how to check if the lists in a list have the same length

Question

I have a list of lists in Java. 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);

How to check if the lists that exist in myList have the same length? I know that I can check the length of each list by something like myList.get(0).size() , so what is an efficient and short way to check if all these lists have the same length (instead of checking one by one)?

Answer 1

You could use Stream.allMatch() by matching any contained list size (for example the first one) with all other contained lists :

boolean isSameLength = 
myList.stream()
      .allMatch(l -> l.size() == myList.get(0).size())

It makes the first comparison helpless as it compares the same contained list but it is more readable that :

boolean isSameLength = 
myList.stream()
      .skip(1)
      .allMatch(l -> l.size() == myList.get(0).size())

Answer 2

You cannot hope to say that n lists have the same length without visiting them. You have to go one-by-one. davidxxx's answer shows a really nice way with streams but visits all the lists anyway. The naive way may be

public static boolean sameLength(List<List> lists){
    if(lists.get(0) == null){ //EDIT: as davidxxx pointed out this if is superfluous
        return true;
    }
    int len = lists.get(0).size();
    for (List list : lists) {
        if(list.size() != len){
            return false;
        }
    }
    return true;
}

My IDE even suggests using functional operators with the above code like davidxxx said.

Answer 3

If you use Java 8, you can use lambda expressions to simplify your application. Look:

if (myList.stream().filter(integers -> integers.size() == myList.get(0).size()).count() == myList.size()) {
    System.out.println("The sub-lists have the same size");
}

Answer 4

You can also check using a counter variable, iterate your myList and get the size of sublist . If size of myList == sublist then assign true else assign false like following:

public static boolean isSameLength(List<List<Integer>> myList) {
    boolean isSame = false;
    for (List<Integer> sublist : myList) {
        if (sublist.size() == myList.size()) {
            isSame = true;
        } else {
            isSame = false;
        }
    }
    return isSame;
}

Answer 5

I propose the following:

myList.stream()
    .map(e -> e.size())
    .distinct()
    .limit(2)  // optimization
    .count() == 1;

Basically checking if there are any 2 lists with a different size.

Answer 6

listOfList.stream().map(list -> list.size()).distinct().count() == 1