在Android中使用SqLite進行身份驗證時出現空指針異常

Question

執行查詢以進行身份​​驗證時，會提供空值。我已經研究了這麼多，但我沒有找到答案。請解決這個，因爲我是在Android的初學者。

我的活動是

public class MainActivity extends ActionBarActivity { 

//private ImageView _imagView; 
//private Timer _timer; 
//private int _index; 
//private MyHandler handler; 

DbHandler db=new DbHandler(this); 
EditText usertxt; 
EditText passtxt; 

String auth=""; 
String utxt=""; 
String ptxt=""; 
long i; 
Cursor cur; 


@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.fragment_main); 

    //handler= new MyHandler(); 
    // _imagView=(ImageView) findViewById(R.id.imageView1); 

    // _index=1; 
    // _timer= new Timer(); 
    // _timer.schedule(new TickClass(), 850, 1500); 

    //......................................// 



     Log.d("Insert: ", "Inserting .."); 


     db.deleteAll(); 

     db.addContact(new ModelAuth(" Sohaib ", " Tariq "," User ")); 

    // Reading all contacts 
    Log.d("Reading: ", "Reading all contacts.."); 
    List<ModelAuth> contacts = db.getAllContacts();  

    for (ModelAuth cn : contacts) { 
     String log = "Id: "+ cn.get_id() + " ,Username: " + cn.getUsername() + " ,Password: " + cn.getPassword() +" ,Auth:" 
       + cn.getAuth(); 
      // Writing Contacts to log 
    Log.d("Name: ", log); 
} 




/* if(i<0 || i == 0) { 
     Toast.makeText(getApplicationContext(), "Not Inserting", Toast.LENGTH_LONG).show(); 
    } 
    else{ 
     Toast.makeText(getApplicationContext(), " Inserted", Toast.LENGTH_LONG).show(); 
    }*/ 


    usertxt= (EditText)findViewById(R.id.editText1); 
    passtxt=(EditText)findViewById(R.id.editText2); 

    utxt = usertxt.getText().toString(); 
    ptxt = passtxt.getText().toString(); 



    Button button=(Button)findViewById(R.id.button1); 
    button.setOnClickListener(new View.OnClickListener() { 

     @Override 
     public void onClick(View v) { 



       utxt = usertxt.getText().toString(); 
       ptxt = passtxt.getText().toString(); 

       auth=db.Authen(utxt,ptxt); 

           if(auth.equals("User")){ 

            Intent intent=new Intent(MainActivity.this,Wheels.class); 
            startActivity(intent); 
             } 

           if(auth.equals("Admin")){ 

            Intent intent=new Intent(MainActivity.this,Admin.class); 
            startActivity(intent); 
       } 

     } 
    }); 



} 

HbHelper類是

public class DbHandler extends SQLiteOpenHelper{ 


// All Static variables 
// Database Version 
private static final int DATABASE_VERSION = 1; 

// Database Name 
private static final String DATABASE_NAME = "carshow"; 

// Contacts table name 
private static final String TABLE_AUTH = "auth_table"; 

// Contacts Table Columns names 
private static final String KEY_ID = "_id"; 
private static final String KEY_USERNAME = "username "; 
private static final String KEY_PASSWORD = "password "; 
private static final String KEY_AUTH = "auth "; 


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

} 

// ......... CRUD操作............. ...... //

@Override 
public void onCreate(SQLiteDatabase db) { 
    String CREATE_AUTH_TABLE = "CREATE TABLE " + TABLE_AUTH + "(" 
      + KEY_ID + " INTEGER PRIMARY KEY," + KEY_USERNAME + " TEXT," 
      + KEY_PASSWORD + " TEXT," + KEY_AUTH + " TEXT" + ");"; 
    db.execSQL(CREATE_AUTH_TABLE); 

} 

@Override 
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 

    db.execSQL("DROP TABLE IF EXISTS " + TABLE_AUTH); 
     onCreate(db); 

} 


public void addContact(ModelAuth authen) { 



    SQLiteDatabase db = this.getWritableDatabase(); 

    ContentValues values = new ContentValues(); 
    values.put(KEY_USERNAME, authen.getUsername()); 
    values.put(KEY_PASSWORD, authen.getPassword()); 
    values.put(KEY_AUTH, authen.getAuth()); 

    // Inserting Row 
    db.insert(TABLE_AUTH, null, values); 
    db.close(); // Closing database connection 


} 

    public String Authen(String usertxt,String passtxt){ 

    SQLiteDatabase db = this.getReadableDatabase(); 

    String aut=""; 

    String query = "SELECT * FROM " + TABLE_AUTH + " WHERE " + KEY_USERNAME + " = '" + usertxt + "' AND " + KEY_PASSWORD + " = '" + passtxt + "' " ; 

    Cursor c= db.rawQuery(query, null); 

    if(c!=null && c.moveToFirst()){ 

      c.getString(3); 

      return aut; 
    } 

     if (c != null && !c.isClosed()) { 
      c.close(); 
     } 
     if (db!=null){ 
      db.close(); 
     } 



    return null; 
     } 

logcat的

05-12 12:01:02.977: E/AndroidRuntime(304): FATAL EXCEPTION: main 
05-12 

12:01:02.977: E/AndroidRuntime(304): java.lang.NullPointerException 
05-12 

12:01:02.977: E/AndroidRuntime(304): at      

com.activity.carsshow.MainActivity$1.onClick      

(MainActivity.java:113) 

05-12 12:01:02.977: E/AndroidRuntime(304): at        

      android.view.View.performClick(View.java:2408) 
05-12 12:01:02.977: 

E/AndroidRuntime(304): at android.view.View      

$PerformClick.run(View.java:8816) 

Answer 1

你應該把支票字符串比較前值：像下面

if(auth != null){ 
    if(auth.equals("User")){ 
     Intent intent=new Intent(MainActivity.this,Wheels.class); 
     startActivity(intent); 
    } 

    if(auth.equals("Admin")){ 
     Intent intent=new Intent(MainActivity.this,Admin.class); 
     startActivity(intent); 
    } 
}