Converting 2d String Array into a Map in Java8-CodePudding

Seems like a very basic question, I want to convert my 2d string array into a Map with List as a Value.

I've tried something like below:

Map<String, List<String>> guestMap = Arrays
    .stream(guests)
    .collect(Collectors.groupingBy(e-> e[1], Collectors.toList()));

My nested array of strings like this

String[][] guests = {
    {"A", "lizzard"},
    {"B", "Gorilla"},
    {"C", "lizzard"},
    {"D", "Gorilla"},
    {"E", "lizzard"},
    {"F", "Gorilla"},
    {"G", "lizzard"},
    {"H", "Monkey"},
    {"I", "lizzard"}
};

I know about a way of solving it using computeIfAbsent(). But I'm looking for a solution purely using Stream API.

CodePudding user response：

Since elements in the stream are of type String[], with your current collector

Collectors.groupingBy(e-> e[1], Collectors.toList())

Map's Values would be of type List<String[]> (whilst you need a List<String>, not a list of arrays). Hence, you need to transform the stream elements received by the downstream Collector before storing them into a List.

As @Rogue has pointed out in the comment you need to use Collector mapping() and the downstream of groupingBy(). And toList() should be in turn provided as the downstream Collector of mapping().

String[][] guests = {
    {"A", "lizzard"},
    {"B", "Gorilla"},
    {"C", "lizzard"},
    {"D", "Gorilla"},
    {"E", "lizzard"},
    {"F", "Gorilla"},
    {"G", "lizzard"},
    {"H", "Monkey"},
    {"I", "lizzard"}
};
        
Map<String, List<String>> guestMap = Arrays.stream(guests)
    .collect(Collectors.groupingBy(
        e -> e[1],
        Collectors.mapping(e -> e[0],
            Collectors.toList())
    ));
    
guestMap.forEach((k, v) -> System.out.println(k   " -> "   v));

Output:

Monkey -> [H]
lizzard -> [A, C, E, G, I]
Gorilla -> [B, D, F]