Flatten nested string arrays

Question

I'm trying to use streams to flatten a multidimensional string array. I could do it with loops but streams seems to be the more idiomatic way. I'm aware of this question but that uses integer specific methods. The following returns an object array instead of the expected String[].

String[][][] sources = new String[][][]{
           {{"a","b","c"},{"d","e","f"}}
           {{"g","h","i"},{"j","k","l"}}
};
String[] values = Arrays.stream(sources[0])
            .flatMap(Arrays::stream)
            .toArray();

"instead of the expected string[]" had you read [the Javadoc](https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#toArray--), you wouldn't be expecting that. — Andy Turner, Jun 11 '18 at 17:25
Thanks, I totally overlooked that and thought the problem was upstream — A. Scientist, Jun 11 '18 at 17:28
your array should be an array of char `char[][][] sources = new char[][][]` not of String!! — Youcef LAIDANI, Jun 11 '18 at 17:29
@YCF_L or the array elements are actually declared with `"`s instead of `'`s. — Andy Turner, Jun 11 '18 at 17:30

Ousmane D. · Accepted Answer · 2018-06-11T17:46:00.770

To make your current example compile, the toArray should be:

String[] values = Arrays.stream(sources[0])
                        .flatMap(Arrays::stream)
                        .toArray(String[]::new); // <----------------------

If you want to do it for sources you'll need to use flatMap twice then collect to an array with .toArray(String[]::new):

String[] values = Arrays.stream(sources)
                        .flatMap(Arrays::stream)
                        .flatMap(Arrays::stream)
                        .toArray(String[]::new);

Flatten nested string arrays

1 Answers1