Remove Last Comma

Question

I want to join strings together, but when doing this it often happens that there is a comma too many, and therefore I need to remove that comma. In this code, I use the substring to delete the two last characters.

How can this become more elegant?

List<String> paramList = new ArrayList<String>( );
paramList.add( "param1" );
paramList.add( "param2" );
StringBuilder result = new StringBuilder();
for ( String p : paramList )
{
 result.append( p ).append( ", " );
}
String withoutLastComma = result.substring( 0, result.length( ) - ", ".length( ) );
System.err.println( withoutLastComma );

• jimjim: How about instead of adding comma and then removing it just not add comma to the last item? using the index based approach would have been fine. for (int i = 0; i < paramList.size()-1; i++) { result.append( p ).append( ", " ); }
• David Harkness: Swapping the order of appends--result.append(", ").append(p)--allows you to use the simpler extraction: result.substring(", ".length())
• Aaron Hall: Apparently a new best answer is below: codereview.stackexchange.com/a/58588/23451
• phil294: If you don't want to use an external library, your solution provides the best readability imo. From all possibilities, I always use this one (although usually with result.deleteCharAt(result.length) which doesn't fit in here)
• mkumawat10: Rather than using any external library for this purpose, it is better to use Streams library provided in Java 8. Use this - paramList.stream().map(String::valueOf).collect(Collectors.joining())

Answer 1

One may use string utility methods such as StringUtil.join to concatenate elements in an array or a collection object. Consult the StringUtil API's StringUtil.join entry.

For example:

StringUtils.join(["a", "b", "c"], "--") // => "a--b--c"

• Athas: I don't see how something this simple can justify adding a library.
• Adeel Zafar Soomro: @Wes: If you are working with critical production code and do not want to take on unnecessary dependencies (or have a policy requirement not to use external libraries) and have the time to write/test/debug/optimize your own methods, then by all means, roll your own. I made the assumption that OP is looking for a simple solution for a non-critical task.
• David Harkness: Guava has Joiner: Joiner.on(", ").join(paramList). It provides helpful options such as skipping null values, too. Oops, already an answer.
• Kolargol00: Java 8 now also has a String.join() method, see my answer below.
• Bananeweizen: @Athas: By repeating that statement for every new utility method you end up with a self made copy of all the methods of that library. And I don't even ask for the unit tests that you wrote for "something this simple".

Answer 2

for ( String p : paramList )
{
 if (result.length() > 0) result.append( ", " );
 result.append( p );
}

• hometoast: I've always used this method. It's clear. It looks nearly the same in any language. I've never seen where the extra comparison would ever have enough weight per iteration to make a difference.
• Kyralessa: I used this solution in .NET, until I discovered that string has a static Join method. I'm surprised Java doesn't have something similar built-in.
• Andrew Hagner: I like how this answer actually shows how to solve the problem, not just avoid it by using a library method. Not that using the library method is wrong though, its just you still don't see how it is actually solved.
• would_like_to_be_anon: It is exactly what StringUtils.join seems to be doing.

Answer 3

Java 8 provides a String.join() method, so you can do it without depending on an external library.

List<String> paramList = new ArrayList<String>();
paramList.add("param1");
paramList.add("param2");
String withoutLastComma = String.join(", ", paramList);

• rolfl: This answer is being discussed on meta
• Kolargol00: Thanks for the explanation @rolfl. Sorry, I didn't notice the question was so old. :(
• Vogel612: On a realated note you can use Collectors.joining() as well.
• Sᴀᴍ Onᴇᴌᴀ: There was a proposed edit for this answer which was rejected but could be a viable solution - it appears to have the same output
• Alexander Ivanchenko: You can drop the type argument on the right side of assignment: new ArrayList<String>() -> new ArrayList<>(), it is not mandatory since Java 7

Answer 4

I believe it's better to know how to write it and then use a library. I usually prefer to make a check before the loop, thus avoiding to have to check every time in the loop:

int size = paramList.size();
if (size > 0) {
 result.append(paramList.get(0));
 for (int i = 1; i < size; ++i) {
 result.append(", ").append(paramList.get(i));
 }
}

• klaar: Now you have two lines where you append your items to your result, instead of one. This is bad style. Don't duplicate lines for the sake of easiness. It will make it harder to maintain this code properly and faultlessly.

