java strings concat

Question

I have got a list of string and now I want to take the list into a String with comma(,) separation is concat method the only way or there is some easy and good way

StringBuilder s = null;
List<String> listObjs (5 strings in it)
for(String listObj : listObjs)
s.append(listObj);
s.append(",");

EDIT: guys at SO rocks....slew of answers in seconds..wow!!

• Less: I think you should use StringBuilder (as mentioned many times) for this kind of string manipulations

Answer 1

You can do

StringBuilder sb = new StringBuilder();
String sep = "";
for (String s : listObjs) {
 sb.append(sep).append(s);
 sep = ", ";
}
System.out.println(sb);

• assylias: Not sure if that's more or less efficient than the boolean check (probably counting nanoseconds here!)
• Peter Lawrey: @assylias its quite a bit shorter. ;) It is useful when the first "sep" is not empty e.g. String sep = "[ ";

Answer 2

you may want to use Apache Commons StringUtils.join method.

Answer 3

Indeed use a StringBuilder:

StringBuilder sb = new StringBuilder();
for (String string : listObjs) {
 sb.append(string);
 sb.append(", ");
}
// Remove the dangling ", "
int length = sb.length();
sb.delete(length - 2, length);
String combined = sb.toString();

Answer 4

Use a StringBuilder.

StringBuilder sb = new StringBuilder();
for (String obj : listObjs) {
 sb.append(String.format("%s,", obj));
 //or without format
 //sb.append(obj).append(",");
}
sb.deleteCharAt(sb.length()-1); // The last ","
System.out.println(sb.toString());

• assylias: you need to remove the trailing comma.
• Peter Lawrey: Format is rather expensive. Do you really want a trailing ", "?

Answer 5

concat() does not modify your s reference (which is null by the way, so you should expect a NullPointerException for s.concat()). You should initialize s with "" and then append to it using s = s.concat(other) or s += other. Or better, use a StringBuilder which is far more efficient for this kind of usecase:

StringBuilder sb = new StringBuilder();
for (String obj : objList) {
 if (sb.length() == 0) {
 sb.append(',');
 }
 sb.append(obj);
}

• lowLatency: thnx for the info ...even i knew it theoretically..but practically you enlightened me

Answer 6

Using concat is not a good idea as it will create a new String at each loop. You can use a StringBuilder instead which is more efficient from a memory perspective.

public static String getCsvFrom(java.util.Collection<?> objects) {
 if (objects == null || objects.isEmpty()) {
 return "";
 }
 StringBuilder s = new StringBuilder();
 boolean firstLoop = true;
 for (Object o : objects) {
 if (!firstLoop) {
 s.append(",");
 } else {
 firstLoop = false;
 }
 s.append(o);
 }
 return s.toString();
}

Answer 7

I wonder why you haven't tried this:

String str = "";
str += listObjs.get(0);
for (int i = 1; i < listObjs.size() - 1; i++ ) {
 str += "," + listObjs.get(i);
}
str += listObjs.get(listObjs.size() - 1);