Generic function to swap rows and columns in an array

Question

I want to make a function which can take any type of array (int, String, boolean, etc) and return an array of the same type with the rows and columns switched.

public static String[][] swap_rows_columns_str_arr(String[][] arr) {
    String[][] new_arr = new String[arr[0].length][arr.length];
    for (int col = 0; col < arr[0].length; col++) {
        for (int row = 0; row < arr.length; row++) {
            new_arr[col][row] = arr[row][col];
        }
    }
    return new_arr;
}

I've managed to get it work for singular datatypes, but how would I get it to work for any datatype?

Welcome to the rabbit hole of type erasure. Unfortunately, to my knowledge, you'll have to dip into a bit of reflection to allocate an array of generic type. It's far more complicated than it has any right to be. — Silvio Mayolo, Dec 26 '21 at 18:59
Better to work with nested Lists of generic type, `List>` than with arrays. — DontKnowMuchBut Getting Better, Dec 26 '21 at 19:06

score 1 · Answer 1 · answered Dec 26 '21 at 20:12

You can use a generic type <T>, but you have to use Array.newInstance() to create the array (see What's the reason I can't create generic array types in Java?). The source code can look like this:

public static <T> T[][] swap_rows_columns_str_arr(T[][] arr) {
    @SuppressWarnings("unchecked")
    T[][] new_arr = (T[][]) Array.newInstance(
            arr.getClass().getComponentType().getComponentType(),
            arr[0].length, 
            arr.length);
    for (int col = 0; col < arr[0].length; col++) {
        for (int row = 0; row < arr.length; row++) {
            new_arr[col][row] = arr[row][col];
        }
    }
    return new_arr;
}

This only works for non-primitive types, as primitive types cannot be used in generics. For the primitive types you have to create eight overloads of this method.

Generic function to swap rows and columns in an array

1 Answers1