The simplest way to comma-delimit a list?
JavaPretty PrintJava Problem Overview
What is the clearest way to comma-delimit a list in Java?
I know several ways of doing it, but I'm wondering what the best way is (where "best" means clearest and/or shortest, not the most efficient.
I have a list and I want to loop over it, printing each value. I want to print a comma between each item, but not after the last one (nor before the first one).
List --> Item ( , Item ) *
List --> ( Item , ) * Item
Sample solution 1:
boolean isFirst = true;
for (Item i : list) {
if (isFirst) {
System.out.print(i); // no comma
isFirst = false;
} else {
System.out.print(", "+i); // comma
}
}
Sample solution 2 - create a sublist:
if (list.size()>0) {
System.out.print(list.get(0)); // no comma
List theRest = list.subList(1, list.size());
for (Item i : theRest) {
System.out.print(", "+i); // comma
}
}
Sample solution 3:
Iterator<Item> i = list.iterator();
if (i.hasNext()) {
System.out.print(i.next());
while (i.hasNext())
System.out.print(", "+i.next());
}
These treat the first item specially; one could instead treat the last one specially.
Incidentally, here is how List
toString
is implemented (it's inherited from AbstractCollection
), in Java 1.6:
public String toString() {
Iterator<E> i = iterator();
if (! i.hasNext())
return "[]";
StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
E e = i.next();
sb.append(e == this ? "(this Collection)" : e);
if (! i.hasNext())
return sb.append(']').toString();
sb.append(", ");
}
}
It exits the loop early to avoid the comma after the last item. BTW: this is the first time I recall seeing "(this Collection)"; here's code to provoke it:
List l = new LinkedList();
l.add(l);
System.out.println(l);
I welcome any solution, even if they use unexpected libraries (regexp?); and also solutions in languages other than Java (e.g. I think Python/Ruby have an intersperse function - how is that implemented?).
Clarification: by libraries, I mean the standard Java libraries. For other libraries, I consider them with other languages, and interested to know how they're implemented.
EDIT toolkit mentioned a similar question: https://stackoverflow.com/questions/285523/last-iteration-of-for-loop-in-java
And another: https://stackoverflow.com/questions/156650/does-the-last-element-in-a-loop-deserve-a-separate-treatment
Java Solutions
Solution 1 - Java
Java 8 and later
Using StringJoiner
class, and forEach
method :
StringJoiner joiner = new StringJoiner(",");
list.forEach(item -> joiner.add(item.toString());
return joiner.toString();
Using Stream
, and Collectors
:
return list.stream().
map(Object::toString).
collect(Collectors.joining(",")).toString();
Java 7 and earlier
See also #285523
String delim = "";
for (Item i : list) {
sb.append(delim).append(i);
delim = ",";
}
Solution 2 - Java
org.apache.commons.lang3.StringUtils.join(list,",");
Solution 3 - Java
Java 8 provides several new ways to do this:
- The
join
method onString
. - A
joining
collector for streams from theCollectors
class. - The
StringJoiner
class.
Example:
// Util method for strings and other char sequences
List<String> strs = Arrays.asList("1", "2", "3");
String listStr1 = String.join(",", strs);
// For any type using streams and collectors
List<Object> objs = Arrays.asList(1, 2, 3);
String listStr2 = objs.stream()
.map(Object::toString)
.collect(joining(",", "[", "]"));
// Using the new StringJoiner class
StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.setEmptyValue("<empty>");
for (Integer i : objs) {
joiner.add(i.toString());
}
String listStr3 = joiner.toString();
The approach using streams assumes import static java.util.stream.Collectors.joining;
.
Solution 4 - Java
Joiner.on(",").join(myList)
Solution 5 - Java
If you use the Spring Framework you can do it with StringUtils:
public static String arrayToDelimitedString(Object[] arr)
public static String arrayToDelimitedString(Object[] arr, String delim)
public static String collectionToCommaDelimitedString(Collection coll)
public static String collectionToCommaDelimitedString(Collection coll, String delim)
Solution 6 - Java
Based on Java's List toString implementation:
Iterator i = list.iterator();
for (;;) {
sb.append(i.next());
if (! i.hasNext()) break;
ab.append(", ");
}
It uses a grammar like this:
List --> (Item , )* Item
By being last-based instead of first-based, it can check for skip-comma with the same test to check for end-of-list. I think this one is very elegant, but I'm not sure about clarity.
