java - 我想比较用户名是否存在于数据库中

Question

我想从数据库中获取数据并将其与一些输入的文本进行比较，即想检查用户名是否已存在于数据库中。

这是我的 DBHelper 代码。

public Boolean checkusername(String name) throws SQLException {

    Cursor mCursor =
            db.query(true,Login_Table, new String[] { KEY_ROWID, KEY_USERNAME,KEY_STATUS }, KEY_USERNAME

                    + "=" + "'" + name + "'", null, null, null, null, null);



    if (mCursor.moveToFirst()) {

        return true;

    }

    return false;

}

进行比较的代码

public class Registration extends Activity implements OnClickListener, OnItemSelectedListener


{
 // Variable Declaration should be in onCreate()
 private Button mSubmit;
 private Button mCancel;

 private EditText mFname;
 private EditText mLname;
 private EditText mUsername;
 private EditText mPassword;
 private EditText mEmail;
 private Spinner mGender;
 private String Gen;
 boolean check;

private DBHelper DB = new DBHelper(Registration.this);

 @Override
 protected void onCreate(Bundle savedInstanceState) {


  super.onCreate(savedInstanceState);
  setContentView(R.layout.register);

  //Assignment of UI fields to the variables
  mSubmit = (Button)findViewById(R.id.submit);
  mSubmit.setOnClickListener(this);

  mCancel = (Button)findViewById(R.id.cancel);
  mCancel.setOnClickListener(this);

  mFname = (EditText)findViewById(R.id.efname);
  mLname = (EditText)findViewById(R.id.elname);

  mUsername = (EditText)findViewById(R.id.reuname);
  mPassword = (EditText)findViewById(R.id.repass);
  mEmail = (EditText)findViewById(R.id.eemail);


  mGender = (Spinner)findViewById(R.id.spinner1);

  // Spinner method to read the on selected value
  ArrayAdapter<State> spinnerArrayAdapter = new ArrayAdapter<State>(this,
               android.R.layout.simple_spinner_item, new State[] { 
            new State("Male"), 
            new State("Female")});
  mGender.setAdapter(spinnerArrayAdapter);
  mGender.setOnItemSelectedListener(this); 



 }

 public static boolean isvalidEmail(String email) {
        boolean isValid = false;

        String expression = "^[\\w\\.-]+@([\\w\\-]+\\.)+[A-Z]{2,4}$";
        CharSequence inputStr = email;

        Pattern pattern = Pattern.compile(expression, Pattern.CASE_INSENSITIVE);
        Matcher matcher = pattern.matcher(inputStr);
        if (matcher.matches()) {
            isValid = true;
        }
        return isValid;
    }




 public void onClick(View v) 
 {

  switch(v.getId()){

  case R.id.cancel:
   Intent i = new Intent(getBaseContext(), Home.class);
   startActivity(i);
   //finish();
   break;

  case R.id.submit:


   String fname = mFname.getText().toString();
   String lname = mLname.getText().toString();

   String uname = mUsername.getText().toString();
   String pass = mPassword.getText().toString();
   String email = mEmail.getText().toString();
   String status="s";


   boolean invalid = false;
  boolean checkEmail=isvalidEmail(email);



   if(fname.equals(""))
   {
    invalid = true;
    Toast.makeText(getApplicationContext(), "Enter your Firstname", Toast.LENGTH_SHORT).show();
   }
   else
       if(fname.matches("[0-9]+" ))
               {
           invalid = true;
            Toast.makeText(getApplicationContext(), "Enter valid name", Toast.LENGTH_SHORT).show();


