Mapping a list to Map Java 8 stream and groupingBy

Question

I have this simple Bean class:

public class Book {     

public Book(Map attribute) {
    super();
    this.attribute = attribute;
}
//         


        

Answer 1

You could do it like this:

Map<String, List<String>> library = 
    books.stream()
         .flatMap(b -> b.getAttribute().entrySet().stream())
         .collect(groupingBy(Map.Entry::getKey, 
                             mapping(Map.Entry::getValue, toList())));

From the Stream<Book>, you flat map it with the stream of each map it contains so that you have a Stream<Entry<String, String>>. From there you group the elements by the entries' key and map each entry to its value that you collect into a List for the values.