Solution 7 - Java
There is a pretty way to achieve this using Java 8:
List<String> list = Arrays.asList(array);
String joinedString = String.join(",", list);
Solution 8 - Java
for(int i=0, length=list.size(); i<length; i++)
result+=(i==0?"":", ") + list.get(i);
Solution 9 - Java
String delimiter = ",";
StringBuilder sb = new StringBuilder();
for (Item i : list) {
sb.append(delimiter).append(i);
}
sb.toString().replaceFirst(delimiter, "");
Solution 10 - Java
One option for the foreach loop is:
StringBuilder sb = new StringBuilder();
for(String s:list){
if (sb.length()>0) sb.append(",");
sb.append(s);
}
Solution 11 - Java
StringBuffer result = new StringBuffer();
for(Iterator it=list.iterator; it.hasNext(); ) {
if (result.length()>0)
result.append(", ");
result.append(it.next());
}
Update: As Dave Webb mentioned in the comments this may not produce correct results if the first items in the list are empty strings.
Solution 12 - Java
I usually do this :
StringBuffer sb = new StringBuffer();
Iterator it = myList.iterator();
if (it.hasNext()) { sb.append(it.next().toString()); }
while (it.hasNext()) { sb.append(",").append(it.next().toString()); }
Though I think I'll to a this check from now on as per the Java implementation ;)
Solution 13 - Java
If you can use Groovy (which runs on the JVM):
def list = ['a', 'b', 'c', 'd']
println list.join(',')
Solution 14 - Java
(Copy paste of my own answer from here.) Many of the solutions described here are a bit over the top, IMHO, especially those that rely on external libraries. There is a nice clean, clear idiom for achieving a comma separated list that I have always used. It relies on the conditional (?) operator:
Edit: Original solution correct, but non-optimal according to comments. Trying a second time:
int[] array = {1, 2, 3};
StringBuilder builder = new StringBuilder();
for (int i = 0 ; i < array.length; i++)
builder.append(i == 0 ? "" : ",").append(array[i]);
There you go, in 4 lines of code including the declaration of the array and the StringBuilder.
2nd Edit: If you are dealing with an Iterator:
List<Integer> list = Arrays.asList(1, 2, 3);
StringBuilder builder = new StringBuilder();
for (Iterator it = list.iterator(); it.hasNext();)
builder.append(it.next()).append(it.hasNext() ? "," : "");
Solution 15 - Java
I usually use something similar to version 3. It works well c/c++/bash/... :P
Solution 16 - Java
This is very short, very clear, but gives my sensibilities the creeping horrors. It's also a bit awkward to adapt to different delimiters, especially if a String (not char).
for (Item i : list)
sb.append(',').append(i);
if (sb.charAt(0)==',') sb.deleteCharAt(0);
Inspired by: https://stackoverflow.com/questions/285523/last-iteration-of-for-loop-in-java/669221#669221
Solution 17 - Java
I didn't compile it... but should work (or be close to working).
public static <T> String toString(final List<T> list,
final String delim)
{
final StringBuilder builder;
builder = new StringBuilder();
for(final T item : list)
{
builder.append(item);
builder.append(delim);
}
// kill the last delim
builder.setLength(builder.length() - delim.length());
return (builder.toString());
}
Solution 18 - Java
StringBuffer sb = new StringBuffer();
for (int i = 0; i < myList.size(); i++)
{
if (i > 0)
{
sb.append(", ");
}
sb.append(myList.get(i));
}
Solution 19 - Java
I somewhat like this approach, which I found on a blog some time ago. Unfortunately I don't remember the blog's name/URL.
You can create a utility/helper class that looks like this:
private class Delimiter
{
private final String delimiter;
private boolean first = true;
public Delimiter(String delimiter)
{
this.delimiter = delimiter;
}
@Override
public String toString()
{
if (first) {
first = false;
return "";
}
return delimiter;
}
}
Using the helper class is simple as this:
StringBuilder sb = new StringBuilder();
Delimiter delimiter = new Delimiter(", ");
for (String item : list) {
sb.append(delimiter);
sb.append(item);
}
Solution 20 - Java
I like this solution:
String delim = " - ", string = "";
for (String item : myCollection)
string += delim + item;
string = string.substring(delim.length());
I assume it can make use of StringBuilder too.
