How to convert List to int[] in Java? [duplicate]

Answer 1

If you are simply mapping an Integer to an int then you should consider using parallelism, since your mapping logic does not rely on any variables outside its scope.

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

Just be aware of this

Note that parallelism is not automatically faster than performing operations serially, although it can be if you have enough data and processor cores. While aggregate operations enable you to more easily implement parallelism, it is still your responsibility to determine if your application is suitable for parallelism.

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There are two ways to map Integers to their primitive form:

1. Via a ToIntFunction.

   mapToInt(Integer::intValue)
   

2. Via explicit unboxing with lambda expression.

   mapToInt(i -> i.intValue())
   

3. Via implicit (auto-) unboxing with lambda expression.

   mapToInt(i -> i)
   

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Given a list with a null value

List list = Arrays.asList(1, 2, null, 4, 5);

Here are three options to handle null:

1. Filter out the null values before mapping.

   int[] arr = list.parallelStream().filter(Objects::nonNull).mapToInt(Integer::intValue).toArray();
   

2. Map the null values to a default value.

   int[] arr = list.parallelStream().map(i -> i == null ? -1 : i).mapToInt(Integer::intValue).toArray();
   

3. Handle null inside the lambda expression.

   int[] arr = list.parallelStream().mapToInt(i -> i == null ? -1 : i.intValue()).toArray();