import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class ArrayHolder {
    public static void main(final String... args) {

      // An array of objects that have an array as a field...
      final ArrayHolder[] holders = {new ArrayHolder(), new ArrayHolder(), new ArrayHolder(), new ArrayHolder()};

      // Go through each element in each array the pre-Java 8 way:
      for (ArrayHolder holder : holders) {
         for (Object object : holder.getArray()) {
            // do the thing
         }
      }

      // Do it with Java 8 Streams:
      Arrays.stream(holders)
            .map(ArrayHolder::getArray)
            .flatMap(Arrays::stream)
            .forEach(System.out::println);

      // Works with lists, too.
      final List<ArrayHolder> holderList = Arrays.asList(holders);

      holderList.stream()
              .map(ArrayHolder::getArray)
              .flatMap(Arrays::stream)
              .forEach(System.out::println);

      // If your class has lists instead of arrays,
      // call the List.stream() method in the flatMap
      // method instead:
      holderList.stream()
               .map(ArrayHolder::getList)
               .flatMap(List::stream)
               .forEach(System.out::println);

      // flatMap has compilation errors
      // The method flatMap(Function<? super ArrayHolder,? extends Stream<? extends R>>)
      // in the type Stream<ArrayHolder> is not applicable for the arguments (ArrayHolder::getArray)
      //
      // The type of getArray() from the type ArrayHolder is Object[],
      // this is incompatible with the descriptor's return type: Stream<? extends R>
      holderList.stream()
              .flatMap(ArrayHolder::getArray);

      // flatMap has compilation errors
      // The method flatMap(Function<? super ArrayHolder,? extends Stream<? extends R>>)
      // in the type Stream<ArrayHolder> is not applicable for the arguments (ArrayHolder::getList)
      //
      // The type of getList() from the type ArrayHolder is List<Object>,
      // this is incompatible with the descriptor's return type: Stream<? extends R>
      holderList.stream()
              .flatMap(ArrayHolder::getList);

      // Works with 2D arrays as well:
      final Object[][] objects = new Object[2][2];

      Arrays.stream(objects)
            .flatMap(Arrays::stream);
   }

   public Object[] getArray() {
      return new Object[5];
   }

   public List<Object> getList() {
      return new ArrayList<>();
   }
}