Java 8 groupingby with custom key

Question

I have a series of input Strings in the following format:

typeA:code1,
typeA:code2,
typeA:code3,
typeB:code4,
typeB:code5,
typeB:code6,
typeC:code7,
...

and I need to get a Map<String, List<String>> with the following structure:

typeA, [code1, code2, code3]
typeB, [code4, code5, code6]
typeC, [code7, code8, ...]

The catch is that to generate each type I need to call a function like this one on each input String:

public static String getType(String code)
{
  return code.split(":")[0];  // yes this is horrible code, it's just for the example, honestly
}

I'm pretty confident that Streams and Collectors can do this, but I'm struggling to get the right incantation of spells to make it happen.

Answer 1

Here is one way to do it (assuming the class is named A):

Map<String, List<String>> result = Stream.of(input)
                          .collect(groupingBy(A::getType, mapping(A::getValue, toList())));

If you want the output sorted you can use a TreeMap instead of the default HashMap:

.collect(groupingBy(A::getType, TreeMap::new, mapping(A::getValue, toList())));

Full example:

public static void main(String[] args) {
  String input[] = ("typeA:code1," +
                "typeA:code2," +
                "typeA:code3," +
                "typeB:code4," +
                "typeB:code5," +
                "typeB:code6," +
                "typeC:code7").split(",");

  Map<String, List<String>> result = Stream.of(input)
                    .collect(groupingBy(A::getType, mapping(A::getValue, toList())));
  System.out.println(result);
}

public static String getType(String code) {
  return code.split(":")[0];
}
public static String getValue(String code) {
  return code.split(":")[1];
}

Answer 2

The code becomes simple if you consider what you have omitted, that you need the second part of the split string as well:

Map<String, List<String>> result = Stream.of(input).map(s->s.split(":", 2))
    .collect(groupingBy(a->a[0], mapping(a->a[1], toList())));

(assuming you have a import static java.util.stream.Collectors.*;)

There is nothing wrong with splitting a String into an array, the implementation has even a “fast-path” for the common case you are splitting using a single simple character instead of a complicate regular expression.

Answer 3

Although I was too slow, here is an MCVE showing how this can be solved with Collectors#groupingBy.

There are obviously different options for defining the "classifier" and "mapper". Here I'm simply using String#substring to find the part before and after the ":".

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.toList;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

public class GroupingBySubstringsTest
{
    public static void main(String[] args)
    {
        List<String> strings = new ArrayList<String>();
        strings.add("typeA:code1");
        strings.add("typeA:code2");
        strings.add("typeA:code3");
        strings.add("typeB:code4");
        strings.add("typeB:code5");
        strings.add("typeB:code6");
        strings.add("typeC:code7");

        Map<String, List<String>> result = strings.stream().collect(
            groupingBy(s -> s.substring(0, s.indexOf(":")), 
                mapping(s -> s.substring(s.indexOf(":")+1), toList())));

        for (Entry<String, List<String>> entry : result.entrySet())
        {
            System.out.println(entry);
        }
    }
}