Java : Joining String from List<String> in reverse order

Question

I have List<String> list = Arrays.asList("A", "B", "C");
I want to join all string inside list with delimiter , in reverse order :

//result
String joinedString = "C,B,A";

what is the best approach to achieve this?
currently I use index loop :

String joinedString = "";
List<String> list = Arrays.asList("A", "B", "C");
for (int i = list.size() - 1; i >= 0; i--) {
     String string = list.get(i);
     joinedString = joinedString + string + ",";
}
//to remove ',' from the last string
if(joinedString.length() > 0) {
    joinedString = joinedString.substring(0, joinedString.length() - 1);
}

//Output    
C,B,A

is it? but could I achieve it better without using index loop? — m fauzan abdi
– m fauzan abdi, Commented Jan 26, 2019 at 2:11
You might find How to get a reversed list view on a list in Java? useful if you don't want to modify the original list. — Kevin Ji
– Kevin Ji, Commented Jan 26, 2019 at 3:33

Andreas · Accepted Answer · 2019-01-26 02:24:58Z

5

If you don't want to modify the list by calling Collections.reverse(List<?> list) on it, iterate the list in reverse.

If you don't know the list type, use a ListIterator to iterate the list backwards without loss of performance, e.g. an normal i = 0; i < size() loop over a LinkedList would otherwise perform badly.

To join the values separated by commas, use a StringJoiner (Java 8+).

List<String> list = Arrays.asList("A", "B", "C");

StringJoiner joiner = new StringJoiner(",");
for (ListIterator<String> iter = list.listIterator(list.size()); iter.hasPrevious(); )
    joiner.add(iter.previous());
String result = joiner.toString();

System.out.println(result); // print: C,B,A

answered Jan 26, 2019 at 2:24

Andreas

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3 Comments

Ravindra Ranwala Over a year ago

stackoverflow.com/questions/322715/…

Andrey Lavrukhin Over a year ago

the same IDEA using streams:

String result= Stream.iterate(list.listIterator(list.size()), it -> it)                         /*.takeWhile(ListIterator::hasPrevious) // java 9+ */                         .limit(list.size()) /*java 8 workaround*/                         .map(ListIterator::previous)                         .collect(Collectors.joining(","));

Andreas Over a year ago

@AndreyLavrukhin That is extremely bad stream logic, because you're violating the contract of map() when you call previous(), because it is not a stateless function. To do it correctly, see How to convert an iterator to a stream?

RaminS · Accepted Answer · 2019-01-26 02:13:09Z

3

The simplest is to use Collections#reverse and String#join.

List<String> list = Arrays.asList("A", "B", "C");
Collections.reverse(list);
String joinedString = String.join(",", list);

answered Jan 26, 2019 at 2:13

RaminS

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3 Comments

Andreas Over a year ago

Good thing OP used Arrays.asList(...) and not List.of(...), since this solution wouldn't work otherwise.

RaminS Over a year ago

Can't find List.of by Googling. Got a link to documentation? @Andreas

Andreas Over a year ago

Added in Java 9: List.of(E...) --- Or this link to see all the overloads.

Prashant Pandey · Accepted Answer · 2019-01-26 07:35:15Z

2

Here's one solution using Java 8:

Collections.reverse(list);

//res contains the desired string
String res = list.stream().collect(Collectors.joining(","));

answered Jan 26, 2019 at 7:35

Prashant Pandey

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Comments

Andrey Lavrukhin · Accepted Answer · 2020-07-12 15:21:07Z

2

It may be usefull joining-like collector:

    public static Collector<CharSequence, ?, String> joiningReversed(CharSequence delimiter, CharSequence prefix, CharSequence suffix) {
    return Collector.of(StringBuilder::new,
            (sb, item) -> {
                if (sb.length() != 0) {
                    sb.insert(0, delimiter);
                }
                sb.insert(0, item);
            },
            (a, b) -> {
                throw new UnsupportedOperationException();
            },
            sb -> sb.insert(0, prefix).append(suffix).toString());
    }

and use it in streams:

    List<String> list = Arrays.asList("A", "B", "C");
    String result = list.stream()
                .collect(joiningReversed(",", "", ""));
    System.out.println(result);

answered Jul 12, 2020 at 15:21

Andrey Lavrukhin

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1 Comment

Andrey Lavrukhin Over a year ago

Note: it works for any sequential stream, even undetermined length.

Traian GEICU · Accepted Answer · 2019-01-26 03:58:22Z

1

Using a Supplier

List<String> list = Arrays.asList("A", "B", "C");
int i[] = { list.size() };
Supplier<String> supplier = () -> (i[0] > 0) ? list.get(--i[0]) : null;
String s, reverse = "";
while ((s = supplier.get()) != null) {
        reverse +=","+s;
}
reverse = (reverse.length() > 0) ? reverse = reverse.substring(1): "" ;

or insted of while loop

reverse = list.stream().map(l->supplier.get()).collect(Collectors.joining(","));

edited Jan 26, 2019 at 3:58

answered Jan 26, 2019 at 3:03

Traian GEICU

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Comments

mogbee · Accepted Answer · 2019-01-26 02:24:07Z

0

Another way, using Comparator.

List<String> list = Arrays.asList("A", "B", "C");
list.sort(Comparator.reverseOrder());
String joinedString = String.join(",", list);

answered Jan 26, 2019 at 2:24

mogbee

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2 Comments

Andreas Over a year ago

OP doesn't not want value sorted, just reverse of current order. The fact that the example shown has values in ascending order is arbitrary.

Witold Kaczurba Over a year ago

this is an overkill. it is sufficient to reverse it

TongChen · Accepted Answer · 2019-01-26 03:26:16Z

0

One method use StringBuilder

    StringBuilder sb = new StringBuilder();
    List<String> strs = Arrays.asList("A", "b", "c","d");
    for (int i = 0; i < strs.size(); i++) {
        sb.append(strs.get(i));
        if (i != strs.size() - 1) {
            sb.append(",");
        }
    }
    System.out.println(sb.reverse().toString());

answered Jan 26, 2019 at 3:26

TongChen

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