获得一组数字的最大差异

Question

我的代码有问题，这是错误：

  

线程中的异常＆＃34; main＆＃34; java.lang.NullPointerException at   javaPractice.Difference.computeDifference（Difference.java:24）at   javaPractice.diffSolution.main（diffSolution.java:18）

class Difference {

    private int[] elements;
    public int maximumDifference;

    public Difference(int[] a) {
        this.elements = a;
    }

    List<Integer> maximumDifferenceList;

    public void computeDifference() {
        for(int i = 0; i < elements.length; i++) {
            for(int j = 0; j < elements.length; j++) {
                if(i != j) {
                    maximumDifferenceList.add(Math.abs(elements[i] - elements[j]));
                }
            }
        }
        Collections.sort(maximumDifferenceList);
        maximumDifference = (int) Collections.max(maximumDifferenceList);
    }

}

public class diffSolution {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        sc.close();

        Difference difference = new Difference(a);

        difference.computeDifference();

        System.out.print(difference.maximumDifference);
    }

}

我试图打印出元素数组，它似乎有值，但它没有评估下面的行：

  

maximumDifferenceList.add（Math.abs（elements [i] - elements [j]））;

先谢谢你了！