How to convert an ArrayList containing Integers to primitive int array?

java

prompt-engineering

performance

best-practices

byAlex Kataev·Sep 30, 2024

Jumping straight into the action! Use Java 8's Stream API for an elegant and efficient solution to convert an ArrayList<Integer> to a primitive int[]. You'll need the mapToInt function to unwrap the Integer objects to int and the toArray() function to gather the results.

ArrayList<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3));
// Unboxing like a pro, getting rid of those pesky Integer objects
int[] array = list.stream().mapToInt(i -> i).toArray();

In Java 8, stream() kick-starts the conversion sequence, mapToInt(i -> i) transforms each Integer to int, and toArray() collects the int values into the array. It's like a factory line for producing int[]. Efficiency and elegance in a single line, and who does not love that?

Null-land? No worries!

Null references in your ArrayList are like unwanted party crashers. They sneak in and things get messy. To prevent them from making a scene (causing NullPointerException), apply a filter before the conversion process.

// The magic line that removes party crashers
int[] array = list.stream().filter(Objects::nonNull).mapToInt(i -> i).toArray();

Embrace method references

Just like maple syrup is to pancakes, method references are to code: they make it sweeter and more readable. Use Integer::intValue in place of mapToInt to achieve this.

// To be or not to be (readable), that's no longer a question!
int[] array = list.stream().mapToInt(Integer::intValue).toArray();

Old school rocks: Manual iteration

If you're feeling nostalgic or working with older Java versions, manual iteration is your friend to convert the ArrayList into an int array.

// Who needs Java 8 when you got loops
ArrayList<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3));
int[] array = new int[list.size()];
for (int i = 0; i < list.size(); i++) {
    Integer value = list.get(i);
    // Guess we got to deal with those party crashers here as well
    array[i] = (value != null) ? value : 0;
}

Library magic: Guava and Apache Commons

Get familiar with Google Guava and Apache Commons Lang. Trust me; these libraries rock when it comes to array conversion.

Guava's Ints.toArray(Collection<Integer>): Short and sweet!
Apache's ArrayUtils.toPrimitive(Object[]): Regards to nulls 🕶️

// Using Google Guava
// Guava for the win!
int[] array = Ints.toArray(list);

// Using Apache Commons
// Apache to the rescue
int[] array = ArrayUtils.toPrimitive(list.toArray(new Integer[0]));

Performance: The need for speed

When dealing with large lists or when performance is a priority:

Remember, LinkedList and streams aren't best friends due to sequential access.
Bet on list iterators over get(i) for better performance when iterating ArrayList.

Say NO to direct casting

You might think, "Why not cast toArray() to int[] directly?" It's an Object[] returned by toArray(), which simply doesn't gel with a primitive array. So, keep things classy with iteration or streams.

A few more tips before you go

Type checking and casting

You can't directly cast ArrayList to int[], but you can cast from Integer[] to Object[]. It's tricky, but remember, unboxing is still necessary.

Pre-sizing your array

When going old school with manual iteration, size your int[] array to the ArrayList's size upfront:

// Alloting space for the array, like reserving a table for your guests
int[] array = new int[list.size()];

Clarity vs brevity

Yes, method references like Integer::intValue offer readability. However, sometimes, an explicit lambda (i -> i.intValue()) helps with precision and debugging.

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