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Converting a Map to a List in Java 8, Groovy and Ruby

Posted by rnaufal on 16th July 2016

Some days ago I was developing a task on a Gradle project and I faced with a situation where I had to convert a Map < String, List < String >> to List < Pair >, each pair containing the key and one element from the List.

I decided to compare the solution in three different languages: Java 8 (using lambdas and the Streams API), Groovy and Ruby to see how concise and expressive they would be. Then, I created the Groovy code and it looked like this:

#!/usr/bin/env groovy

def map = ["a" : ["1", "2", "3"], "b" : ["4", "5", "6"], "c" : ["7"]]

println map.collectMany { key, values -> values.collect {value -> [key, value]}}

Running the above code, the result is below:

[[a, 1], [a, 2], [a, 3], [b, 4], [b, 5], [b, 6], [c, 7]]

The Ruby version looked like this:

#!/usr/bin/env ruby

map = { "a" => ["1", "2", "3"], "b" => ["4", "5", "6"], "c" => ["7"] }

p map.collect {|key, values| {|value| [key, value] } }.flatten(1)

The Ruby program generated the following output:

[["a", "1"], ["a", "2"], ["a", "3"], ["b", "4"], ["b", "5"], ["b", "6"], ["c", "7"]]

Below is the Java 8 version, using lambdas, Streams and the Collectors API:

        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"));

        List< Pair > result = values.
                collect(ArrayList< Pair >::new,
                        (pairs, entry) -> entry.
                                forEach(value ->
                                        pairs.add(new Pair(entry.getKey(), value))), List< Pair >::addAll);

private static class Pair {

    private final String first;
    private final String second;

    public Pair(String first, String second) {
        this.first = first;
        this.second = second;

    public String toString() {
        return new StringBuilder("Pair{")
                .append(", second='")

Running the Java 8 version produced the following output:

[Pair{first='a', second='1'}, Pair{first='a', second='2'}, 
Pair{first='a', second='3'}, Pair{first='b', second='4'}, 
Pair{first='b', second='5'}, Pair{first='b', second='6'}, Pair{first='c', second='7'}]

The Groovy and Ruby version are very expressive and concise. Note the use of the collectMany method on the Groovy version and the use of the flatten method on the Ruby version to flatten the result list into a single list of pairs.
The Java 8 version made use of the collect method of the Stream API, to collect the results in a list of Pair instances, each one holding the key and value of each element from the List< String >.

What do you think about this comparison? Leave your comments here!

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Posted in arrays, groovy, java, java8, lambdas, programming, ruby | 2 Comments »