查找所有包含最大值的索引

Question

I have a list of lists in Java. It is supposed to get the last item of each list and place it in another list named lastItems , then it will get the max value of lastItems and its index. Here is the code:

List<List<Integer>> myList = new ArrayList<>();
myList.add(new ArrayList<Integer>());
myList.add(new ArrayList<Integer>());
myList.add(new ArrayList<Integer>());

myList.get(0).add(1);
myList.get(0).add(2);
myList.get(0).add(3);
myList.get(1).add(4);
myList.get(1).add(5);
myList.get(1).add(6);
myList.get(2).add(7);
myList.get(2).add(8);
myList.get(2).add(9); 
if (!myList.contains(Collections.EMPTY_LIST) && !myList.contains(null) && myList.stream().allMatch(l -> l.size() == L.get(0).size())) {             
     int[] lastItems = myList.stream().mapToInt(x -> x.get(x.size() - 1)).toArray();
     int maxValue = Arrays.stream(lastItems).max().getAsInt();
     int index = IntStream.range(0, lastItems.length).filter(i -> maxValue == lastItems[i]).findFirst().orElse(-1));
     System.out.println(maxValue);
}

but the problem is that IntStream.range(0, lastItems.length).filter(i -> maxValue == lastItems[i]).findFirst().orElse(-1)) finds only the index of the first maxValue in the list, but I want to find all the indexes that may contain maxValue (for example in list = [9, 5, 9] , index = 0 and index=2 contain the maxValue ). How can I do this?

Answer 1

Why not just keep it simple? After finding the maxValue , use a simple for-loop to find out the index of the largest values.

int[] lastItems = myList.stream().mapToInt(x -> x.get(x.size() - 1)).toArray();
int maxValue = Arrays.stream(lastItems).max().getAsInt();

for (int i = 0; i < lastItems.length; i++) {
    if (maxValue == lastItems[i])
        System.out.println("MAx value = " + lastItems[i] + " with index " + i);
}

Answer 2

You may do it like so,

int[] maxIndices = IntStream.range(0, lastItems.length)
    .filter(i -> lastItems[i] == maxValue)
    .toArray();

There's no point of using findFirst if you want to get the indices of all of the max values. Just collect the matching values into a container instead.

If you just want to print it, then it would be,

IntStream.range(0, lastItems.length)
.filter(i -> lastItems[i] == maxValue)
.forEach(System.out::println);