Convert List to Set in Java

This post will discuss how to convert a list to a set in Java. As the set collection contains no duplicate elements, it will discard any repeated elements in the list.

There are three general-purpose implementations of the Set interface provided by JDK — HashSet, TreeSet, and LinkedHashSet. This post uses HashSet, which is very fast and offers constant-time operations.

1. Naive solution

A naive solution is to create an empty set and add every element of the specified list to it.

// Generic method to convert list to set

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

{

// create an empty set

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

// push each list element into the set

for (T e: list) {

items.add(e);

}

// return the set

return items;

}

2. Using Constructor

We can use the HashSet constructor, which can take another collection object to construct a new set containing the elements of the specified list.

// Generic method to convert list to set

public static <T> Set<T> convertToSet(List<T> list) {

return new HashSet<>(list);

}

The constructor will throw NullPointerException if the specified list is null. We can also use TreeSet that gives a sorted set:

// Generic method to convert list to set

public static <T> Set<T> convertToSet(List<T> list) {

return new TreeSet<>(list);

}

This only works if all the list elements implement the Comparable interface and are mutually comparable, i.e., for any pair of elements (a, b) in the list, a.compareTo(b) does not throw a ClassCastException.

If all elements of the list don’t implement Comparable, then we can specify our own Comparator. For this method to work, all the list elements must be mutually comparable using the specified comparator.

// Generic method to convert list to set

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

{

Set<T> items = new TreeSet<>((Comparator<T>)Comparator.reverseOrder());

items.addAll(list);

return items;

}

3. Using Java 8

In Java 8, we can use the Stream to convert a list to a set by converting the specified list to a sequential Stream using List.stream() and using a Collector to accumulate the input elements into a new Set.

// Generic method to convert list to set

public static <T> Set<T> convertToSet(List<T> list) {

return list.stream().collect(Collectors.toSet());

}

Collectors.toSet() doesn’t guarantee on the type of the set returned. We can use Collectors.toCollection() to specify the desired Collection:

// Generic method to convert a list to `HashSet`

public static <T> Set<T> convertToSet(T[] arr) {

return list.stream().collect(Collectors.toCollection(HashSet::new));

}

4. Using Guava Library

We can also use Guava API to convert a list to a set.

1. Sets.newHashSet() creates a mutable HashSet instance containing the specified list elements. We should use this method to add or remove elements later, or some of the elements can be null.

// Generic method to convert list to set

public static <T> Set<T> convertToSet(List<T> list) {

return Sets.newHashSet(list);

}

2. We can also use ImmutableSet.copyOf that returns an immutable set containing the specified list elements. This should be preferred when mutability is not required.

// Generic method to convert list to set

public static <T> Set<T> convertToSet(List<T> list) {

return ImmutableSet.copyOf(list);

}

3. Guava also provides TreeSet implementation, which we can use to create a mutable, empty TreeSet instance sorted according to a Comparator provided at set creation time. We can pass Guava’s Ordering.natural() for the natural ordering of keys.

// Generic method to convert list to set

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

{

TreeSet<T> treeSet = Sets.newTreeSet((Comparator<T>) Ordering.natural());

treeSet.addAll(list);

return treeSet;

}

That’s all about converting List to Set in Java.

Convert List to Set in Java

1. Naive solution

2. Using Constructor

3. Using Java 8

4. Using Guava Library

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