How to Efficiently Convert a List to an int[] in Java?

Converting List to int[] in Java

When attempting to convert a List to int[], it's common to encounter confusion due to the limitations of List.toArray(). While it returns an Object[], this array cannot be directly cast to int[].

To address this issue, a common approach is to use a loop, as shown in the question. However, Java 8 introduces a more efficient and concise solution using streams.

Using Streams

With the introduction of IntStream, Java 8 provides a convenient way to handle primitive types like int. The following code demonstrates how to convert a List to int[] using streams:

int[] example1 = list.stream().mapToInt(i -> i).toArray();
// OR
int[] example2 = list.stream().mapToInt(Integer::intValue).toArray();

Thought Process

• The simple Stream#toArray returns an Object[] array, which is not desired.
• IntStream is a special stream designed for int values.
• To obtain an IntStream from Stream, we use the mapToInt method.
• mapToInt takes a ToIntFunction, which maps Integer to int. This can be implemented as a lambda or method reference.
• Integer#intValue unboxes the Integer to obtain the underlying int value.
• Alternatively, we can use simple unboxing to replace the lambda in mapToInt, resulting in the more concise example1.

This stream-based approach simplifies the conversion process and eliminates the need for manual loops.

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