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, ], 
   ]