Using lambda to format Map into String

I think you're looking for something like this:

import java.util.*;
import java.util.stream.*;
public class Test {

    public static void main(String[] args) throws Exception {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "foo");
        map.put(2, "bar");
        map.put(3, "baz");
        String result = map.entrySet()
            .stream()
            .map(entry -> entry.getKey() + " - " + entry.getValue())
            .collect(Collectors.joining(", "));
        System.out.println(result);
    }
}

To go through the bits in turn:

• entrySet() gets an iterable sequence of entries
• stream() creates a stream for that iterable
• map() converts that stream of entries into a stream of strings of the form "key - value"
• collect(Collectors.joining(", ")) joins all the entries in the stream into a single string, using ", " as the separator. Collectors.joining is a method which returns a Collector which can work on an input sequence of strings, giving a result of a single string.

Note that the order is not guaranteed here, because HashMap isn't ordered. You might want to use TreeMap to get the values in key order.