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static String A[] = {"1123", "3312", "0145" };
static String B[] = {"1213", "1110", "3240" };
static String C[] = {"1121", "0101", "0431" };
static String ALL[][] = {A, B, C };
static String List[] = Arrays.stream(ALL).flatMap(pl ->Arrays.stream(pl)).toArray(String[]::new);

In the last line
static String List[] = Arrays.stream(ALL).flatMap(pl ->Arrays.stream(pl)).toArray(String[]::new);
What does the code for mean?
I didn't understand it when I looked it up, so could anyone tell me?