How do I get length of list of lists in Java?

If I have a List<List<String>> data in Java, I can get the length of the first list via code:

int lengthData = data.get(0).size(); 

But how do I get the number of lists in the structure without traversing the list of lists to find out?

Maybe I've been a bit unclear. I have the structure:

List<List<String>> data 

And I see that:

int i = data.size(); 

Will equal 1 because it is the root list. So what I want to know is how many sublists there are. Traversal of the structure like this:

for (List<String> l : data) { total ++; } 

Only gives me a result of 1 which I find odd.

I have data of the form:

List 1 ==> 1, 2, 3, 4 List 2 ==> 3, 8. 9, 1 

And so on where these are sublists of the root list.

4 Answers

Just use

int listCount = data.size(); 

That tells you how many lists there are (assuming none are null). If you want to find out how many strings there are, you'll need to iterate:

int total = 0; for (List<String> sublist : data) { // TODO: Null checking total += sublist.size(); } // total is now the total number of strings 

Java 8

import java.util.Arrays; import java.util.List; import java.util.ArrayList; public class HelloWorld{ public static void main(String []args){ List<List<String>> stringListList = new ArrayList<>(); stringListList.add(Arrays.asList(new String[] {"(0,0)", "(0,1)"} )); stringListList.add(Arrays.asList(new String[] {"(1,0)", "(1,1)", "(1,2)"} )); stringListList.add(Arrays.asList(new String[] {"(2,0)", "(2,1)"} )); int count=stringListList.stream().mapToInt(i -> i.size()).sum(); System.out.println("stringListList count: "+count); } } 

import java.util.ArrayList;

public class TestClass {

public static void main(String[] args) { ArrayList<ArrayList<String>> listOLists = new ArrayList<ArrayList<String>>(); ArrayList<String> List_1 = new ArrayList<String>(); List_1.add("1"); List_1.add("2"); listOLists.add(List_1); ArrayList<String> List_2 = new ArrayList<String>(); List_2.add("4"); List_2.add("5"); List_2.add("10"); List_2.add("11"); listOLists.add(List_2); for (int i = 0; i < listOLists.size(); i++) { System.out.print("list " + i + " :"); for (int j = 0; j < listOLists.get(i).size(); j++) { System.out.print(listOLists.get(i).get(j) + " ;"); } System.out.println(); } } 

}

I hope this solution gives a better picture of list if lists

count of the contained lists in the outmost list

int count = data.size(); 

lambda to get the count of the contained inner lists

int count = data.stream().collect( summingInt(l -> l.size()) );