Convert list to array in Java

The Java way: Using toArray()

There are two flavors of the built-in toArray() method in Java: the plane-jane toArray() which spits out a plain old Object[], and the more customizable toArray(T[] a) which accepts a type-specific array to determine the returned array's type.

List<Fruit> fruits = List.of(new Fruit("Apple"), new Fruit("Banana"));
Fruit[] fruitsArray = fruits.toArray(new Fruit[0]); // 0-sized but mighty
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Who needs extra peeling, right?

Concurrency issues? No problem!

If there are multiple threads possibly altering your list simultaenously, normal array conversion can lead to access errors or unexpected results.

List<SpaceShip> spaceFleet = Collections.synchronizedList(new ArrayList<>());
// ... hypothetical multithread scenario with lasers
SpaceShip[] spaceFleetArray = spaceFleet.toArray(new SpaceShip[0]); // one fleet, no clashes
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The ConcurrentModificationException ghost doesn't stand a chance!

Primitive clashes: manual boxing

Primitive type lists, such as List<int> or List<long>, do not convert directly to arrays, autoboxing is not their best friend. The solution is to manually box them:

List<Integer> integerList = Arrays.asList(1, 2, 3);
int[] integerArray = integerList.stream().mapToInt(Integer::intValue).toArray(); // Kibosh the boxing!
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It's like putting boxing gloves on integers. Pretty punchy huh?

Expert corner: Advanced tips and Caveats

Still here? Let's dive into the deep end with some additional tips and concerns.

Reflection magic for typed array creation

For those who fancy the darker arts, you can use Array.newInstance() for a more flexible approach:

Integer[] integerArray = integerList.toArray((Integer[]) Array.newInstance(Integer.class, integerList.size()));
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Remember, with great power comes great responsibility!

Java 8 method references ftw

Feeling streamy? Java 8's streams paired with method references give you a neat one-liner:

Double[] doubleArray = doubleList.stream().toArray(Double[]::new); // Slick as a whistle, ain't it?
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Null handling: The gatekeeper

Always, ALWAYS, check for null before you convert a list to an array!

if(yourList == null) return null; // Careful with those null pointers, they bite!
Element[] array = yourList.toArray(new Element[0]);
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Type casting: Get it right!

When it's generics, remember to cast to the correct type:

Element[] array = (Element[]) yourList.toArray(new Element[0]); // Innit, type matters!
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