我需要使用ArrayList类对数组进行排列。 我不知道怎么了

Question

so im taking an ap comp sci class in school, and in the class we're learning about the basics of java. For this assignment we have to make permutations by taking numbers from one one-dimensional array, and putting in another, then deleting that number so it can't be picked again. The numbers in the array can't repeat. We have to use the ArrayList Class too. And I can't figure out what's wrong! This is the method that creates the permutations:

 public static ArrayList<Integer> createPerm()
    {
        ArrayList<Integer> list = new ArrayList<Integer>(10);
        Integer x = 1, remove = 0;
        for (Integer i = 0; i < 10; i++)
        {
            list.add(x);
            x++;
        }

        ArrayList<Integer> perm = new ArrayList<Integer>(10);

        for(Integer i = 0; i < 10; i++)
         {
            Integer r = (int)(Math.random() * 10) + 1;
            for (Integer j = 0; j <= list.size() - 1; j++)
            {
                if (list.get(j) == r)
                {
                    remove = j + 1;
                    list.remove(remove);
                    perm.add(r);
                }
            }
        }

        return perm;

}

Answer 1

I think that you (and I also:)) got a lttle bit confused because you are using Integer-objects as index and as list elements.
That is no problem with the List.get method, because there is only one get method which is expecting an int and Java converts the Integer to int.
The problem is in your usage of list.remove(). There are two methods, one expects an object and one an int.
So if you pass an Integer object, the remove(Object) method is called. But you pass the index, not the r-matching object, so the remove method fails sometimes, because it is random if the element is in your list if remove was called before. And if the method not fails, you have removed the element with the value of your index(+1), not the one who matches r.

     for(Integer i = 0; i < 10; i++)
     {
        Integer r = (int)(Math.random() * 10) + 1;
        for (Integer j = 0; j <= list.size() - 1; j++)
        {
            if (list.get(j) == r)
            {
                remove = j + 1;
                list.remove(remove);//the main error is here you found 4 
                                    //on index 2 and removes 3 (because of +1)
                perm.add(r);
            }
        }
    }

The next thing is, that random can deliver the same number more than once,
so you should not loop only 10 times. Loop until the list is empty.
I have corrected the code as below, the original lines are commented before the correction.

    //for (Integer i = 0; i < 10; i++) {
    while (!list.isEmpty()) {
        Integer r = (int) (Math.random() * 10) + 1;
        for (Integer j = 0; j <= list.size() - 1; j++) {
            //if (list.get(j) == r) {
            if (list.get(j).equals(r)) {
                //remove = j + 1;
                remove = list.get(j);
                list.remove(remove);
                perm.add(r);
            }
        }
    }

And here I put the code somewhat more clearly, so that it is easier to read

public static ArrayList<Integer> createPerm() {
    ArrayList<Integer> list = new ArrayList<Integer>(10);
    for (int i = 0; i < 10; i++) {
        list.add(i+1);
    }

    ArrayList<Integer> perm = new ArrayList<Integer>(10);

    while (!list.isEmpty()) {
        int r = (int) (Math.random() * 10) + 1;
        for (int j = 0; j < list.size(); j++) {
            if (list.get(j).intValue() == r) {
                perm.add(list.remove(j));
            }
        }
    }

    return perm;
}