Java Program to Combine Two List by Alternatively Taking Elements

A list is an ordered sequence of elements stored together to form a collection. A list can contain duplicate as well as null entries. A list allows us to perform index-based operations, that is additions, deletions, manipulations, and positional access. Java provides an in-built interface <<java.util>> to perform list as well as other class-based functions. 

Cases: There can occur two different scenarios while doing so as per the length of lists

1. If list 2 gets exhausted while adding elements alternatively then the remaining elements of list 1 are the second list is remaining elements of list 1 to be added in the same sequence of occurrence.
2. If list 1 gets exhausted and so on as discussed in the above case vice-versa

So, the Aim is to completely remove the elements from second one and add to first one list and whatever is left will be the second list.

Approach: The following approach is adopted to store elements alternatively in a merged list. 

• Two lists are declared and initialized with a set of elements.
• Two counters, i and j are maintained to iterate over the length of the lists. The loop runs until the shorter length of both the lists.
• An empty list is maintained to store the merged contents of both the lists, in order of list1 followed by list2.
• At the end of the loop, one of the list, that is a shorter one is exhausted. A loop is then used to iterate over the remaining elements of the longer list and store them at the end one by one.
• The data type of the merged list should be similar to the individual lists.

Implementation: Two examples are discussed below considering both integer list and string lists

Example 1: String Lists

Java

// Java Program to Combine Two List
// by Alternatingly Taking Elements

// importing required packages
import java.io.*;
import java.util.*;
import java.util.Iterator;

// Class to access alternate elements
class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating(declaring) list1
        List<String> list1 = new ArrayList<String>();
        // Adding elements to list1
        // Custom inputs
        list1.add("Geeks");
        list1.add("Geeks");
        list1.add("portal");

        // Creating(declaring) list2
        List<String> list2 = new ArrayList<String>();

        // Adding elements to list2
        // Custom inputs
        list2.add("for");
        list2.add("is CSE");
        list2.add("portal");

        // Display message
        System.out.print("List1 contents: ");

        // Iterating over List1
        Iterator iterator = list1.iterator();

        // Condition check using hasNext() which holds true
        // till there is single element remaining in the
        // List
        while (iterator.hasNext()) {

            // Printing elements of List 1
            System.out.print(iterator.next() + " ");
        }

        // Next Line
        System.out.println();

        // Display message
        System.out.print("List2 contents: ");

        // Iterating over List 2
        iterator = list2.iterator();

        // Condition check using hasNext() which holds true
        // till there is single element remaining in the
        // List
        while (iterator.hasNext()) {

            // Printing elements of List 2
            System.out.print(iterator.next() + " ");
        }

        // Declaring counters
        int i = 0;
        int j = 0;

        // Creating(declaring) merged List
        List<String> merged_list = new ArrayList<String>();

        // Iterating over both the lists until
        // the elements of shorter List are exhausted
        while (i < list1.size() && j < list2.size()) {

            // Step 1: Adding List1 element
            merged_list.add(list1.get(i));

            // Step 2: Adding List2 element
            merged_list.add(list2.get(j));

            // Incrementing counters
            i++;
            j++;
        }

        // Iterating over the remaining part of List1
        while (i < list1.size()) {
            merged_list.add(list1.get(i));

            // Incrementing List1 counter
            i++;
        }

        // Iterating over the remaining part of List2
        while (j < list2.size()) {
            merged_list.add(list2.get(j));

            // Incrementing List1 counter
            j++;
        }

        // Next line
        System.out.println();

        // Display message
        System.out.print("Merged List contents: ");

        // Iterators
        iterator = merged_list.iterator();

        // Iterating over merged List using hasNext() method
        // which holds true till there is single element
        // remaining
        while (iterator.hasNext()) {

            // Printing merged list contents
            System.out.print(iterator.next() + " ");
        }
    }
}

Output

List1 contents: Geeks Geeks portal 
List2 contents: for is CSE portal 
Merged List contents: Geeks for Geeks is CSE portal portal 

Example 2: Integer Lists

Java

// Java Program to Combine Two List
// by Alternatingly Taking Elements

// importing required packages
import java.io.*;
import java.util.*;
import java.util.Iterator;

// Class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {
        // Creating(declaring) List1
        List<Integer> list1 = new ArrayList<Integer>();
        // Adding elements to List1
        // Custom inputs
        list1.add(2);
        list1.add(4);
        list1.add(6);

        // Creating(declaring) List2
        List<Integer> list2 = new ArrayList<Integer>();

        // Adding elements to List2
        // Custom inputs
        list2.add(1);
        list2.add(3);
        list2.add(5);
        list2.add(7);

        // Display message
        System.out.print("List1 contents: ");

        // Iterating over List1
        Iterator iterator = list1.iterator();

        // ConditionCheck using hasNext() method which hold
        // true till single element in remaining List
        while (iterator.hasNext()) {

            // Printing List1 contents
            System.out.print(iterator.next() + " ");
        }

        // New line
        System.out.println();

        // Display message
        System.out.print("List2 contents: ");
        iterator = list2.iterator();

        // ConditionCheck using hasNext() method which hold
        // true till single element in remaining List
        while (iterator.hasNext()) {

            // Printing List2 contents
            System.out.print(iterator.next() + " ");
        }
        // Setting counters to zeros
        int i = 0;
        int j = 0;

        // Creating(declaring) merged list
        List<Integer> merged_list
            = new ArrayList<Integer>();

        // Iterating over both the lists
        // until the shorter list
        while (i < list1.size() && j < list2.size()) {

            // Step 1: Adding List2 element
            merged_list.add(list2.get(j));

            // Step 2: Adding List1 element
            merged_list.add(list1.get(i));

            // Incrementing counters
            i++;
            j++;
        }

        // Iterating over the remaining part of List1

        // Case 1: Input: ShorterList following BiggerList
        while (i < list1.size()) {

            // Merge remaining List to List1, and
            // making List2 final as NULL List
            merged_list.add(list1.get(i));
            i++;
        }

        // Case 2: Input: BiggerList following ShorterList
        while (j < list2.size()) {

            // Merge remaining List to List1,an d
            // making List2 -> NULL List
            merged_list.add(list2.get(j));
            j++;
        }

        // New line
        System.out.println();

        // Display message
        System.out.print("Merged List contents: ");

        // Iterating over merged list
        iterator = merged_list.iterator();

        // Condition check using hasNext() method which
        // holds true till there is single element remaining
        // in the List
        while (iterator.hasNext()) {

            // Printing merged List contents i.e
            // FinalList = List1 + List2(Null final List)
            System.out.print(iterator.next() + " ");
        }
    }
}

Output

List1 contents: 2 4 6 
List2 contents: 1 3 5 7 
Merged List contents: 1 2 3 4 5 6 7