Ajuda com :Android e Servlet (WebService)!

public void executaRequisicaoAoServidor() {
        //obtem post que será enviado
        HttpPost post = obtenhaPost();
        JSONObject jsonResposta = null;

        try {
            jsonResposta = executeChamadaComRespostaJSON(httpPost);
            //..faça o que quiser com o json aqui
        } catch (Exception e) {
        }
    }

    public HttpPost obtenhaPost() {
        //requisicao com a url do webservice
        HttpPost httpPost = new HttpPost("http://foo.bar");

        //parâmetros da requisição
        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
        nameValuePairs.add(new BasicNameValuePair("username", usuario));
        nameValuePairs.add(new BasicNameValuePair("password", senha));

        try {
            httpPost.setHeader("Accept", "application/json");
            httpPost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }

        return httpPost;
    }

    public JSONObject executeChamadaComRespostaJSON(HttpPost httpPost) throws IOException, JSONException {
        JSONObject resposta = null;
        HttpResponse response = null;

        response = executePost(httpPost);
        resposta = new JSONObject(EntityUtils.toString(response.getEntity())); //se a resposta do webservice retornar um JSON ele é pego aqui

        return resposta;
    }

    public HttpResponse executePost(HttpPost httpPost) throws IOException, JSONException {
        DefaultHttpClient client = new DefaultHttpClient();
        return client.execute(httpPost); //retorna a resposta do webservice
    }