安卓 java在不重新启动的情况下运行活动

Question

我创建了一个简单的应用程序，可以登录到DB并从中获取数据。但是现在我发现了一个问题：如果你第一次写错了登录数据，然后第二次尝试登录，那么它将无法工作。你必须重新启动它

我认为这段代码有一个问题：已编辑

public class MyMoodleApplicationActivity extends Activity {
/** Called when the activity is first created. */
EditText username;
EditText password;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    username = (EditText)findViewById(R.id.username);
    password = (EditText)findViewById(R.id.password);
    final Button loginButton = (Button)findViewById(R.id.login);      
    loginButton.setOnClickListener(loginListener);
    final Button clearButton = (Button)findViewById(R.id.clear);
    clearButton.setOnClickListener(clearListener);
}
private OnClickListener loginListener = new OnClickListener(){

    public void onClick(View v){
        String usr = username.getText().toString();
        String psw = password.getText().toString();
        System.out.println("Username: "+usr);
        System.out.println("Password: "+psw);
        ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
        nameValuePairs.add(new BasicNameValuePair("usern",""+usr));
        nameValuePairs.add(new BasicNameValuePair("passw",""+psw));
        InputStream is = null; 
        String result = "";
        //http post
        try{
                HttpClient httpclient = new DefaultHttpClient();
                HttpPost httppost = new HttpPost("http://ik.su.lt/*****");
                httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
                HttpResponse response = httpclient.execute(httppost);
                HttpEntity entity = response.getEntity();
                is = entity.getContent();
        }catch(Exception e){
                Log.e("log_tag", "Error in http connection "+e.toString());
        }
        //convert response to string
        try{
                BufferedReader reader = new BufferedReader(new InputStreamReader(is,"iso-8859-1"),8);
                StringBuilder sb = new StringBuilder();
                String line = null;
                while ((line = reader.readLine()) != null) {
                        sb.append(line + "\n");
                }
                is.close();

                result=sb.toString();
        }catch(Exception e){
                Log.e("log_tag", "Error converting result "+e.toString());
        }
        String usernameFromDB = "";
        String firstnameFromDB = "";
        String lastnameFromDB = "";
        String emailFromDB = "";
        String phoneFromDB = "";
        String skypeFromDB = "";
        String cityFromDB = "";
        String descriptionFromDB = "";
        try{
                JSONArray jArray = new JSONArray(result);
                for(int i=0;i<jArray.length();i++){
                        JSONObject json_data = jArray.getJSONObject(i);
                        usernameFromDB = json_data.getString("username");
                        firstnameFromDB = json_data.getString("firstname");
                        lastnameFromDB = json_data.getString("lastname");
                        emailFromDB = json_data.getString("email");
                        phoneFromDB = json_data.getString("phone1");
                        skypeFromDB = json_data.getString("skype");
                        cityFromDB = json_data.getString("city");
                        descriptionFromDB = json_data.getString("description");

                        System.out.println(usernameFromDB+ "  " + firstnameFromDB+"  "+lastnameFromDB+"  "
                                + emailFromDB + "  " + phoneFromDB +"  " + skypeFromDB+ "  " + cityFromDB + "  "+
                                descriptionFromDB);
                }
        }
        catch(JSONException e){
            AlertDialog alertDialog = new AlertDialog.Builder(MyMoodleApplicationActivity.this).create();
            alertDialog.setTitle("Klaida!");
            alertDialog.setMessage("Toks vartotojas neegzistuoja");
            alertDialog.setButton("OK", new DialogInterface.OnClickListener() {

                @Override
                public void onClick(DialogInterface dialog, int which) {
                    // TODO Auto-generated method stub
                    return;
                }
            });
            alertDialog.show();
                Log.e("log_tag", "Error parsing data "+e.toString());
        }
        if(usr.length()== 0){
            AlertDialog alertDialog = new AlertDialog.Builder(MyMoodleApplicationActivity.this).create();
            alertDialog.setTitle("Klaida!");
            alertDialog.setMessage("Jūs neįvedėte slapyvardžio");
            alertDialog.setButton("OK", new DialogInterface.OnClickListener() {

                @Override
                public void onClick(DialogInterface dialog, int which) {
                    // TODO Auto-generated method stub
                    return;
                }
            });
            alertDialog.show();
        }
        else if(psw.length()==0){
            AlertDialog alertDialog = new AlertDialog.Builder(MyMoodleApplicationActivity.this).create();
            alertDialog.setTitle("Klaida!");
            alertDialog.setMessage("Jūs neįvedėte slaptažodžio");
            alertDialog.setButton("OK", new DialogInterface.OnClickListener() {

                @Override
                public void onClick(DialogInterface dialog, int which) {
                    // TODO Auto-generated method stub
                    return;
                }
            });
            alertDialog.show();
        }
        else if (usr.equals(usernameFromDB)){
            Intent in = new Intent(getApplicationContext(), LoggedUser.class);
            in.putExtra("firstname", firstnameFromDB);
            in.putExtra("lastname", lastnameFromDB);
            in.putExtra("email", emailFromDB);
            in.putExtra("phone1", phoneFromDB);
            in.putExtra("skype", skypeFromDB);
            in.putExtra("city", cityFromDB);
            in.putExtra("description", descriptionFromDB);

            startActivity(in);
            finish();
    }
}};

private OnClickListener clearListener = new OnClickListener(){
    @Override
    public void onClick(View v){
        username.setText("");
        password.setText("");
    }
};

在这些if语句中，我检查您输入的用户名和密码是否为空用户名和数据库中的用户名不相等。 如何重写代码以使其正常工作

编辑： 我在数据库中有用户john，但如果我在login字段中写john并按下login按钮，它不会说没有john用户。在LogCat中，我看到json流被读取。有没有办法建立一个规则，规定大写字母和小写字母是不同的