Java Set to List

In this section, we will discuss how to convert Set (HashSet or TreeSet) into List (ArrayList or LinkedList).

Java Set to List

There are the following ways to convert Set to List in Java:

Native Approach
Using Constructor (ArrayList or LinkedList)
Using ArrayList addAll() Method
Using Stream in Java

Native Approach

The logic for the native approach is very simple. In this approach, we simply create a Set (HashSet or TreeSet) and iterate over the Set and add all the elements of the Set to the list with the help of a loop.

SetToListExample1.java

  import java.util.*;  public class SetToListExample1  {  public static void main(String args[])   {  //creating a Set of type String   Set set = new HashSet();  //adding elements to the Set  set.add(“Bordeaux”);  set.add(“Cannes”);  set.add(“Chamonix”);  set.add(“Chartres”);  set.add(“Clermont-Ferrand”);  set.add(“Limoges”);  set.add(“Marseille”);  set.add(“Nice”);  set.add(“Paris”);  //determines the size of Set  int n = set.size();  //creates an ArrayList of size n  List list = new ArrayList(n);  //loop iterates over Set  for (String x : set)  //adds elements to the list  list.add(x);  System.out.println(“ArrayList is: “);  //loop iterates over List  for (String x : list)  //prints the elements  System.out.println(x);  }  }  

Output:

 ArrayList is:   Cannes  Bordeaux  Marseille  Nice  Clermont-Ferrand  Chartres  Limoges  Chamonix  Paris

Using Constructor (ArrayList or LinkedList Class)

The logic is the same as above. The only difference is that we have used the constructor of the ArrayList and LinkedList class and passed set elements to the constructor.

SetToListExample2.java

  import java.util.*;  public class SetToListExample2   {  public static void main(String args[])   {  //creating a HashSet of type String  Set set= new HashSet();  //adding elements to the Set  set.add(“Sharjah”);  set.add(“Dubai”);  set.add(“Abu Dhabi”);  set.add(“Kalba”);  set.add(“Dhaid”);  set.add(“Hatta”);  //creates an ArrayList using constructor and passing elements to the constructor   List list = new ArrayList(set);  System.out.println(“ArrayList is: “);  //loop iterates over the list  for (String x : list)  //prints the ArrayList  System.out.println(x);  System.out.println(“nLinkedList is: “);  //creates LinkedList using constructor and passing elements to the constructor   List lList = new LinkedList(set);  //loop iterates over LinkedList  for (String x : lList)  //prints the LinkedList  System.out.println(x);  }  }  

Output:

ArrayList is:   Sharjah  Dhaid  Kalba  Hatta  Dubai  Abu Dhabi    LinkedList is:   Sharjah  Dhaid  Kalba  Hatta  Dubai  Abu Dhabi

Using ArrayList.addAll() Method

The ArrayList.addAll() method appends all the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection’s Iterator. It overrides the addAll() method of the AbstractCollection class.

SetToListExample3.java

  import java.util.*;  public class SetToListExample3  {  public static void main(String args[])   {  //converting HashSet to ArrayList  //creating a HashSet of type String  Set set = new HashSet();  //adding elements to the Set  set.add(“Texas”);  set.add(“California”);  set.add(“Illinois”);  set.add(“Dallas”);  set.add(“San Jose”);  set.add(“California”);  set.add(“Austin”);  set.add(“Columbus”);  //creates a constructor of the ArrayList class  List list = new ArrayList();  //adding the set elements to the list using the addAll() method  list.addAll(set);  System.out.println(“ArrayList is: “);  //loop iterates over the LinkedList  for (String x : list)  //prints the ArrayList  System.out.println(x);  //converting HashSet to LinkedList  List linkList = new LinkedList();  //adding Set elements to the LinkedList using the addAll() method  linkList.addAll(set);  System.out.println(“nLinkedList is: “);  //loop iterates over the LinkedList  for (String x : linkList)  //prints the LinkedList  System.out.println(x);  }  }  

Output:

ArrayList is:   Texas  Illinois  Columbus  California  Austin  Dallas  San Jose    LinkedList is:   Texas  Illinois  Columbus  California  Austin  Dallas  San Jose

Using Stream in Java

If we use Stream to convert Set to List, first, we will convert Set to stream and then convert the stream to list. It works only in Java 8 or later versions.

stream(): The method stream() returns a regular object stream of a set or list.

Stream.collect(): The collect() method of the Stream class is used to accumulate elements of any Stream into a Collection.

Collectors.toList(): The method returns a Collector that collects the input elements into a new List.

Let’s see an example.

SetToListExample4.java

  import java.util.*;  import java.util.stream.*;  public class SetToListExample4  {  public static void main(String args[])   {  //Creating a hash set of strings  Set set = new HashSet();  //adding elements to the Set  set.add(“London”);  set.add(“England”);  set.add(“Wales”);  set.add(“Scotland”);  set.add(“Bristol”);  set.add(“Cambridge”);  //converts set to stream and then stream to list  List list = set.stream().collect(Collectors.toList());  System.out.println(“List is: “);  //loop iterates over the list  for (String x : list)  //prints the list elements   System.out.println(x);  }  }  

Output:

List is:   Cambridge  Bristol  Wales  London  England  Scotland

Next TopicLevel Order Traversal of a Binary Tree in Java

Java Set to List

Java Set to List

Native Approach

Using Constructor (ArrayList or LinkedList Class)

Using ArrayList.addAll() Method

Using Stream in Java

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