How do I convert List to Set in Java

Published May 09, 2022

In Java, the ArrayList class allows duplicate entries while maintaining their order, while the HashSet class uses a Hashtable (an implementation class of Set) to prevent duplicate entries. Here are two ways you can use to convert a list to a set:

a) Using a constructor

b) Using the naïve method

Using a constructor

The simplest method for converting a list into a HashSet. Using this method, we create an ArrayList object and pass it to the constructor of HashSet.

Example

package convert_list_to_set_using_constructor;

import java.util.Arrays;

import java.util.HashSet;

import java.util.List;

import java.util.Set;

public class convert_list_to_set_using_constructor {

public static <T> Set<T> convertListToSet(List<T> list)

{

return new HashSet<>(list);

}

public static void main(String[] args) {

List<String> list = Arrays.asList("Toyota",

"Mazda",

"Mitsubishi",

"Nissan",

"Subaru",

"audi");

System.out.println("Our List: " + list);

Set<String> set = convertListToSet(list);

System.out.println("The Converted Set from List: " + set);

}

Output

Our List: [Toyota, Mazda, Mitsubishi, Nissan, Subaru, audi]

The Converted Set from List: [audi, Toyota, Subaru, Mazda, Mitsubishi, Nissan]

The naïve approach

The naive approach involves creating an empty set and then adding each List element to the newly created Set.

Example

import java.util.Arrays;

import java.util.HashSet;

import java.util.List;

import java.util.Set;

public class Naive_approach_method_to_convert_list_to_set {

public static <T> Set<T> convertListToSet(List<T> list)

{

Set<T> set = new HashSet<>();

for (T t : list)

set.add(t);

return set;

}

public static void main(String[] args) {

List<String> list = Arrays.asList("Mangoes",

"Oranges",

"Apples",

"Kiwi",

"Citrus",

"lemon");

System.out.println(" Our List: " + list);

Set<String> set = convertListToSet(list);

System.out.println("The converted Set: " + set);

}

Output

Our List: [Mangoes, Oranges, Apples, Kiwi, Citrus, lemon]

The converted Set: [Apples, Citrus, lemon, Kiwi, Mangoes, Oranges]

Conclusion

To conclude, we have examined two ways to turn a List in Java into a Set. You may choose to use either approach according to your needs.

Keyword: Convert List to a set

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