Calling a non-static method in a static class - java

Question

I am trying to write a code that generates a List containing all the possible permutations of a given int array.

I have found online a method ("nextPermutation" in the code below) that allows to do that, and I am trying to implement it into a basic code, but it does not work.

The problem is that when I try to dynamically add the array containing the new permutation to the list, all the previous permutations already stored in the list are replaced with the new one.

I guess the problem is somehow related with the fact that my "nextPermutation" is non-static, but I have no idea about what I should do to fix it.

Any suggestion?

package lang_dist;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class lang_dist {

    public boolean nextPermutation(int[] array) {
        // Find longest non-increasing suffix
        int i = array.length - 1;
        while (i > 0 && array[i - 1] >= array[i])
            i--;
        // Now i is the head index of the suffix


        // Are we at the last permutation already?
        if (i <= 0)
            return false;

        // Let array[i - 1] be the pivot
        // Find rightmost element that exceeds the pivot
        int j = array.length - 1;
        while (array[j] <= array[i - 1])
            j--;
        // Now the value array[j] will become the new pivot
        // Assertion: j >= i

        // Swap the pivot with j
        int temp = array[i - 1];
        array[i - 1] = array[j];
        array[j] = temp;

        // Reverse the suffix
        j = array.length - 1;
        while (i < j) {
            temp = array[i];
            array[i] = array[j];
            array[j] = temp;
            i++;
            j--;
        }

        // Successfully computed the next permutation
        return true;
    }

    public static void main( String[] args )
    {


    int[] array = {0, 0, 1, 1, 1, 1};


    List<int[]> rowList = new ArrayList<int[]>();
    List<int[]> results = new ArrayList<int[]>();

    lang_dist d=new lang_dist();

    while (d.nextPermutation(array)){

         System.out.println("Permutation:" + Arrays.toString(array));

         results = Arrays.asList(array);

         rowList.add(results.get(0));


    };

    System.out.println("---");
    for (int[] row : rowList) {
        System.out.println("Row = " + Arrays.toString(row));
    }
    }


}

There are *static* methods and there are *instance* methods. Create an instance of `lang_dist` (`new`) and call your method on it. — Elliott Frisch, Mar 15 '16 at 15:33
yes, that's what i tried to do with the bit `lang_dist d=new lang_dist(); while (d.nextPermutation(array)){ ...` of the code above, but it doesn't solve the problem. Or this is not the right way to create an instance? Sorry, I am very new to java and i could be missing something very obvious.. — yamayama, Mar 15 '16 at 15:51

Julien Lopez · Accepted Answer · 2017-10-06T14:49:40.027

The (main) problem is that you store your results in the same array at every permutation. Therefore, rowList contains n references to the same array.

To (quickly) fix the problem, you need to create a new array for every permutation:

results = Arrays.asList(array.clone());

Besides, results here is redundant, use rowList or results to store your permutations.

I suggest you have a look at: Are arrays passed by value or passed by reference in Java? and Is Java "pass-by-reference" or "pass-by-value"?

Calling a non-static method in a static class - java

1 Answers1