Solution 21 - Java
Because your delimiter is ", " you could use any of the following:
public class StringDelim {
public static void removeBrackets(String string) {
System.out.println(string.substring(1, string.length() - 1));
}
public static void main(String... args) {
// Array.toString() example
String [] arr = {"Hi" , "My", "name", "is", "br3nt"};
String string = Arrays.toString(arr);
removeBrackets(string);
// List#toString() example
List<String> list = new ArrayList<String>();
list.add("Hi");
list.add("My");
list.add("name");
list.add("is");
list.add("br3nt");
string = list.toString();
removeBrackets(string);
// Map#values().toString() example
Map<String, String> map = new LinkedHashMap<String, String>();
map.put("1", "Hi");
map.put("2", "My");
map.put("3", "name");
map.put("4", "is");
map.put("5", "br3nt");
System.out.println(map.values().toString());
removeBrackets(string);
// Enum#toString() example
EnumSet<Days> set = EnumSet.allOf(Days.class);
string = set.toString();
removeBrackets(string);
}
public enum Days {
MON("Monday"),
TUE("Tuesday"),
WED("Wednesday"),
THU("Thursday"),
FRI("Friday"),
SAT("Saturday"),
SUN("Sunday");
private final String day;
Days(String day) {this.day = day;}
public String toString() {return this.day;}
}
}
If your delimiter is ANYTHING else then this isn't going to work for you.
Solution 22 - Java
In my opinion, this is the simplest to read and understand:
StringBuilder sb = new StringBuilder();
for(String string : strings) {
sb.append(string).append(',');
}
sb.setLength(sb.length() - 1);
String result = sb.toString();
Solution 23 - Java
In Python its easy
",".join( yourlist )
In C# there is a static method on the String class
String.Join(",", yourlistofstrings)
Sorry, not sure about Java but thought I'd pipe up as you asked about other languages. I'm sure there would be something similar in Java.
Solution 24 - Java
You can also unconditionally add the delimiter string, and after the loop remove the extra delimiter at the end. Then an "if list is empty then return this string" at the beginning will allow you to avoid the check at the end (as you cannot remove characters from an empty list)
So the question really is:
"Given a loop and an if, what do you think is the clearest way to have these together?"
Solution 25 - Java
if (array.length>0) // edited in response Joachim's comment
sb.append(array[i]);
for (int i=1; i<array.length; i++)
sb.append(",").append(array[i]);
Based on https://stackoverflow.com/questions/668952/clearest-way-to-comma-delimit-a-list-java/669831#669831
Using this idea: https://stackoverflow.com/questions/156650/does-the-last-element-in-a-loop-deserve-a-separate-treatment
Solution 26 - Java
public static String join (List<String> list, String separator) {
String listToString = "";
if (list == null || list.isEmpty()) {
return listToString;
}
for (String element : list) {
listToString += element + separator;
}
listToString = listToString.substring(0, separator.length());
return listToString;
}
Solution 27 - Java
public String toString(List<Item> items)
{
StringBuilder sb = new StringBuilder("[");
for (Item item : items)
{
sb.append(item).append(", ");
}
if (sb.length() >= 2)
{
//looks cleaner in C# sb.Length -= 2;
sb.setLength(sb.length() - 2);
}
sb.append("]");
return sb.toString();
}
Solution 28 - Java
None of the answers uses recursion so far...
public class Main {
public static String toString(List<String> list, char d) {
int n = list.size();
if(n==0) return "";
return n > 1 ? Main.toString(list.subList(0, n - 1), d) + d
+ list.get(n - 1) : list.get(0);
}
public static void main(String[] args) {
List<String> list = Arrays.asList(new String[]{"1","2","3"});
System.out.println(Main.toString(list, ','));
}
}
Solution 29 - Java
private String betweenComma(ArrayList<String> strings) {
String united = "";
for (String string : strings) {
united = united + "," + string;
}
return united.replaceFirst(",", "");
}
Solution 30 - Java
Method
String join(List<Object> collection, String delimiter){
StringBuilder stringBuilder = new StringBuilder();
int size = collection.size();
for (Object value : collection) {
size --;
if(size > 0){
stringBuilder.append(value).append(delimiter);
}
}
return stringBuilder.toString();
}
Usage
Given the array of [1,2,3]
join(myArray, ",") // 1,2,3