Find common elements in two ArrayLists in Java
Last Updated :
05 Dec, 2022
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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]
- Using Collections.retainAll() method
Syntax:
Collections1.retainAll(Collections2) This method keeps only the common elements of both Collection in Collection1.
Approach:- Get the two ArrayLists.
- Find the common elements in both the Lists using Collection.retainAll() method. This method keeps only the common elements of both Collection in Collection1.
- The List 1 now contains the common elements only.
Java // Java Program to find common elements // in two ArrayLists // Using retainAll() method // import ArrayList package import java.util.ArrayList; public class GFG { // main method public static void main(String[] args) { // create ArrayList list1 ArrayList<String> list1 = new ArrayList<String>(); // Add values in ArrayList list1.add("Hii"); list1.add("Geeks"); list1.add("for"); list1.add("Geeks"); // print list 1 System.out.println("List1: " + list1); // Create ArrayList list2 ArrayList<String> list2 = new ArrayList<String>(); // Add values in ArrayList list2.add("Hii"); list2.add("Geeks"); list2.add("Gaurav"); // print list 2 System.out.println("List2: " + list2); // Find the common elements list1.retainAll(list2); // print list 1 System.out.println("Common elements: " + list1); } }
Output:Program 2: By retaining the contents of List1.List1: [Hii, Geeks, for, Geeks] List2: [Hii, Geeks, Gaurav] Common elements: [Hii, Geeks, Geeks]
Java // Java Program to find common elements // in two ArrayLists // Using retainAll() method // import ArrayList package import java.util.ArrayList; public class GFG { // main method public static void main(String[] args) { // create ArrayList list1 ArrayList<String> list1 = new ArrayList<String>(); // Add values in ArrayList list1.add("Hii"); list1.add("Geeks"); list1.add("for"); list1.add("Geeks"); // print list 1 System.out.println("List1: " + list1); // Create ArrayList list2 ArrayList<String> list2 = new ArrayList<String>(); // Add values in ArrayList list2.add("Hii"); list2.add("Geeks"); list2.add("Gaurav"); // print list 2 System.out.println("List2: " + list2); // Create ArrayList list3 ArrayList<String> list3 = new ArrayList<String>(list1); // Store the comparison output // in ArrayList list3 list3.retainAll(list2); // print list 3 System.out.println("Common elements: " + list3); } }
Output:List1: [Hii, Geeks, for, Geeks] List2: [Hii, Geeks, Gaurav] Common elements: [Hii, Geeks, Geeks]
- Using Stream filter
Syntax:
list1.stream() .filter(list2::contains) .collect(Collectors .toList())); This method returns element if found in second list.Approach:- First create two ArrayList and add values of list.
- Convert the ArrayList to Stream using stream() method.
- Set the filter condition to be distinct using contains() method.
- Collect the filtered values as List using collect() method. This list will be return common element in both list.
- Print list3
Java // Java Program to find common elements // in two ArrayLists // Using Stream filter method // import ArrayList package import java.util.*; import java.util.stream.*; public class GFG { // main method public static void main(String[] args) { // create ArrayList list1 ArrayList<String> list1 = new ArrayList<String>(); // Add values in ArrayList list1.add("Hii"); list1.add("Geeks"); list1.add("for"); list1.add("Geeks"); // print list 1 System.out.println("List1: " + list1); // Create ArrayList list2 ArrayList<String> list2 = new ArrayList<String>(); // Add values in ArrayList list2.add("Hii"); list2.add("Geeks"); list2.add("Gaurav"); // print list 2 System.out.println("List2: " + list2); // Find common elements 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]
- Naive approach:
- First create two ArrayList and add values of list.
- Create a temporary ArrayList to contain common elements.
- Iterate through the list1 and check if that element is present in the list2 using ArrayList.contains() method.
- If found, add it to the list3
- Print the common elements from list3
Java // Java Program to find common elements // in two ArrayLists // Using Stream filter method // import ArrayList package import java.util.ArrayList; public class GFG { // main method public static void main(String[] args) { // create ArrayList list1 ArrayList<String> list1 = new ArrayList<String>(); // Add values in ArrayList list1.add("Hii"); list1.add("Geeks"); list1.add("for"); list1.add("Geeks"); // print list 1 System.out.println("List1: " + list1); // Create ArrayList list2 ArrayList<String> list2 = new ArrayList<String>(); // Add values in ArrayList list2.add("Hii"); list2.add("Geeks"); list2.add("Gaurav"); // print list 2 System.out.println("List2: " + list2); // Create ArrayList list3 ArrayList<String> list3 = new ArrayList<String>(); // Find common elements // while iterating through list1 for (String temp : list1) { // Check if the element is // present in list2 or not if (list2.contains(temp)) { // Since present, add it to list3 list3.add(temp); } } // print common elements from list 3 System.out.println("Common elements: " + list3); } }
Output:List1: [Hii, Geeks, for, Geeks] List2: [Hii, Geeks, Gaurav] Common elements: [Hii, Geeks, Geeks]
