How to run N test cases when the program has global variables?

Since the program has some global variables, I think it is not looping N times, accepting N from the user, where N is the number of test cases.

I tried accepting N in main and then adding a for loop running N times the body of main, but this doesn't work and always gives an error.

Here is the code:(To print the permutation of a string which is next in the lexographical order)

import java.util.*; 
public class Permutation
{ 
    // Returns true if str[curr] does not matches with any of the  
    // characters after str[start]  
    static String x[];static int i=0;
    static String sort(String s)
    {
        char a[]=s.toCharArray();
        Arrays.sort(a);
        return String.valueOf(a);
    }

    static boolean shouldSwap(char str[], int start, int curr) { 
        for (int i = start; i < curr; i++) { 
            if (str[i] == str[curr]) { 
                return false; 
            } 
        } 
        return true; 
    }
    // Prints all distinct permutations in str[0..n-1]  
    static void findPermutations(char str[], int index, int n) { 

        if (index >= n) { 
            x[i++]=String.valueOf(str);
        } 

        for (int i = index; i < n; i++) { 

            // Proceed further for str[i] only if it  
            // doesn't match with any of the characters  
            // after str[index]  
            boolean check = shouldSwap(str, index, i); 
            if (check) { 
                swap(str, index, i); 
                findPermutations(str, index + 1, n); 
                swap(str, index, i); 
            } 
        } 
    } 

    static void swap(char[] str, int i, int j) { 
        char c = str[i]; 
        str[i] = str[j]; 
        str[j] = c; 
    } 

    // Driver code 
    public static void main(String[] args) { 
        Scanner sc=new Scanner(System.in);
        String s1=sc.nextLine();
        String s=sort(s1);
        char str[]=s.toCharArray();
        int n = str.length; int nf=1;
        for(int i=1;i<=n;i++)
        {
            nf=nf*i;
        }
        x=new String[nf];
        Arrays.fill(x," ");
        findPermutations(str,0,n);

        for(int i=0;i<x.length;i++)
        {
            if(x[i].equals(s1))
            {
                System.out.println(x[i+1]);
            }
        }
    } 

How shall I solve this error? I don't want to reprogram the entire thing.