Answer 5

One of possible ways is a using Joiner from Google Guava library:

result = Joiner.on(", ").join(paramList);

Answer 6

That's strange that nobody has mentioned iterator-wise approach so far.

So here it goes:

public static <E> String join(Iterable<E> iterable, String delim) {
 Iterator<E> iterator = iterable.iterator();
 if (!iterator.hasNext()) {
 return "";
 }
 StringBuilder builder = new StringBuilder(iterator.next().toString());
 while (iterator.hasNext()) {
 builder.append(delim).append(iterator.next().toString());
 }
 return builder.toString();
}

No messing with indexes, substringing, etc, etc.

And let's use it:

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7);
System.out.println(join(list, ", "));

Update: NPE-safe approach would be avoid using toString() on next() (thanks @David Harkness):

public static <E> String join(Iterable<E> iterable, String delim) {
 Iterator<E> iterator = iterable.iterator();
 if (!iterator.hasNext()) {
 return "";
 }
 StringBuilder builder = new StringBuilder(iterator.next());
 while (iterator.hasNext()) {
 builder.append(delim).append(iterator.next());
 }
 return builder.toString();
}

• David Harkness: There's no need for the toString() calls on the elements since StringBuilder does that automatically. Plus you're risking an NPE.

Answer 7

What I use is a variable that I initialize as empty and then set inside the loop.

List<String> paramList = new ArrayList<String>( );
paramList.add("param1");
paramList.add("param2");
String separator = "";
StringBuilder result = new StringBuilder();
for (String p : paramList)
{
 result.append(separator)
 result.append(p);
 separator = ", ";
}
System.err.println(result.toString());

• Anthony Simmon: I often use this for languages without "join" functions, like PL/SQL...

Answer 8

A couple of alternate options here - any of such can simply only resolve to making the intentions clear, although it's hardly an issue.

It might be a little clearer, albeit not much, using lastIndexOf:

String withoutLastComma = result.substring(0, result.lastIndexOf(","));

Or just refactor a little, which could be more explanatory:

StringBuilder result = new StringBuilder();
for (int i = 0; i < paramList.size(); i++)
{ 
 result.append(paramList.get(i));
 if (i + 1 != paramList.size())
 result.append(", ");
}
System.err.println(result);

Or, lastly, use a string utility library as linked in other answers provided; though, the thought of doing so brings the term 'sledgehammer to crack a nut' to mind, but it might well be justified depending on what other operations you require to carry out.

• Gertjan: This makes the code more complex. Replacing the foreach with a for statement makes it a bit more complex. However your solution works and is correct, but you have to make sure not bringing in extra complexity.
• Grant Thomas: I would argue that complexity in such a trivial construct is far, far from being reached (though understand where you're coming from in terms of 'more complex') - it is clear and self-explanatory for the everyday / professional programmer (IMO). Even just the following of the original warrants such changes, I think: ( 0, result.length( ) - ", ".length( ) ); And, thanks for the input. :)
• Gertjan: for this sample you are right. But imagine a scenario with a couple of nested loops. In that case you get something like result[i].result[j].result[k]. In those cases a foreach might be a more readable solution because it is easy to make a small mistake like using 1 instead of i or mixup the i,j & k variable.
• Grant Thomas: @Gertjan: I agree, luckily we're not in that situation here. ;)

Answer 9

I like this technique:

private String join(Iterable<?> items, String sep) {
 Iterator<?> iter = items.iterator();
 if (!iter.hasNext()) {
 return "";
 }
 StringBuilder builder = new StringBuilder();
 builder.append(iter.next());
 while (iter.hasNext()) {
 builder.append(sep).append(iter.next());
 }
 return builder.toString();
}

What I like about it is there is no wasted if condition inside the loop.

