All Elements to n 

public class ElementN {
    public void changeArray(int[] array, int n) {
        for (int i = 0; i < array.length; i++) {
            array[i] = n;
        }
    }

    public static void main(String[] args) {
        ElementN elementN = new ElementN();
        int[] array = {1, 2, 3, 4, 5};
        System.out.println(Arrays.toString(array));
        elementN.changeArray(array, 10);
        System.out.println(Arrays.toString(array));
    }
}

ElementN.main(null);

[1, 2, 3, 4, 5]
[10, 10, 10, 10, 10]

Average of Array 

public class FindAverage {
    public double findAverage(ArrayList<Integer> arr) {
        int sum = 0;
        for (int i =0; i< arr.size(); i++) {
            sum += arr.get(i);
        }
        return (double) sum / arr.size();
    }

    public static void main(String[] args) {
        FindAverage average = new FindAverage();
        ArrayList<Integer> arr = new ArrayList<>();
        arr.add(1);
        arr.add(2);
        arr.add(3);
        System.out.println(arr);
        System.out.println(average.findAverage(arr));
    }
}

FindAverage.main(null);

[1, 2, 3]
2.0

Find Average of Diagonal 

public class AverageDiagonal {
    public double findAverage(int[][] arr) {
        int sum = 0;
        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr[0].length - i; j++) {
                if (i == j) {
                    sum += arr[i][j];
                }
            }
        }

        return (double) sum / arr.length;
    }

    public static void main(String[] args) {
        AverageDiagonal average = new AverageDiagonal();
        int[][] arr = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
        System.out.println(Arrays.deepToString(arr));
        System.out.println(average.findAverage(arr));
    }
}

AverageDiagonal.main(null);

[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
2.0