Compare two ArrayList and remove duplicates in Java

Prerequisite: ArrayList in Java

Given two ArrayLists, the task is to print all common elements in both the ArrayLists in Java .

Examples:

Input: List1 = ["Hii", "Geeks", "for", "Geeks"], List2 = ["Hii", "Geeks", "Gaurav"] Output: [Hii, Geeks, Geeks] Input: List1 = ["a", "b", "c", "d", "e", "f"], List2 = ["b", "d", "e", "h", "g", "c"] Output:[b, c, d, e]

1. Using Collections.retainAll[] method

   Syntax:

   Collections1.retainAll[Collections2] This method keeps only the common elements of both Collection in Collection1.

   Approach:

   
   
   
   1. Get the two ArrayLists.
   2. Find the common elements in both the Lists using Collection.retainAll[] method. This method keeps only the common elements of both Collection in Collection1.
   3. The List 1 now contains the common elements only.

   Below is the implementation of the above approach:
   Program: By modifying the contents of List1.

   import java.util.ArrayList; public class GFG {     public static void main[String[] args]     {         ArrayList             list1 = new ArrayList[];         list1.add["Hii"];         list1.add["Geeks"];         list1.add["for"];         list1.add["Geeks"];         System.out.println["List1: "                            + list1];         ArrayList             list2 = new ArrayList[];         list2.add["Hii"];         list2.add["Geeks"];         list2.add["Gaurav"];         System.out.println["List2: "                            + list2];         list1.retainAll[list2];         System.out.println["Common elements: "                            + list1];     } }

   Output: List1: [Hii, Geeks, for, Geeks] List2: [Hii, Geeks, Gaurav] Common elements: [Hii, Geeks, Geeks]

   Program 2: By retaining the contents of List1.

   import java.util.ArrayList; public class GFG {     public static void main[String[] args]     {         ArrayList             list1 = new ArrayList[];         list1.add["Hii"];         list1.add["Geeks"];         list1.add["for"];         list1.add["Geeks"];         System.out.println["List1: "                            + list1];         ArrayList             list2 = new ArrayList[];         list2.add["Hii"];         list2.add["Geeks"];         list2.add["Gaurav"];         System.out.println["List2: "                            + list2];         ArrayList             list3 = new ArrayList[list1];         list3.retainAll[list2];         System.out.println["Common elements: "                            + list3];     } }

   Output: List1: [Hii, Geeks, for, Geeks] List2: [Hii, Geeks, Gaurav] Common elements: [Hii, Geeks, Geeks]

2. Using Stream filter

   Syntax:

   list1.stream[] .filter[list2::contains] .collect[Collectors .toList[]]]; This method returns element if found in second list.

   Approach:

   1. First create two ArrayList and add values of list.
   2. Convert the ArrayList to Stream using stream[] method.
   3. Set the filter condition to be distinct using contains[] method.
   4. Collect the filtered values as List using collect[] method. This list will be return common element in both list.
   5. Print list3

   Below is the implementation of the above approach:

   Program:

   import java.util.*; import java.util.stream.*; public class GFG {     public static void main[String[] args]     {         ArrayList             list1 = new ArrayList[];         list1.add["Hii"];         list1.add["Geeks"];         list1.add["for"];         list1.add["Geeks"];         System.out.println["List1: "                            + list1];         ArrayList             list2 = new ArrayList[];         list2.add["Hii"];         list2.add["Geeks"];         list2.add["Gaurav"];         System.out.println["List2: "                            + list2];         System.out.print["Common elements: "];         System.out.println[list1.stream[]                                .filter[list2::contains]                                .collect[Collectors                                             .toList[]]];     } }

   Output: List1: [Hii, Geeks, for, Geeks] List2: [Hii, Geeks, Gaurav] Common elements: [Hii, Geeks, Geeks]

3. Naive approach:
   1. First create two ArrayList and add values of list.
   2. Create a temporary ArrayList to contain common elements.
   3. Iterate through the list1 and check if that element is present in the list2 using ArrayList.contains[] method.
   4. If found, add it to the list3
   5. Print the common elements from list3

   Below is the implementation of the above approach:

   import java.util.ArrayList; public class GFG {     public static void main[String[] args]     {         ArrayList             list1 = new ArrayList[];         list1.add["Hii"];         list1.add["Geeks"];         list1.add["for"];         list1.add["Geeks"];         System.out.println["List1: "                            + list1];         ArrayList             list2 = new ArrayList[];         list2.add["Hii"];         list2.add["Geeks"];         list2.add["Gaurav"];         System.out.println["List2: "                            + list2];         ArrayList             list3 = new ArrayList[];         for [String temp : list1] {             if [list2.contains[temp]] {                 list3.add[temp];             }         }         System.out.println["Common elements: "                            + list3];     } }

   Output: List1: [Hii, Geeks, for, Geeks] List2: [Hii, Geeks, Gaurav] Common elements: [Hii, Geeks, Geeks]

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