Java Set to List

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

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<String> set = new HashSet<String>();
//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<String> list = new ArrayList<String>(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.

List<String> list = new ArrayList<String>(set);
List<String> lList = new LinkedList<String>(set);	

SetToListExample2.java

import java.util.*;
public class SetToListExample2 
{
public static void main(String args[]) 
{
//creating a HashSet of type String
Set<String> set= new HashSet<String>();
//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<String> list = new ArrayList<String>(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<String> lList = new LinkedList<String>(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<String> set = new HashSet<String>();
//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<String> list = new ArrayList<String>();
//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<String> linkList = new LinkedList<String>();
//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<String> set = new HashSet<String>();
//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<String> 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

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