Reimplement of Python's itertools.permutations in Java

PyItertoolPermutation.java

public class PyItertoolPermutation {
    public static void main(String[] args) {
        char[] arr = {'A', 'B', 'C', 'D'};
        int r = 2;
        dfs(0, arr, r);

    }

    public static void dfs(int changing_idx, char[] arr, int r) {
        if (changing_idx == r){
            printFirstN(arr, r);
            return;
        }

        // assume the input is already sorted

        // initial trigger
        dfs(changing_idx + 1, arr, r);

        int remaining_choice = arr.length - 1 - changing_idx;
        // tick
        for (int i = 1; i <= remaining_choice; i++) {
            int swap_idx = changing_idx + i;
            swap(arr, changing_idx, swap_idx);
            // trigger here
            dfs(changing_idx + 1, arr, r);
        }

        // reset to keep the order
        moveToLast(arr, changing_idx);
    }

    public static void swap(char[] arr, int i, int j){
        char tmp = arr[i]; arr[i] = arr[j]; arr[j] = tmp;
    }

    public static void moveToLast(char[] arr, int idx) {
        char tmp = arr[idx];
        for (int i = idx; i < arr.length - 1; i++) {
            arr[i] = arr[i+1];
        }
        arr[arr.length - 1] = tmp;
    }

    public static void printFirstN(char[] arr, int n){
        for (int i = 0; i < n; i++) {
            System.out.print(arr[i]);
        }
        System.out.println();
    }
}