1. Overview
In this quick tutorial, we’ll take a look at the conversion between a List and a Set, starting with Plain Java, using Guava and the Apache Commons Collections library, and finally with Java 10.
This article is part of the “Java – Back to Basic” series here on Baeldung.
2. Convert List to Set
2.1. With Plain Java – Using the Constructor
Let’s start with converting a List to a Set using Java:
public void givenUsingCoreJava_whenListConvertedToSet_thenCorrect() {
List<Integer> sourceList = Arrays.asList(0, 1, 2, 3, 4, 5);
Set<Integer> targetSet = new HashSet<>(sourceList);
}
As we can see, the conversion process is type-safe and straightforward since the constructors of each collection do accept another collection as a source.
2.2. With Plain Java – Using Loop
Alternatively, we can convert a List to Set using a loop:
public void givenUsingCoreJava_whenUsingLoopConvertToSet_thenCorrect() {
List<Integer> sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
Set<Integer> targetSet = new HashSet<>();
for (Integer element : sourceList) {
targetSet.add(element);
}
}
In this example, we first initialized a new Set and used a for-each loop to add() all List elements to it.
2.3. With Plain Java – Using the addAll() Method
Instead, we can add all elements from a List to a Set using the addAll() method:
public final void givenUsingCoreJava_whenUsingAddAllToSet_thenCorrect() {
List<Integer> sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
Set<Integer> targetSet = new HashSet<>();
targetSet.addAll(sourceList);
}
It’s worth mentioning that addAll() is a method from the Collection interface. Therefore, it can also be used to convert a List to a Set. We’ll see an example later.
2.4. With Plain Java – Using Stream API
Stream API was introduced in Java 8. It provides many handy methods that allow us to deal with collections easily. Next, let’s convert List to Set using Stream API:
public void givenUsingCoreJava_whenUsingStreamToSet_thenCorrect() {
List<Integer> sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
Set<Integer> targetSet = sourceList.stream().collect(Collectors.toSet());
}
In this example, we use the standard toSet() Collector to convert the List to a Set.
2.5. With Guava
Let’s do the same conversion using Guava:
public void givenUsingGuava_whenListConvertedToSet_thenCorrect() {
List<Integer> sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
Set<Integer> targetSet = Sets.newHashSet(sourceList);
}
2.6. With Apache Commons Collections
Next, let’s use the Commons Collections API to convert between a List and a Set:
public void givenUsingCommonsCollections_whenListConvertedToSet_thenCorrect() {
List<Integer> sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
Set<Integer> targetSet = new HashSet<>(6);
CollectionUtils.addAll(targetSet, sourceList);
}
2.7. With Java 10
One additional option is to use the Set.copyOf static factory method introduced in Java 10:
public void givenUsingJava10_whenListConvertedToSet_thenCorrect() {
List sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
Set targetSet = Set.copyOf(sourceList);
}
Note that a Set created this way is unmodifiable.
3. Convert Set to List
3.1. With Plain Java – Using the Constructor
Now let’s do the reverse conversion, *from a Set to a List, using ArrayList‘s constructor*:
public void givenUsingCoreJava_whenSetConvertedToList_thenCorrect() {
Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
List<Integer> targetList = new ArrayList<>(sourceSet);
}
3.2. With Plain Java – Using Loop
Similarly, we can convert a Set to List using a loop:
public void givenUsingCoreJava_whenUsingLoop_thenCorrect() {
Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
List<Integer> targetList = new ArrayList<>();
for (Integer element : sourceSet) {
targetList.add(element);
}
}
As the code above shows, we loop through the given Set and add() each element to a pre-initialized List.
3.3. With Plain Java – Using the addAll() Method
As we’ve mentioned, since addAll() is a method from the Collection interface, and both Set and List are subtypes of Collection, addAll() is available for Set and List:
public void givenUsingCoreJava_whenUsingAddAll_thenCorrect() {
Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
List<Integer> targetList = new ArrayList<>();
targetList.addAll(sourceSet);
}
As the code shows, addAll() is handy to fill all elements from one Collection to another.
3.4. With Plain Java – Using Stream API
Next, let’s convert Set to List using Stream API:
public void givenUsingCoreJava_whenUsingStream_thenCorrect() {
Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
List<Integer> targetList = sourceSet.stream().collect(Collectors.toList());
}
This time, we use the standard toList() Collector to convert the Set to a List.
3.5. With Guava
We can do the same using the Guava solution:
public void givenUsingGuava_whenSetConvertedToList_thenCorrect() {
Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
List<Integer> targetList = Lists.newArrayList(sourceSet);
}
This is very similar to the java approach, only with a little less duplicated code.
3.6. With Apache Commons Collections
Now let’s see the Commons Collections solution to convert between a Set and a List:
public void givenUsingCommonsCollections_whenSetConvertedToList_thenCorrect() {
Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
List<Integer> targetList = new ArrayList<>(6);
CollectionUtils.addAll(targetList, sourceSet);
}
3.7. With Java 10
Finally, we can use the List.copyOf that’s been introduced in Java 10:
public void givenUsingJava10_whenSetConvertedToList_thenCorrect() {
Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
List<Integer> targetList = List.copyOf(sourceSet);
}
We need to keep in mind that the resulting List is unmodifiable.
4. Conclusion
As always, the implementation of all of these examples and code snippets can be found over on GitHub.