How to flatten a Nested List in Java

Points To Remember

• See How to create a Nested List from a String input. We use this class to create a nested list and then we try to flatten this list.

Program : Flatten a Nested List 

Here we write a Java function that will accept a nested ist upto any level of nesting and flatten this list.

import java.util.List;
import java.util.LinkedList;
import java.util.Scanner;

class FlattenList{

  public static void main(String args[]){

    CreateList obj = new CreateList();

    // Get the nested list sample
    List<Object> nestedList = obj.createNestedList();
    System.out.println("-----------------------------------");

    // Print the nested list
    System.out.println("Nested List  = " + nestedList);  
    // Print the sample nested list after flattening the list 
    System.out.println("Flatten List = " + new FlattenList().flattenList(nestedList));

  }
  
  public List<Integer> flattenList(List<Object> nestedList){
     List<Integer> flatList = new LinkedList<Integer>();
     for(Object obj : nestedList){
        if(obj instanceof List) // If the value is a List
          for(Integer integer : flattenList((List)obj)) // traverse the returned list and add it to the list.
            flatList.add(integer); 
        if(obj instanceof Integer) // If the value is an integer number add it to list
          flatList.add((Integer)obj);
     } 
    return flatList;
  }

}

The above program will give the following output

Enter the Nested List
[1,2,3,[4,5],6,[7,[8,9]],10,[11,[12,[13,[14,15],16]]]]
-----------------------------------
Nested List  = [1, 2, 3, [4, 5], 6, [7, [8, 9]], 10, [11, [12, [13, [14, 15], 16]]]]
Flatten List = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]

Algorithm

1. We traverse the List from start to end.
2. At each element we check if the element is a List or Integer.
1. If it is a Integer we add it to the output list.
2. If it is a list we use recursion to flatten this list.

Using the above steps any level of nested list can be flatten.