How to convert List to Set (ArrayList to HashSet)

By mkyong | Updated: August 30, 2012

Viewed: 5,872 (+16 pv/w)

Tags: arraylist hashset java

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.

Convert List to Set

Set set = new HashSet(list);

Convert Set to List

List list = new ArrayList(set);

1. List To Set Example

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class ConvertListToSet
{
    public static void main( String[] args )
    {
    	System.out.println("List values .....");
    	List<String> list = new ArrayList<String>();
        list.add("1");
        list.add("2");
        list.add("3");
        list.add("4");
        list.add("1");
        
        for (String temp : list){
        	System.out.println(temp);
        }
        
        Set<String> set = new HashSet<String>(list);
        
        System.out.println("Set values .....");
        for (String temp : set){
        	System.out.println(temp);
        }
    }
}

Output

List values .....
1
2
3
4
1
Set values .....
3
2
1
4

After the conversion, all the duplicated values in List will just ignore, because the Set does not allow duplicated values.

2. Set To List Example

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class ConvertSetToList
{
    public static void main( String[] args )
    {
    	System.out.println("Set values .....");
        Set<String> set = new HashSet<String>();
        set.add("1");
        set.add("2");
        set.add("3");
        set.add("4");
        
        for (String temp : set){
        	System.out.println(temp);
        }
        
        System.out.println("List values .....");
    	List<String> list = new ArrayList<String>(set);
        
        for (String temp : list){
        	System.out.println(temp);
        }
    }
}

Output

Set values .....
3
2
1
4
List values .....
3
2
1
4