为什么这样写会出现OutOfMemory错误，而第二段那样写就不会？

URL url = new URL(urlStr);
HttpURLConnection urlConn = (HttpURLConnection) url.openConnection();
BufferedReader buffer = new BufferedReader(new InputStreamReader(urlConn.getInputStream()));
line = buffer.readLine();
while(line != null){
sb.append(line);
}

URL url = new URL(urlStr);
HttpURLConnection urlConn = (HttpURLConnection) url.openConnection();
BufferedReader buffer = new BufferedReader(new InputStreamReader(urlConn.getInputStream()));
while ((line = buffer.readLine()) != null) {
sb.append(line);
}