How to convert List to Set (ArrayList to HashSet)

Collection object has a constructor that accept a Collection object to initial the value. Since both Set and List are extend the Collection, the conversion is quite straightforward. It’s just pass a List into Set constructor or vice verse.
List to Set in Java

Given a list (ArrayList or LinkedList), convert it into a set(HashSet or TreeSet) of strings in Java.

Method 1 (Simple)
We simply create an list. We traverse the given set and one by one add elements to the list.

// Java program to demonstrate conversion of // list to set using simple traversal import java.util.*; class Test {     public static void main(String[] args)     {         // Creating a list of strings         List<String> aList = Arrays.asList("lea", "for",                      "learnhowtocode", "learnhowtocode.info", "GFG");         Set<String> hSet = new HashSet<String>();         for (String x : aList)             hSet.add(x);         System.out.println("Created HashSet is");         for (String x : hSet)             System.out.println(x);         // We can created TreeSet same way     } } Method 2 (Using HashSet or TreeSet Constructor) // Java program to demonstrate conversion of // list to set using constructor import java.util.*; class Test {     public static void main(String[] args)     {         // Creating a list of strings         List<String> aList = Arrays.asList("Geeks", "for",                      "Quiz", "learnhowtocode", "GFG");         // Creating a hash set using constructor         Set<String> hSet = new HashSet<String>(aList);         System.out.println("Created HashSet is");         for (String x : hSet)             System.out.println(x);         System.out.println("Created TreeSet is");         Set<String> tSet = new TreeSet<String>(aList);         System.out.println("Created TreeSet is");         for (String x : tSet)             System.out.println(x);     } } Method 3 (Using addAll method) // Java program to demonstrate conversion of // Set to array using addAll() method. import java.util.*; class Test {     public static void main(String[] args)     {         // Creating a list of strings         List<String> aList = Arrays.asList("Geeks", "for",                     "GeeksQuiz", "learnhowtocode", "GFG");         Set<String> hSet = new HashSet<String>(aList);         hSet.addAll(aList);         System.out.println("Created HashSet is");         for (String x : hSet)             System.out.println(x);         Set<String> tSet = new TreeSet<String>(aList);         tSet.addAll(aList);         System.out.println("Created TreeSet is");         for (String x : tSet)             System.out.println(x);     } } Method 4 (Using stream in Java) We use stream in Java to convert given list to stream, then stream to set. This works only in Java 8 or versions after that. // Java program to demonstrate conversion of // Set to list using stream import java.util.*; import java.util.stream.*; class Test {     public static void main(String[] args)     {         // Creating a list of strings         List<String> aList = Arrays.asList("Geeks", "for",                     "GeeksQuiz", "learnhowtocode", "GFG");         // Converting to set using stream         Set<String> set = aList.stream().collect(Collectors.toSet());         for (String x : set)             System.out.println(x);     } }

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About Mariano

I'm Ethan Mariano a software engineer by profession and reader/writter by passion.I have good understanding and knowledge of AngularJS, Database, javascript, web development, digital marketing and exploring other technologies related to Software development.