สอบถาม Error จาก Android Check Login Username Password หน่อยครับ

package me.sensu.payap.payap;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
import android.app.AlertDialog;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.StatusLine;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONException;
import org.json.JSONObject;

import android.os.Bundle;
import android.os.StrictMode;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.util.Log;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private EditText username;
    private EditText password;
    private Button btnLogin;

    private static final String url = "http://payap.sensu.me/rests/signin";

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

        if (android.os.Build.VERSION.SDK_INT > 9) {
            StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
            StrictMode.setThreadPolicy(policy);
        }

        username = (EditText) findViewById(R.id.username);
        password = (EditText) findViewById(R.id.password);
        btnLogin = (Button) findViewById(R.id.btnLogin);

        btnLogin.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
         final AlertDialog.Builder ad = new AlertDialog.Builder(this);

        switch (v.getId()) {
            case R.id.btnLogin :
                List<NameValuePair> params = new ArrayList<NameValuePair>();
                params.add(new BasicNameValuePair("username", this.username.getText().toString()));
                params.add(new BasicNameValuePair("password", this.password.getText().toString()));

                String ServerResponse  = getHttpPost(this.url,params);

                String status = "";
                String userid = "";
                String errors = "";

                JSONObject c;

                try{
                    c = new JSONObject(ServerResponse);
                } catch (JSONException e) {
                    e.printStackTrace();
                }

                if (status == "warning") {
                    ad.setTitle("Error");
                    ad.setPositiveButton("Close", null);
                    ad.setMessage(errors);
                    ad.show();

                    username.setText("");
                    password.setText("");
                } else {
                    // Success
                }

                break;
        }
    }

    public String getHttpPost(String url,List<NameValuePair> params) {
        StringBuilder str = new StringBuilder();
        HttpClient client = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(url);

        try {
            httpPost.setEntity(new UrlEncodedFormEntity(params));
            HttpResponse response = client.execute(httpPost);
            StatusLine statusLine = response.getStatusLine();
            int statusCode = statusLine.getStatusCode();
            if (statusCode == 200) { // Status OK
                HttpEntity entity = response.getEntity();
                InputStream content = entity.getContent();
                BufferedReader reader = new BufferedReader(new InputStreamReader(content));
                String line;
                while ((line = reader.readLine()) != null) {
                    str.append(line);
                }
            } else {
                Log.e("Log", "Failed to download result..");
            }
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str.toString();
    }

}

<uses-permission android:name="android.permission.INTERNET" />