Java 8 Streams int and Integer

Question

Was just getting my hands dirty with Java 8 and stumbled on a behaviour as below -

public static void main(String... args){
    System.out.println("[Start]");
    int[] ints = {1, 2, 3, 4};
    Stream.of(ints).forEach(i->System.out.println("Int : "+i));


    Integer[] integerNums = {1, 2, 3, 4};
    Stream.of(integerNums).forEach(i->System.out.println("Integer : "+i));

    System.out.println("[End]");
}

And the output is :

[Start]
Int : [I@5acf9800
Integer : 1
Integer : 2
Integer : 3
Integer : 4
[End]

Whereas I was expecting the code to print all int's and Integer's in both the cases? Any insights on this would be greatly helpful...

Answer 1

Generics don't work with primitive types. The Stream.of method is declared as

static <T> Stream<T> of(T... values)

When you use

Stream.of(ints)

where ints is an int[], Java doesn't see a number of int elements (primitive types), it sees a single int[] element (which is a reference type). So the single int[] is bound as the only element in the array represented by T... values.

So values becomes an int[][] and each of its elements is an int[], which print like

[I@5acf9800