               }
   else
       if (fname.matches(".*\\s+.*"))
       {
           invalid = true;
            Toast.makeText(getApplicationContext(), "Spaces are not allowed in Firstname", Toast.LENGTH_SHORT).show();
       }
   else

   if(lname.equals(""))
   {
    invalid = true;
    Toast.makeText(getApplicationContext(), "Please enter your Lastname", Toast.LENGTH_SHORT).show();
   }
   else
       if (fname.matches(".*\\s+.*"))
       {
           invalid = true;
            Toast.makeText(getApplicationContext(), "Spaces are not allowed in Lastname", Toast.LENGTH_SHORT).show();
       }
   else

    if(uname.equals(""))
    {
     invalid = true;
     Toast.makeText(getApplicationContext(), "Please enter your Username", Toast.LENGTH_SHORT).show();
    }
    else
           if (uname.matches(".*\\s+.*"))
           {
               invalid = true;
                Toast.makeText(getApplicationContext(), "Spaces are not allowed in Username", Toast.LENGTH_SHORT).show();
           }
    else


      if(pass.equals(""))
     {
      invalid = true;
      Toast.makeText(getApplicationContext(), "Please enter your Password", Toast.LENGTH_SHORT).show();

   }
      else
       if (pass.matches(".*\\s+.*"))
       {
           invalid = true;
            Toast.makeText(getApplicationContext(), "Spaces are not allowed in Password", Toast.LENGTH_SHORT).show();
       }
     else 
      if(email.equals(""))
   {
    invalid = true;
    Toast.makeText(getApplicationContext(), "Please enter your Email ID", Toast.LENGTH_SHORT).show();
   }
      else if(checkEmail==false)
      {
            invalid = true;
            Toast.makeText(getApplicationContext(), "Please enter valid Email ID", Toast.LENGTH_SHORT).show();
      }
      else

DB.open();

  if (DB.checkusername(uname)) {

      invalid = true;
      Toast.makeText(getApplicationContext(), "username alredy in use enter another", Toast.LENGTH_SHORT).show();

  }

     else
      if(invalid == false)
      {
       addEntry(fname, lname, Gen, uname, pass, email);
       login( uname, pass, status);
       Intent i_register = new Intent(Registration.this, Login.class);
       startActivity(i_register);
       //finish();
      }

      break;
      }


    }





 public void onDestroy()
 {
  super.onDestroy();
  DB.close();
 }

private void login(String username,String password,String status)
{

      SQLiteDatabase db = DB.getWritableDatabase();
      ContentValues values = new ContentValues();
      values.put("username", username); 
      values.put("password", password);
      values.put("status", status);
      try
      {
       db.insert(DBHelper.Login_Table, null, values);
      }
      catch(Exception e)
      {
       e.printStackTrace();
      }
}


 private void addEntry(String fname, String lname, String Gen, String uname, String pass, String email) 
 {

  SQLiteDatabase db = DB.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put("firstname", fname);
  values.put("lastname", lname);
  values.put("gender", Gen);
  values.put("username", uname);
  values.put("password", pass);
  values.put("email", email);


  try
  {
   db.insert(DBHelper.DATABASE_TABLE_NAME, null, values);

   Toast.makeText(getApplicationContext(), "your details submitted Successfully...", Toast.LENGTH_SHORT).show();
  }
  catch(Exception e)
  {
   e.printStackTrace();
  }
 }



}

这是错误日志

07-07 02:02:48.623: E/AndroidRuntime(8297): FATAL EXCEPTION: main
07-07 02:02:48.623: E/AndroidRuntime(8297): java.lang.NullPointerException
07-07 02:02:48.623: E/AndroidRuntime(8297):     at com.google.data.DBHelper.checkusername(DBHelper.java:164)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at com.google.data.Registration.onClick(Registration.java:209)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at android.view.View.performClick(View.java:2408)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at android.view.View$PerformClick.run(View.java:8816)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at android.os.Handler.handleCallback(Handler.java:587)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at android.os.Handler.dispatchMessage(Handler.java:92)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at android.os.Looper.loop(Looper.java:123)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at android.app.ActivityThread.main(ActivityThread.java:4627)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at java.lang.reflect.Method.invokeNative(Native Method)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at java.lang.reflect.Method.invoke(Method.java:521)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-07 02:02:48.623: E/AndroidRuntime(8297):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

似乎没有打开您正在执行查询的数据库。在执行任何操作之前，您需要先打开数据库。

示例:db.open()