Java 8 fill array with supplier

Question

Is there a way to fill an array using java 8 Supplier ?

I would like to write:

Supplier<Object> supplier = () -> new Object();
Object[] array = new Object[size];
Arrays.fill(array, supplier);

Note: I know i could write my own method.

Solution 1

In case you want to create new array filled with results generated by Supplier you can use

Object[] array = Stream.generate(supplier)
                       .limit(arraySize)
                       .toArray(); // will generate new *Object[]* array

For different types than Object[] you can use toArray(IntFunction<YourType[]> generator); like toArray(YourType[]::new) (credits to @Holger).

String[] array  = Stream.generate(supplier)
                        .limit(10)
                        .toArray(String[]::new); //now *String[]* array will be returned

Streams also allow us to work with most "popular" primitive types which are int long and double. For instance we can use IntStream#toArray to create int[] holding elements from IntStream. To "fill" IntStream with elements from supplier we can use IntStream.generate(intSupplier) like

int[] array = IntStream.generate(()->1)
                       .limit(5)
                       .toArray(); //returns `new Int[]{1,1,1,1,1}

---

In case when you want to fill already existing array with data from Supplier see answer posted by Stuart Marks based on Arrays.setAll(array, supplier) which aside from handling arrays of objects also supports some arrays of primitive types: double[] int[] and long[] .

Other alternative is to use use creative solution from @Hogler's comment:

Arrays.asList(array).replaceAll(x -> supplier.get()); 
//you can even overwrite a range using `subList`

just be aware of its problems with array of primitive types explained by Jon Skeet at https://stackoverflow.com/a/1467940.

Solution 2

In java.util.Arrays there is

<T> void Arrays.setAll(T[] array, IntFunction<T> generator)

This doesn't take a supplier; instead it takes an IntFunction whose input argument is the array index being filled. If your objects aren't dependent upon the destination array index, you can disregard the parameter and call a supplier like this:

Arrays.setAll(array, i -> supplier.get());

There are overloads for arrays of primitives as well as arrays of reference type. There is also a corresponding family of methods parallelSetAll() that does the same thing, except in parallel. (It uses streams internally.)

Comments

[gontard]

Thanks, but i should have been more specific in my question. I know i could write method with java.

[gontard]

Thanks, this is very close to what i am looking for. I gave my example with Object but what if i want to use another type like Integer or Date ?

[Holger]

@gontard: For generating an array of MyType, use toArray(MyType[]::new)

[gontard]

Ok. Could you add this alternative to your answer ? A last question: is it possible to do the same thing with a generic type ? I know it is not possible to create an array of generics but may be...

[Syam S]

@Pshemo: I got. I thought he wanted a usual filling with same object.

[Holger]

As a remark, you can fill an already existing array in the following “creative” way: Arrays.asList(array).replaceAll(x->supplier.get());. You can even overwrite a range using subList.

[Timofey Gorshkov]

In this solution array created twice.

[Norbert Madarász]

Using the creative way one should be careful with primitive types. First example is ok: System.out.println(Arrays.asList( new Integer[]{1,2,3,4,5})); [1, 2, 3, 4, 5] Second example is a list containing one single element which is the whole array: System.out.println(Arrays.asList( new int[]{1,2,3,4,5})); [[I@53b32d7]