计算数组的所有排列

Question

I am trying to write a method to permute an array into all it's possible permutations. I am taking each array in the form of an ArrayList, flipping two elements, and then returning the ArrayList to an ArrayList of ArrayLists. If I print each array to the screen after flipping the two elements, it is printing as expected. [1,2,3] with the first two elements flipped prints as [2,1,3] but when I add the permuted ArrayList to the other ArrayList, they are all printing out as [1,2,3]

The code:

public static void arrays() {
    
    //convert the input Lists to ArrayLists
    ArrayList<Integer> a = new ArrayList<Integer>();
    a.add(1);
    a.add(2);
    a.add(3);
    //ArrayList<Integer> b = new ArrayList<Integer>(B);
    
    //an array with 3 elements will have 1 * 2 * 3 permutations
    Integer numberOfPossiblePermutations = numberOfPermutations(a);
    
    ArrayList<ArrayList<Integer>> allPermutations = new ArrayList<ArrayList<Integer>>();
    allPermutations.add(a);
    
    System.out.println("The properly rearranged arrays");
    while (allPermutations.size() < numberOfPossiblePermutations) {
        //get the last array in the list of arrays
        ArrayList<Integer> lastArray = allPermutations.get(allPermutations.size()-1);
        ArrayList<Integer> arrayToAdd = lastArray;
        
        for (int i=0; i+1<lastArray.size(); i++) {
            //flip two elements in the array
            
            int x = lastArray.get(i);
            int y = lastArray.get(i+1);
            arrayToAdd.set(i+1, x);
            arrayToAdd.set(i, y);
            System.out.println(arrayToAdd);
            allPermutations.add(arrayToAdd);
        }
        
    }
    
    System.out.println();
    System.out.println("but when I add each ArrayList to another ArrayList they are coming out like this");
    
    for (int i=0; i<allPermutations.size(); i++) {
        System.out.println(allPermutations.get(i));
    }
    
}

//accepts an array and returns the possible number of permutations
public static Integer numberOfPermutations(ArrayList<Integer> a) {
    Integer numberOfPermutations = 1;
    for (int i=1; i<=a.size(); i++) {
        numberOfPermutations *= i;
    }
    return numberOfPermutations;    
}

The output:

• The properly rearranged arrays

• [2, 1, 3]

• [2, 3, 1]

• [3, 2, 1]

• [3, 1, 2]

• [1, 3, 2]

• [1, 2, 3]

• but when I add each ArrayList to another ArrayList they are coming out like this

• [1, 2, 3]

• [1, 2, 3]

• [1, 2, 3]

• [1, 2, 3]

• [1, 2, 3]

• [1, 2, 3]

• [1, 2, 3]

Answer 1

You are only interacting with a single ArrayList instance, but calling it by different names. So when you print it, you're printing it at different stages of that object's lifetime, but it's still the same instance.

This line:

ArrayList<Integer> arrayToAdd = lastArray;

does not make a copy of the object; it's just giving the same object a different name. You need to explicitly copy it. For example:

ArrayList<Integer> arrayToAdd = new ArrayList<Integer>();
arrayToAdd.addAll(lastArray);