Find the min and max number from a Collection using Java streams

Question

Below is the code snippet which, as expected, is failing at compile time.

What I really want to do is to find the min and max from all the lists using streams.

public class Delete {

   public static void main(String[] args) {

      List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 677, 0);
      List<Integer> list1 = Arrays.asList(11, 12, 23, 34, 25, 77);
      List<Integer> list2 = Arrays.asList(12, 21, 30, 14, 25, 67);
      List<Integer> list3 = Arrays.asList(41, 25, 37, 84, 95, 7);

      List<List<Integer>> largeList = Arrays.asList(list, list1, list2, list3);

      System.out.println(largeList.stream().max(Integer::compare).get());
      System.out.println(largeList.stream().min(Integer::compare).get());
   }

}

Answer 1

You have to flatten the elements of all the List s into a single Stream<Integer> in order for your code to work :

System.out.println(largeList.stream().flatMap(List::stream).max(Integer::compare).get());
System.out.println(largeList.stream().flatMap(List::stream).min(Integer::compare).get());

This is not very efficient, though, since you process the List s twice in order to find both min and max , and you can get the same data (and more) in a single processing by using IntStream::summaryStatistics() :

IntSummaryStatistics stats = largeList.stream().
                                      .flatMap(List::stream)
                                      .mapToInt(Integer::intValue)
                                      .summaryStatistics();
System.out.println(stats.getMin());
System.out.println(stats.getMax());

Answer 2

尝试这个：

largeList.stream().flatMap(List::stream).max(Integer::compare).get();