關於字元編碼的問題

在一個activity裡面有一個文本輸入框，我在後台通過
EditText et_username = (EditText) findViewById(R.id.username);
String username = new String(et_username.getText().toString());
這樣拿到的字串不是utf-8.我嘗試這樣：
username = new String(et_username.getText().toString().getBytes("UTF-8"));
拿到的字串依然不是utf-8希望牛人賜教怎麼能夠從activity擷取utf-8字元集的字串？

 

String keyword = new String(keyword_EditText.getText()
                                .toString().getBytes("UTF-8"));
                        keyword = URLEncoder.encode(keyword_EditText.getText().toString());

 

 

httpost = new HttpPost(API_DefaultURL.replace("{0}", _page));
                httpost.setHeader("Content-Type", "text/plain; charset=utf-8");
                StringEntity entity = new StringEntity(strdata);
                httpost.setEntity(entity);
                DefaultHttpClient client = new DefaultHttpClient();
                HttpResponse response = client.execute(httpost);
                source = EntityUtils.toString(response.getEntity(), "UTF-8");