I learnt something about Java’s string over here.

Basically, it says,

String data = "";
String s;
while ( (s=reader.readLine()) != null) {
    data += s + " ";
//  After compiler:
//    data = new StringBuffer(data).append(s).append(" ").toString();

in other words, all String objects are not mutable — you can only create a new one with modified content and delete the old one. Hence string manipulation involves a lot of malloc-free operations and consequently, inefficient. Thus the better way to do is to use StringBuffer objects which is expandable dynamically. For example:

StringBuffer data = new StringBuffer();
String s;
while ( (s=reader.readLine()) != null) {
    data.append(s+" ");

Note, the problem of inefficiency happens only when you manipulate the string, hence the concatenation in the append() function in the code above doesn’t matter.