You can collect the entries into a single String as follows:

  String sorted =
        map.entrySet().stream()
           .sorted(Collections.reverseOrder(Map.Entry.comparingByValue()))
           .map(e-> e.getKey().toString() + "=" + e.getValue().toString())
           .collect(Collectors.joining (","));

One simple way is to append your list items in a StringBuilder

List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);

StringBuilder b = new StringBuilder();
list.forEach(b::append);

System.out.println(b);

you can also try:

String s = list.stream().map(e -> e.toString()).reduce("", String::concat);

Explanation: map converts Integer stream to String stream, then its reduced as concatenation of all the elements.

Note: This is normal reduction which performs in O(n²)

for better performance use a StringBuilder or mutable reduction similar to F. Böller's answer.

String s = list.stream().map(Object::toString).collect(Collectors.joining(","));

Ref: Stream Reduction

The official documentation for what you want to do: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html

 // Accumulate names into a List
 List<String> list = people.stream().map(Person::getName).collect(Collectors.toList());

 // Convert elements to strings and concatenate them, separated by commas
 String joined = things.stream()
                       .map(Object::toString)
                       .collect(Collectors.joining(", "));

For your example, you would need to do this:

 // Convert elements to strings and concatenate them, separated by commas
 String joined = persons.stream()
                       .map(Person::getName) // This will call person.getName()
                       .collect(Collectors.joining(", "));

The argument passed to Collectors.joining is optional.

Refer to @jubobs solution link. That is, you could do it this way in your case:

Stream<Character> testStream = Stream.of('a', 'b', 'c');

String result = testStream.collect(Collector.of(
    StringBuilder::new,
    StringBuilder::append,
    StringBuilder::append,
    StringBuilder::toString));

This is more performant then map/castping each character to a String first and then joining, as StringBuilder#append(char c) will cut out that intermediate step.