Here are some unit tests to go with it:

@Test
public void testEmptyCollection() {
 Assert.assertTrue(join(Collections.emptyList(), ", ").isEmpty());
}
@Test
public void testJoinSingleItem() {
 String item = "hello";
 Assert.assertEquals(item, join(Collections.singletonList(item), ", "));
}
@Test
public void testJoinTwoItems() {
 Integer item1 = 4;
 Integer item2 = 9;
 String sep = ", ";
 String expected = item1 + sep + item2;
 Assert.assertEquals(expected, join(Arrays.asList(item1, item2), sep));
}

• David Harkness: Then you'll love Alexey's answer above. ;)
• janos: Ah, too many answers, I overlooked that one. Normally I would have just left some suggestions in comments, instead of a full-blown answer. Too late now, I'll just keep this anyway, for the few small extras I added.

Answer 10

String listString = Arrays.toString(paramList.toArray());
System.err.println( listString );

Will return:

[param1, param2]

This has the added benefit of using the String.valueOf(object) method which will print 'null' in the case of null objects. The Arrays.toString() method is also pretty straight forward if you just want to reimplement it. Removing the brackets:

 int iMax = paramList.size() - 1;
 if (iMax == -1) {
 return "";
 }
 String[] params = paramList.toArray();
 StringBuilder b = new StringBuilder();
 for (int i = 0; ; i++) {
 String param = params[i];
 b.append(param);
 if (i == iMax) {
 return b.toString();
 }
 b.append(", ");
 }

Answer 11

Surprised no one has contributed a unit test specification:

• a useful result should contain max(0, paramList.length() - 1) commas.
• a robust solution should not throw IndexOutOfBoundsException if the list is empty.
• an efficient solution would provide a realistic StringBuilder capacity estimate.

The result can be misleading or useless if any parameter contains a comma. Java8 String.join should be redesigned to flag this "delimiter collision" possibility at compile time, and only accept strings to be joined that can be split again afterwards because they have already been escaped or quoted or do not or cannot contain the delimiter.

• Solomon Ucko: There are a few problems with this (old) answer: 1. This would greatly decrease performance. 2. This is usually unnecessary. 3. "Can be split again" might rely on heuristics which can lead to random errors. 4. There is not necessarily a good way to fix the error flagged.

Answer 12

If you are using java 8, you can use StringJoiner like;

StringJoiner sj = new StringJoiner(",");
for ( String p : paramList )
{
 sj.add(p);
}

Answer 13

When we use stream we do have more flexibility, like
map ⟶ convert any array object to string
filter ⟶ remove when it is empty
join ⟶ add joining character

 //Deduplicate the comma character in the input string
 String[] splits = input.split("\\s*,\\s*");
 return Arrays.stream(splits).filter(StringUtils::isNotBlank).collect(Collectors.joining(", "));

• Alexander Ivanchenko: You can ditch the intermediate variable splits
• Alexander Ivanchenko: "join ⟶ add joining character" - a small correction: joining expects a delimiter of type CharSequence (a super-type of String, StringBuilder, CharBuffer)

Answer 14

As an alternative of manually going through the loop to construct the comma-separated contents of the List, you can take advantage of the List's toString() method alongside with substring method of String.

String contents = paramList.toString(); //returns [param 1, param2]
//remove `[` and `]`
System.out.println(contents.substring(1, contents.length()-1));

Answer 15

Here's a more efficient alternative to the delete method, which just breaks the StringBuilder without asking for chars or positions:

@Test
public void appendTest (){
 final String comma = ", ";
 List<String> paramList = new ArrayList<>();
 paramList.add( "param1" );
 paramList.add( "param2" );
 StringBuilder result = new StringBuilder();
 for (String s : paramList) {
 result.append(s).append(comma);
 }
 if (!paramList.isEmpty()){
 result.setLength(result.length() - comma.length());
 }
 System.out.println(result);
}