Create Map from two lists with some filtering

September 18, 2022

I got two Lists as follows

List<String> keys = List.of("A1", "B1", "A2", "B2");
List<List<String>> values = List.of(List.of("A1", "B1"), List.of("A2", "B2"), List.of("A1", "B2"), List.of("A2", "B1"));

And I want to make a Map from those two Lists at the declaration time.

Map<String, List<List<String>>> result = Map.ofEntries(
    Map.entry("A1", List.of(List.of("A1", "B1"), List.of("A1", "B2"))),
    Map.entry("A2", List.of(List.of("A2", "B2"), List.of("A2", "B1"))),
    Map.entry("B1", List.of(List.of("A1", "B1"), List.of("A2", "B1"))),
    Map.entry("B2", List.of(List.of("A2", "B2"), List.of("A1", "B2")))
)

As you can see, each map entry‘s value gathers values entry containing the key’s value.

I tried to make this Map object using stream api, map method and filter method.

But I couldn’t make it.

Show me the best way to make this one.

>Solution :

First stream the keys, and use the toMap collector. You specify what the keys and values of the map are, in terms of an element in the keys list.

The keys of the map are just the elements of the keys list, unchanged, and that the values of the map are the (inner) lists of values that contains the element in the keys list. This is where you can stream the values list and filter.

var result = keys.stream().collect(Collectors.toMap(
    key -> key, // or Function.identity()
    key -> values.stream().filter(value -> value.contains(key)).toList()
));