How to Efficiently Convert an ArrayList of Integers to a Primitive int Array in Java?

Converting ArrayList of Integers to Primitive int Array

In Java, there are multiple approaches to convert an ArrayList containing Integer objects to a primitive int[] array.

One method, as attempted in the code snippet provided, is to use the toArray() method. However, this will throw a compile-time error because of incompatible array types. To resolve this, specify the type of array to be returned, as shown below:

int[] n = (int[])x.toArray(new int[x.size()]);

An alternative approach, particularly useful with Java 8, is to leverage the Stream API:

int[] arr = list.stream().mapToInt(i -> i).toArray();

This method converts each element in the list to its corresponding int value, yielding the desired array.

You can also use the intValue method to explicitly convert each Integer object:

int[] arr = list.stream().mapToInt(Integer::intValue).toArray();

To handle potential null references, add a filter to the stream pipeline:

int[] arr = list.stream().filter(Objects::nonNull).mapToInt(i -> i).toArray();

With these techniques, you can efficiently convert an ArrayList of Integers to a primitive int array in Java.

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