/**
     * this method used to open AlertDialog
     */
    private void openAlertDialog(){
        final AlertDialog.Builder builder = new AlertDialog.Builder(this);
        LayoutInflater inflater = getLayoutInflater();
        View view = inflater.inflate(R.layout.layout_alert_dialog, null);
        builder.setView(view);

        final AlertDialog alertDialog = builder.show();

        final EditText etUsername = (EditText) view.findViewById(R.id.edit_text_username);
        final EditText etPassword = (EditText) view.findViewById(R.id.edit_text_password);
        Button   btnLogin   = (Button)   view.findViewById(R.id.button_login);

        btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                if(TextUtils.isEmpty(etUsername.getText().toString())){
                    Toast.makeText(MainActivity.this, "Username is required", Toast.LENGTH_SHORT).show();
                }else if(TextUtils.isEmpty(etPassword.getText().toString())){
                    Toast.makeText(MainActivity.this, "Password is required", Toast.LENGTH_SHORT).show();
                }else{
                    editor = sharedPreferences.edit();
                    editor.putString("username", etUsername.getText().toString());
                    editor.putString("password", etPassword.getText().toString());
                    editor.commit();
                    alertDialog.dismiss();

                    startActivity(new Intent(MainActivity.this, MainActivity.class));
                    overridePendingTransition(0,0);
                    finish();
                }
            }
        });
    }