checking if arraylist is sorted or not-CodePudding

I need to check if the elements in the arraylist are arranged in ascending order or not. and when i use this code, the output shows "not sorted" even for a sorted array. why is it showing the wrong output?**

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    ArrayList<Integer> array1 = new ArrayList<>();
    ArrayList<Integer> array2 = new ArrayList<>();
    int n = sc.nextInt();
    for (int i = 0; i < n; i  ) {
        array1.add(i, sc.nextInt());
        array2.add(i, array1.get(i));
    }

    Collections.sort(array1);
    if (array1 == array2) {
        System.out.println("sorted");
    }
    else {
        System.out.println("not sorted");
    }
}

CodePudding user response：

"==" checks weather two object identical that means it check weather they point to the same memory location. If your array1 is on @698 location your array2 will be somewhere else like @700 ,so they don't point to the same location that's why it shows they are not equal. You better check it on this way:

    if (array1.equals(array2)){
        System.out.println("They are equal");
    } else{
        System.out.println("They are not equal");

Then it shows the right answer

CodePudding user response：

The error using == instead of equals is clear. For arrays there is Arrays.equals(int[] lhs, int[] rhs).

However sort does to much work, and needs a copy of the array. Better:

static boolean isSorted(List<Integer> list) {
    for (int i = 1; i < list.size();   i) {
        if (list.get(i).compareTo(list.get(i - 1)) < 0) {
            return false;
        }
    }
    return true;
}

Could even be more general:

static <T extends Comparable<T>> boolean isSorted(List<T> list) {