Invertir diagonal principal JAVA

 Dada una matriz cuadrada invertir su diagonal principal


import java.util.Scanner;
public class JavaMatrizMetod2 {

  public static void llenar (int M [] [], int d)
    {
        Scanner Leer = new Scanner(System.in);
    for (int i = 1 ; i <= d ; i++)
    {
        for (int j = 1 ; j <= d ; j++)
        {
        System.out.print("Inserte pos[" + i + "][" + j + "]: ");
        M [i] [j] = Leer.nextInt();
        }
    }
    }
    public static void mostrar (int M [] [], int d)
    {
    for (int i = 1 ; i <= d ; i++)
    {
        System.out.println ();
        for (int j = 1 ; j <= d ; j++)
        {
        System.out.print("[" + M [i] [j] + "]");
        }
    }
    }
    public static void invierte (int M [] [], int d)
    {
    int fin = d;
    for (int i = 1 ; i <= d / 2 ; i++)
    {
        int aux = M [i] [i];
        M [i] [i] = M [d] [d];
        M [d] [d] = aux;
        fin--;
    }
    }
    public static void main (String args [])
    {
        Scanner Leer = new Scanner(System.in);
    int M [] [] = new int [20] [20];
    System.out.print("Inserte dimen. de la matriz cuadrada: ");
    int d = Leer.nextInt();
    llenar (M, d);
    System.out.print("nMATRIZ ORIGINAL: ");
    mostrar (M, d);
    System.out.print("nnMATRIZ CON LA DIAGONAL PRINCIPAL INVERTIDA: ");
    invierte (M, d);
    mostrar (M, d);
    }