Conbinacion Y Permutacion En Java

package matematicas;

import javax.swing.*;

public class Matematicas {

public static int factorial(int x){

int i,f;

f=1;

for(i=1;i<=x;i++)

f=f*i;

return f;

}

//

public static int permuta(int n, int r){

int P;

P=factorial(n)/factorial(n-r);

return P;

}

//

public static int combina(int n, int r){

int c;

c=factorial(n)/(factorial(r)*factorial(n-r));

return c;

}

public static void main(String[] args) {

int N,R,Comb,Perm;

int op=0;

do{

try{

op=Integer.parseInt(JOptionPane.showInputDialog("Combinaciones y Permutaciones\n"+

" ----------Menu--------\n"

+ " 1 - Combinaciones\n"

+ " 2 - Permutaciones\n"

+ " 3 - Salir\n"

+ " Elegir una Opcion\n"));

if(op>=1&&op<=3){

switch(op){

case 1:

JOptionPane.showMessageDialog(null,"Combinaciones");

try{

N=Integer.parseInt(JOptionPane.showInputDialog("Introdusca el valor N:"));

R=Integer.parseInt(JOptionPane.showInputDialog("Introdusca el valor R:"));

Comb=combina(N,R);

JOptionPane.showMessageDialog(null,"Combinacion = "+Comb);

}catch(NumberFormatException ex){

JOptionPane.showMessageDialog(null,"ERROR, Linea : "+ex);

}

break;

case 2:

JOptionPane.showMessageDialog(null,"Permutaciones");

try{

N=Integer.parseInt(JOptionPane.showInputDialog("Introdusca el valor N:"));

...