How do I convert a Java 8 IntStream to a List?

java

java-8

collections

performance

byAlex Kataev·Aug 18, 2024

To swiftly convert a Java 8 IntStream to a List<Integer>, use boxed() followed by collect(Collectors.toList()):

List<Integer> myList = IntStream.of(1, 2, 3).boxed().collect(Collectors.toList());

The boxed() method wraps primitive int values into Integer objects and collect(Collectors.toList()) converts these into the final List<Integer>.

Upgrade for Java 16 users

In Java 16, reduce your typed characters and acrobatics with the smoother toList method :

List<Integer> myList = IntStream.of(1, 2, 3).boxed().toList();

This nifty method provides an unmodifiable List, scaling up your performance game.

Eclipse Collections: The Ferrari of conversions

Consider high performance Eclipse Collections when you need to live life in the fast lane:

MutableIntList intList = IntStream.of(1, 2, 3).collect(
        MutableIntList::new, 
        MutableIntList::add, 
        MutableIntList::addAll);

Why? It shuns the annoying boxing overhead. Bye unnecessary baggage!

Manual driving: custom collectors

If you like to drive manual or just staying classy with functional programming, create a List<Integer> using custom collector functions:

List<Integer> myList = IntStream.of(1, 2, 3)
                                .collect(ArrayList::new, 
                                         ArrayList::add, 
                                         (list1, list2) -> list1.addAll(list2));

Comment: Because some of us like it old school.

When immutability is a religion

To enforce immutability post Java 8 and pre Java 16:

List<Integer> immutableList = Collections.unmodifiableList(
    IntStream.of(1, 2, 3).boxed().collect(Collectors.toList())
);

This shields your List from meddlesome mods, throwing UnsupportedOperationException if any dares.

Before vs. After

Before: 🔢➡️🔢➡️🔢➡️🔢  // Marathon
After :  🚌 [🔢, 🔢, 🔢, 🔢] // Relax, your transit is here!

Stream to List: Faster than your food delivery!

Dealing with int array refugees

If you have an int array begging for conversion, Arrays.stream is your hero:

int[] numbers = {1, 2, 3};
List<Integer> myList = Arrays.stream(numbers).boxed().collect(Collectors.toList());

Comment: Who said arrays can't be elegant?

For the data behemoths

Handling large datasets? Choose your collection based on usage. Index-based access? Definitely List. Else, consider Set/Map based on data uniqueness and access needs.