ListOfPairsJava9.java

public class ListOfPairsJava9 {

    public static void main(String[] args) {
        final Map<String, List<String>> values = new HashMap<>();
        values.put("a", Arrays.asList("1", "2", "3"));
        values.put("b", Arrays.asList("4", "5", "6"));
        values.put("c", Collections.singletonList("7"));

        final List<Map.Entry<String, String>> result = values
                .entrySet()
                .stream()
                .collect(ArrayList::new,
                        (pairs, entry) -> pairs.addAll(entry
                                .getValue()
                                .stream()
                                .map(value -> Map.entry(entry.getKey(), value))
                                .collect(Collectors.toList())),
                        List::addAll);

        System.out.println("result = " + result);
    }
}