How to iterate over a 2D list (list of lists) in Java

Given a 2D list, the task is to iterate this 2D list in Java.

2D list (list of lists)
The 2D list refers to a list of lists, i.e. each row of the list is another list.

[
  [5, 10],
  [1], 
  [20, 30, 40]
] 

Iterate a 2D list: There are two ways of iterating over a list of list in Java.

1. Iterating over the list of lists using loop:
   • Get the 2D list to the iterated
   • We need two for-each loops to iterate the 2D list successfully.
   • In the first for-each loop, each row of the 2D lists will be taken as a separate list

     for (List list : listOfLists)
      { }
     

   • In the second for-each loop, each item of the list in each row will be taken separately

     for (K item : list)
      { }
     

   • Hence we can do any operation on this item. Here we are printing the item.

   Below is the implementation of the above approach:

   
   

   
   
   

   // Java code to demonstrate the concept of // list of lists using loop    import java.util.*; public class List_of_list {        // Iterate the 2D list using loop     // and print each element     public static <K> void     iterateUsingForEach(List<List<K> > listOfLists)     {            // Iterate the 2D list         // and print each element         System.out.println("[");            for (List<K> list : listOfLists) {             System.out.print("  [");                for (K item : list) {                 System.out.print("  "                                  + item                                  + ", ");             }             System.out.println("], ");         }         System.out.println("]");     }        // Driver code     public static void main(String[] args)     {            // List of Lists         ArrayList<List<Integer> > listOfLists             = new ArrayList<List<Integer> >();            // Create N lists one by one         // and append to the list of lists         List<Integer> list1             = new ArrayList<Integer>();         list1.add(5);         list1.add(10);         listOfLists.add(list1);            List<Integer> list2             = new ArrayList<Integer>();         list2.add(1);         listOfLists.add(list2);            List<Integer> list3             = new ArrayList<Integer>();         list3.add(20);         list3.add(30);         list3.add(40);         listOfLists.add(list3);            // Iterate the 2D list         iterateUsingForEach(listOfLists);     } }

   Output:

   [
    [ 5,  10, ], 
    [ 1, ], 
    [ 20,  30,  40, ], 
   ]
   

2. Iterating over the list of lists using iterator:
   • Get the 2D list to the iterated
   • We need two iterators to iterate the 2D list successfully.
   • The first iterator will iterate each row of the 2D lists as a separate list

     Iterator listOfListsIterator = listOfLists.iterator();
     

   • Each row of the 2D list can be obtained with the help of next() method of Iterator

     listOfListsIterator.next();
     

     But the next() method returns the Iterator as an Object’s object. Hence we need to cast this returned object into a list.

     list = (List)listOfListsIterator.next();
     

   • The second iterator will iterate each item of the list in each row separately

     Iterator eachListIterator = list.iterator();
     

   • Hence we can do any operation on this item. Here we are printing the item.

   Below is the implementation of the above approach:

   
   

   
   
   

   // Java code to demonstrate the concept of // list of lists using iterator    import java.util.*;    class List_of_list {        // Iterate the 2D list using Iterator     // and print each element     public static <K> void     iterateUsingIterator(List<List<K> > listOfLists)     {         // Iterator for the 2D list         Iterator listOfListsIterator             = listOfLists.iterator();            System.out.println("[");         while (listOfListsIterator.hasNext()) {                // Type cast next() method             // to convert from Object to List<K>             List<K> list = new ArrayList<K>();                list = (List<K>)listOfListsIterator.next();                // Iterator for list             Iterator eachListIterator                 = list.iterator();                System.out.print("  [");             while (eachListIterator.hasNext()) {                    System.out.print(                     "  "                     + eachListIterator.next()                     + ", ");             }             System.out.println("], ");         }         System.out.println("]");     }        // Driver code     public static void main(String[] args)     {            // List of Lists         ArrayList<List<Integer> > listOfLists             = new ArrayList<List<Integer> >();            // Create N lists one by one         // and append to the list of lists         List<Integer> list1             = new ArrayList<Integer>();         list1.add(5);         list1.add(10);         listOfLists.add(list1);            List<Integer> list2             = new ArrayList<Integer>();         list2.add(1);         listOfLists.add(list2);            List<Integer> list3             = new ArrayList<Integer>();         list3.add(20);         list3.add(30);         list3.add(40);         listOfLists.add(list3);            // Iterate the 2D list         iterateUsingIterator(listOfLists);     } }

   Output:

   [
    [ 5,  10, ], 
    [ 1, ], 
    [ 20,  30,  40, ], 
   ]