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

Converting an ArrayList of Integers to Primitive int Array

In Java, it's possible to convert an ArrayList containing Integer objects to a primitive int[] array. However, attempting to directly cast the ArrayList to an int[], as shown below, will result in a compile-time error:

List<Integer> x = new ArrayList<Integer>();
int[] n = (int[])x.toArray(int[x.size()]);

To successfully convert the ArrayList to a primitive array, there are several ways to approach it:

Using toArray with Primitive Array

One method is to invoke the toArray method of the ArrayList with an array of the desired primitive type as an argument. For instance:

List<Integer> x = new ArrayList<Integer>();
int[] n = new int[x.size()];
x.toArray(n);

This approach involves creating a new primitive array of the appropriate size and passing it to toArray to populate the array with the Integer values.

Using Java 8 Streams

If using Java 8 or later, it's possible to convert the ArrayList to a primitive array using the stream() method with mapToInt:

List<Integer> x = new ArrayList<Integer>();
int[] n = x.stream().mapToInt(Integer::intValue).toArray();

This approach utilizes streams to iterate through the ArrayList, map each Integer object to its primitive int value, and collect the results into an array using toArray.

Other Considerations

When converting an ArrayList containing Integer objects to a primitive array, it's important to consider:

• Null Values: If the ArrayList contains any null values, they will be omitted from the converted array.
• Size: The converted array will have the same size as the ArrayList.
• Boxing/Unboxing: When converting between primitive and object types, boxing and unboxing operations are necessary. This can incur some performance overhead.

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