Set<String> set_1;
Set<String> set_2;

StringBuilder builder = new StringBuilder();
for (String str : set_1) {
  builder.append(str).append(" ");
}

this.string_1 = builder.toString();

builder = new StringBuilder();
for (String str : set_2) {
  builder.append(str).append(" ");
}

this.string_2 = builder.toString();

String str_1 = StringUtils.join(set_1, " ");

Joiner.on(" ").join(set_1);

Joiner.on(" ").skipNulls().join(set_1);
// If 2nd item was null, would produce "1, 3"

Joiner.on(" ").useForNull("<unknown>").join(set_1);
// If 2nd item was null, would produce "1, <unknown>, 3"

List<String> strings = new LinkedList<>();
 strings.add("Java");strings.add("is");
 strings.add("cool");
 String message = String.join(" ", strings);
 //message returned is: "Java is cool"

 Set<String> strings = new LinkedHashSet<>();
 strings.add("Java"); strings.add("is");
 strings.add("very"); strings.add("cool");
 String message = String.join("-", strings);
 //message returned is: "Java-is-very-cool"

mySet.toString().replaceAll("\[|\]","").replaceAll(","," ");

public static String join(Set<String> set, String sep) {
    String result = null;
    if(set != null) {
        StringBuilder sb = new StringBuilder();
        Iterator<String> it = set.iterator();
        if(it.hasNext()) {
            sb.append(it.next());
        }
        while(it.hasNext()) {
            sb.append(sep).append(it.next());
        }
        result = sb.toString();
    }
    return result;
}