java - StringUtils.join() returning pointer

Question

I was trying to join the elements of an array via the StringUtils.join(array, separator) method from the Apache commons library. Can anyone please explain me why I cannot get the resulting string and only a pointer to its location if I use an array of primitive types like int[] ? Please see example code below:

public static void main(String[] args){

    String[] s = new String[]{"Anna","has", "apples"};
    System.out.println(StringUtilities.join(s, " "));

    int[] a = new int[]{1,2,3};
    System.out.println(StringUtilities.join(a, " "));

    Integer[] b = new Integer[]{1,2,3};
    System.out.println(StringUtilities.join(b, " "));
}

Only using Integer arrays works. I understood that arrays of primitives are internally treated differently than ArrayList or other higher order objects, but then why is it possible (same topic more or less but a different question maybe) to instantiate a HashMap<String, int[]> without any warnings, exceptions? Are the arrays internally wrapped in another object? Only for maps? From what I read from the doc you cannot parametrize a map, set, array list etc with primitive types, which I understand, but then... I find it a bit confusing. Any reasonable explanation would be appreciated. Thank you.

Answer 1

Take a look at the signature for int arrays of StringUtils#join:

join(byte[] array, char separator)

You called join using

StringUtils.join(a, " "),

using a String instead of a char. Try using

StringUtils.join(a, ' ')

instead.

What happened is that your call matched another signature:

join(T... elements),

so your arguments get interpreted as two objects, an integer array and a String with a space character. While creating the result string, the method concatenated the string representation of the integer array with the string.

Answer 2

An array of ints (a primitive) is an object.

I suspect the implementation doesn't support primitive arrays, and calls toString() on them, resulting in something like [I@659e0bfd (which translates to single dimensional (only one [) array of Ints and its "memory location").