groupingBy on stream<String[]> - java-stream

Question

I have a situation where i have to generate a Map from List
I have a List as below
List<String> li = new ArrayList<String>();
li.add("A:abc");
li.add("B:xyz");
li.add("C:mno");
li.add("B:bbb");
li.add("A:aaa");
li.add("C:xxx");
I want to generate a Map from this as below
Map<String, String[]> or Map<String, List<String>>
I wrote the below lambda expression
li.stream().map(i->i.split(":")).collect(Collectors.groupingBy(i->i[0]));
but the resultant map is Map<String, List<String[]>> , which is not expected

Answer 1

Try it like this.
stream the list
split the string.
groupingBy on first element of array from split
the mapping to put the second element from array in the list.
List<String> li = new ArrayList<String>();
li.add("A:abc");
li.add("B:xyz");
li.add("C:mno");
li.add("B:bbb");
li.add("A:aaa");
li.add("C:xxx");
Map<String, List<String>> map =
li.stream().map(str -> str.split(":"))
.collect(Collectors.groupingBy(a -> a[0],
Collectors.mapping(a -> a[1],
Collectors.toList())));
map.entrySet().forEach(System.out::println);
prints
A=[abc, aaa]
B=[xyz, bbb]
C=[mno, xxx]

Answer 2

The answer from #WJS is perfect, however, if you want to get a Map<String, String[]> like you asked in the question, you might consider extending it using Collectors.collectingAndThen and List#toArray
Map<String, String[]> result = li.stream()
.map(x -> x.split(":"))
.collect(Collectors.groupingBy(
x -> x[0],
Collectors.mapping(
x -> x[1],
Collectors.collectingAndThen(
Collectors.toList(),
x -> x.toArray(String[]::new)
)
)
));
result.forEach((key, value) -> System.out.println(key + " : " + Arrays.toString(value)));
Prints the expected result
A : [abc, aaa]
B : [xyz, bbb]
C : [mno, xxx]

Answer 3

I want to generate a Map from this as below Map<String, String[]> or
Map<String, List<String>>
You do not need to call .map; just collect the stream using Collectors.groupingBy(s -> s.split(":")[0]).
Demo:
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<String> li = new ArrayList<String>();
li.add("A:abc");
li.add("B:xyz");
li.add("C:mno");
li.add("B:bbb");
li.add("A:aaa");
li.add("C:xxx");
Map<String, List<String>> map =
li.stream().collect(Collectors.groupingBy(s -> s.split(":")[0]));
map.entrySet().forEach(System.out::println);
}
}
Output:
A=[A:abc, A:aaa]
B=[B:xyz, B:bbb]
C=[C:mno, C:xxx]
However, if you are expecting a result like
A=[abc, aaa]
B=[xyz, bbb]
C=[mno, xxx]
you can call .map as you are doing and then group by the first element of the array, obtained as a result of splitting, mapping against the list of the second element of the array.
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<String> li = new ArrayList<String>();
li.add("A:abc");
li.add("B:xyz");
li.add("C:mno");
li.add("B:bbb");
li.add("A:aaa");
li.add("C:xxx");
Map<String, List<String>> map =
li.stream()
.map(s -> s.split(":"))
.collect(
Collectors.groupingBy(
arr -> arr[0], Collectors.mapping(
arr -> arr[1], Collectors.toList()
)
)
);
map.entrySet().forEach(System.out::println);
}
}