Easiest way to convert a List to a Set in Java

Java- addAll

set.addAll(aList);

Java- new Object

new HashSet<>(list)

Java-8

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

Using Guva

 Sets.newHashSet(list)

Apache Commons

CollectionUtils.addAll(targetSet, sourceList);

Java 10

var set = Set.copyOf(list);

Remember that, converting from List to Set will remove duplicates from collection because List supports duplicates but Set does not support duplicates in Java.

Direct Conversion : The most common and simple way to convert a List to a Set

// Creating a list of strings
List<String> list = Arrays.asList("One", "Two", "Three", "Four");

// Converting a list to set
Set<String> set = new HashSet<>(list);

Apache Commons Collections : You may also use the Commons Collections API to convert a List to a Set :-

// Creating a list of strings
List<String> list = Arrays.asList("One", "Two", "Three", "Four");

// Creating a set with the same number of members in the list 
Set<String> set = new HashSet<>(4);

// Adds all of the elements in the list to the target set
CollectionUtils.addAll(set, list);

Using Stream : Another way is to convert given list to stream, then stream to set :-

// Creating a list of strings 
List<String> list = Arrays.asList("One", "Two", "Three", "Four"); 

// Converting to set using stream 
Set<String> set = list.stream().collect(Collectors.toSet()); 

The best way to use constructor

Set s= new HashSet(list);

In java 8 you can also use stream api::

Set s= list.stream().collect(Collectors.toSet());

A more Java 8 resilient solution with Optional.ofNullable

Set<Foo> mySet = Optional.ofNullable(myList).map(HashSet::new).orElse(null);

If you use Eclipse Collections:

MutableSet<Integer> mSet = Lists.mutable.with(1, 2, 3).toSet();
MutableIntSet mIntSet = IntLists.mutable.with(1, 2, 3).toSet();

The MutableSet interface extends java.util.Set whereas the MutableIntSet interface does not. You can also convert any Iterable to a Set using the Sets factory class.

Set<Integer> set = Sets.mutable.withAll(List.of(1, 2, 3));

There is more explanation of the mutable factories available in Eclipse Collections here.

If you want an ImmutableSet from a List, you can use the Sets factory as follows:

ImmutableSet<Integer> immutableSet = Sets.immutable.withAll(List.of(1, 2, 3))

Note: I am a committer for Eclipse Collections

In Java 1.8, the stream API can be used to convert a list to set. For example, the code below shows the conversion of a list to a set:

List<String> empList = Arrays.asList("java", "python", ".net", "javaScript", "php");
Set<String> set = empList.stream().collect(Collectors.toSet());
set.forEach(value -> System.out.printf("%s ", value));

In case the list contains objects and I want to create a set out of it:

List<Employee> empList = Arrays.asList(new Employee(1, 1000, "Chandra Shekhar", 6000),
new Employee(2, 1000, "Rajesh", 8000), new Employee(3, 1004, "Rahul", 9000),
new Employee(4, 1001, "Suresh", 12000), new Employee(5, 1004, "Satosh", 7000));

Set<String> set = empList.stream().map(emp -> emp.getName()).collect(Collectors.toSet());
System.out.println(set);