CODEWITHSUNDEEP
javajava-7
abuka123
abuka123

Reputation: 451

Convert a list of string into a comma separated string without duplications

I have a list of strings in Java. I would like to to insert it into a string with commas, something like:

Input: [option1,option2,option3,option1,option4]
Output: option1,option2,option3,option4

I saw this topic and I really liked this one:

        StringBuilder result = new StringBuilder();
        String delimiter= "";
        for (String i : list) {
            result.append(delimiter).append(i);
            delimiter = ",";
        }
        return result.toString();

But what would be the best way to add support for unique only items? How can I return a string without any duplicates? I could remove duplication from list first but it feels like there is a better solution here.

Upvotes: 2

Views: 7040

Answers (3)

Andreas
Andreas

Reputation: 159135

Extra requirements from comments:

Sorted

If sorting the values is ok, handling case-sensitive is easiest done using a TreeSet with a case-insensitive Comparator, like the String.CASE_INSENSITIVE_ORDER, or by using a Collator for full language support.

static String toString(List<String> list, boolean unique) {
    Collection<String> dataToProcess = list;
    if (unique) {
        dataToProcess = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
        dataToProcess.addAll(list);
    }
    
    StringBuilder result = new StringBuilder();
    String delimiter= "";
    for (String s : dataToProcess) {
        result.append(delimiter).append(s);
        delimiter = ",";
    }
    return result.toString();
}

Test

List<String> list = Arrays.asList("option1", "option3", "OPTION1", "OPTION2", "option4", "option2");
System.out.println(toString(list, false));
System.out.println(toString(list, true));

Output

option1,option3,OPTION1,OPTION2,option4,option2
option1,OPTION2,option3,option4

Notice how the "unique" result is sorted, but the non-unique result is not.

Using Collator

For better language support, use a Collator:

    if (unique) {
        Collator collator = Collator.getInstance(/*Locale.GERMANY*/);
        collator.setStrength(Collator.SECONDARY);
        dataToProcess = new TreeSet<>(collator);
        dataToProcess.addAll(list);
    }

Unsorted

To do it without sorting the values, we'll keep using the TreeSet but build a new list.

    if (unique) {
        Collator collator = Collator.getInstance(/*Locale.GERMANY*/);
        collator.setStrength(Collator.SECONDARY);
        Set<String> set = new TreeSet<>(collator);
        dataToProcess = new ArrayList<>();
        for (String s : list)
            if (set.add(s))
                dataToProcess.add(s);
    }

Output

option1,option3,OPTION1,OPTION2,option4,option2
option1,option3,OPTION2,option4

Upvotes: 0

azro
azro

Reputation: 54168

The best to avoid duplicates is to use a Set

  • TreeSet for alphabetical order

  • LinkedHashSet to keep insertion order (initial order)

StringBuilder result = new StringBuilder();
String delimiter= "";
for (String i : new LinkedHashSet<String>(list)) {
    result.append(delimiter).append(i);
    delimiter = ",";
}
return result.toString();

But you can just do String.join(',', new LinkedHashSet<String>(list))

Back to List

List<String> inputList = (unique) ? new ArrayList<>(new HashSet<>(list)) : list;

Case-insensitive

Set<String> check = new HashSet<String>();
StringBuilder result = new StringBuilder();
String delimiter= "";
for (String i : list) {
    if(!check.contains(i.toLowerCase())){
        result.append(delimiter).append(i);
        delimiter = ",";
        check.add(i.toLowerCase());
    }
}
return result.toString();

Upvotes: 3

deHaar
deHaar

Reputation: 18578

You can stream() the list and collect only distinct values. Afterwards, you can just String.join(...) them to a comma separated String, like this:

public static void main(String[] args) {
    // example values
    List<String> list = new ArrayList<>();
    list.add("option1");
    list.add("option2");
    list.add("option3");
    list.add("option4");
    list.add("option1");
    list.add("option3");
    list.add("option5");

    // stream the list and collect only distinct values in a new list
    List<String> distinctOptions = list.stream()
                                        .distinct()
                                        .collect(Collectors.toList());
    // then create a comma separated String from that list
    String commaSeparatedUniqueStrings = String.join(",", distinctOptions);
    // and print it
    System.out.println(commaSeparatedUniqueStrings);
}

The output is

option1,option2,option3,option4,option5

Upvotes: 0

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