Best way to convert list to comma separated string in java

The Separator you are using is a UI component. You would be better using a simple String sep = ", ".

You could count the total length of the string first, and pass it to the StringBuilder constructor. And you do not need to convert the Set first.

Set<String> abc = new HashSet<String>();
abc.add("A");
abc.add("B");
abc.add("C");

String separator = ", ";
int total = abc.size() * separator.length();
for (String s : abc) {
    total += s.length();
}

StringBuilder sb = new StringBuilder(total);
for (String s : abc) {
    sb.append(separator).append(s);
}

String result = sb.substring(separator.length()); // remove leading separator

Since Java 8:

String.join(",", slist);

From Apache Commons library:

import org.apache.commons.lang3.StringUtils

Use:

StringUtils.join(slist, ',');

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