.getDouble(10),
                    cursor.getDouble(11),
                    cursor.getDouble(12),
                    cursor.getDouble(13),
                    cursor.getDouble(14),
                    cursor.getDouble(15),
                    cursor.getDouble(16),
                    cursor.getDouble(17),
                    cursor.getDouble(18),
                    cursor.getDouble(19));
    }
    cursor.close();
    return users;

}

理想情况下，您可以为您的 Users 类创建另一个构造函数，该构造函数可以仅采用本例中所需的三个参数。

修改后的代码将如下所示:

public Users userLogin(String username) {
    Users users = null;
    SQLiteDatabase db = this.getReadableDatabase();
    String[] projection = {KEY_USER_ID, KEY_USERNAME, KEY_PASSWORD};
    String selection = KEY_USERNAME + " =?";
    String[] selectionargs = {username};
    Cursor cursor = db.query(TABLE_USERS, projection, selection, selectionargs, null, null,null );
    if (cursor.moveToFirst()){

            users = new Users(
                    cursor.getInt(0),
                    cursor.getString(1),
                    cursor.getString(2))
    }
    cursor.close();
    return users;

}

此外，最好检查 userLogin() 函数的 null 返回值:

loginbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                usernameinput = usernameuser.getText().toString();
                passwordinput = passworduser.getText().toString();
                Users userlogin = db.userLogin(usernameinput);
                if (userlogin  != null && usernameinput.equals(userlogin.get_username()) && passwordinput.equals(userlogin.get_password())) {
                    startActivity(new Intent(getApplicationContext(), Home_Page.class));
                }
                else {
                    Toast.makeText(getApplicationContext(), "Error", Toast.LENGTH_SHORT).show();
                }
            }

        });

关于java - 用户使用 SQLite 数据库登录 java.lang.NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/29502471/

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