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.

Use one of the the StringUtils.join methods in Apache Commons Lang.

import org.apache.commons.lang3.StringUtils;

String result = StringUtils.join(list, ", ");

If you are fortunate enough to be using Java 8, then it's even easier...just use String.join

String result = String.join(", ", list);

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

You can't use myString+separator+Map.Entry::getKey because Map.Entry::getKey is not a string but a method reference. You should instead use a lambda:

entry -> myString + separator + entry.getKey()