Java 在Android中使用SqLite进行身份验证时出现空指针异常

当执行查询以进行身份验证时，它给出空值。我已经研究了这么多，但我没有找到答案。请解决这个问题，因为我是android的初学者

我的活动是

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);
                }

        }
    });



}

//。。。。。。。。。积垢操作…………/

@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

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)

---

您应该在比较String:的值之前进行检查，如下所示

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);
    }
}

---

发布您的logcat，因为logcat在这里起主要作用。谢谢，但为什么查询不起作用，因为我的语句非常正确，值正确插入表中。您在用户名和密码中添加了空格。检查

db.addContact（新ModelAuth（“Sohaib”、“Tariq”、“User”）。删除空格，我想你会没事的。现在，在删除空格之后，它又给了我一个错误05-12 13:27:55.587:E/Cursor（380）：结束一个尚未停用或关闭的游标。database=/data/data/com.prac.carsshow/databases/carshow，table=null，query=SELECT*FROM auth_table，其中username='Sohaib'和password='Tariq'完成了！哎哟！很抱歉，我没有放置字符串变量来获取答案。非常感谢，先生！！
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);
    }
}