SoAnswer.java

import static java.lang.System.out;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class SoAnswer {

    public static void main(String[] args) {

        List<List<A>> lists = Arrays.asList(generateAs("hello", 1), generateAs("world", 1),
            generateAs("hello", 2), generateAs("world", 3));

        System.out.println("\ninput");
        lists.stream().flatMap(Collection::stream)
            .forEach(out::println);

        Map<String, List<A>> collect = lists.stream()
            .flatMap(Collection::stream)
            .collect(Collectors.groupingBy(A::getX));

        System.out.println("\noutput");
        collect.entrySet().stream()
            .forEach(out::println);
    }

    public static class A{
        public String x;
        public String y;
        public String z;

        public String getX() {
            return x;
        }

        @Override
        public String toString() {
            return "A{" +
                "x='" + x + '\'' +
                ", y='" + y + '\'' +
                ", z='" + z + '\'' +
                '}';
        }
    }

    private static List<A> generateAs(String x, int num) {
        return IntStream.range(0, num)
            .mapToObj(i -> {
                A a = new A();
                a.x = x;
                return a;
        }).collect(Collectors.toList());
    }

}