Java 8 - best way converting array elements

Question

Starting with Java 8 so need a bit of time to get used to it. It's a classical problem, I've an array of objects that I want to transform.

Before Java8 the ideal code would be (no null pointers):

P[] outputArray = new P[inputArray.length];
for (int i =0; i< inputArray.length; i++ )
{
    outputArray [i] = inputArray[i].transformToP();
}

What is the best version in Java8 ?

Answer 1

Using the Stream API it's quite simple:

P[] outputArray = Arrays.stream(inputArray).map(e -> e.transformToP()).toArray(P[]::new);

Also method reference can be used (suppose that I is the type of input elements):

P[] outputArray = Arrays.stream(inputArray).map(I::transformToP).toArray(P[]::new);

Note that you may have problems if transformToP() method throws checked exceptions. In this case convert them to unchecked ones or consult this question.

Answer 2

Using a stream over an array is a fine technique as described in Tagir Valeev's answer. However, don't forget about Arrays.setAll. This is a handy shortcut for setting all the elements of an array based on index. To transform an array to a new array by some function, you could do this:

P[] outputArray = new P[inputArray.length];
Arrays.setAll(outputArray, i -> inputArray[i].transform());

You don't have to copy it into a new array. If you want to transform the array in-place, you could do this:

Arrays.setAll(array, i -> array[i].transform());

There is also a parallel variation parallelSetAll.

Under the covers this is just an IntStream.range over the indexes of the input array, but it's sometimes darned convenient for quick one-liners.